Reminiscences: A Journey Through Particle Physics 9789814405010, 9789814405003

Citation preview

REMINISCENCES A Journey through Particle Physics

8461.9789814405003-tp.indd 1

2/10/12 2:35 PM

October 4, 2012

4:7

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

This page intentionally left blank

ii

b1432-fm

REMINISCENCES A Journey through Particle Physics Adrian Melissinos University of Rochester, USA

World Scientific NEW JERSEY

•

8461.9789814405003-tp.indd 2

LONDON

•

SINGAPORE

•

BEIJING

•

SHANGHAI

•

HONG KONG

•

TA I P E I

•

CHENNAI

2/10/12 2:35 PM

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

REMINISCENCES A Journey through Particle Physics Copyright © 2013 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher.

ISBN 978-981-4405-00-3

Typeset by Stallion Press Email: [email protected]

Printed in Singapore.

Alvin - Reminiscences.pmd

1

10/2/2012, 2:34 PM

October 4, 2012

4:7

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Preface

This is a personal account of how I remember my involvement in research in Particle Physics from the time of 200 MeV Synchrocyclotrons on University campuses, to the 14 TeV era of the Large Hadron Collider at CERN. During that period I was a member of the Faculty of the University of Rochester, but the research took place at the large accelerators located at Brookhaven National Laboratory, Fermilab, Cornell University, Stanford’s Linear Accelerator (SLAC) and the European Center for Nuclear Research (CERN). The development of high energy physics was made possible by the successful and efficient operation of these machines and by the hospitality the host laboratories extended to University researchers and their students. This trend is clearly evident by the participation and contributions of several thousand researchers from all over the globe, to any one of today’s LHC experiments. I discuss the experiments in which I participated including some technical details, hopefully not so many as to discourage the reader. With hindsight I return to the cases where opportunities were missed. While the presentation is chronological, obviously it is very far from being a comprehensive record of the rapid and spectacular evolution of the field of particle, or high energy, physics during that period. All practitioners remember the fundamental discoveries that took place in those times and their reaction to the mundane but also the unexpected announcements, and how they interpolate with the events described in this account. The digression into lasers and into microwave techniques added an extra dimension to the challenges that could be addressed. v

b1432-fm

October 4, 2012

vi

4:7

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Even in the early years of this story, experiments were collaborative efforts and this required some degree of cooperation and civility within a research group. In fact, lasting friendships developed from sharing night shifts or long analysis discussions. As I look back over these years, the dominant recollection is the enthusiasm and joy of doing physics, shared with many a distinguished colleague and with the many gifted postdocs and graduate students who made the whole enterprise possible. I vividly remember each of them and have proudly followed their achievements, well beyond those recounted here. With my thanks, this book is dedicated to them. The research discussed here was supported over the years by the US Department of Energy (DOE), the successor to the Atomic Energy Commission and the Energy Research and Development Administration. I want to thank Bill Wallenmeyer, Bernie Hildebrand and P. K. Williams for their dedication to research in Particle Physics and for their outstanding support of the scientific programs. Robert Barker of the Air Force Office of Scientific Research, and Beverly Berger of the National Science Foundation, supported some of the experiments described here. I thank the American Physical Society, Elsevier Publishing, and the Particle Data Group, for permission to use figures from their publications. Finally, thank you to my long-time colleague, Professor Thomas Ferbel, for a critical reading of the entire manuscript and his valuable suggestions, to Dr. Melanie Day at the University of Rochester for help with Latex issues, and to my editor at WSPC for assistance with the production process.

Adrian Melissinos October 1, 2012

b1432-fm

October 4, 2012

4:7

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

b1432-fm

Contents

Preface

v

1.

1

Level Crossing

References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.

Strange Particles

13

The Rochester Emulsion Group . . . . . . . . . . . . The Low Energy Λ − p Interaction . . . . . . . . . . Analysis of Bubble Chamber Film at Rochester . . . The Ratio of K + /K − Lifetime; a Test of CP T Invariance . . . . . . . . . . . . . . . . . . . . . K + /K − Backward Scattering and pp Annihilations . References . . . . . . . . . . . . . . . . . . . . . . . . 3.

Deep Inelastic Scattering

. . 13 . . 14 . . 18 . . 21 . . 23 . . 30 31

Muon-Proton Scattering . . . . . . . . . . . . . . . . . . 31 The Sequel: µ − p 2 . . . . . . . . . . . . . . . . . . . . 37 References . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.

The Rising pp Total Cross Section

41

The US-USSR Gas Jet Experiment . . . . . . . . . . . . 41 The Flying Wire . . . . . . . . . . . . . . . . . . . . . . 50 References . . . . . . . . . . . . . . . . . . . . . . . . . . 54

vii

October 4, 2012

4:7

viii

5.

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

55

The Landscape . . . . . . . . . . . . . . . The Rochester–Brookhaven Collaboration The European Muon Collaboration . . . . References . . . . . . . . . . . . . . . . . . 6.

b1432-fm

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

The Cornell B-Factory

55 59 66 68 69

References . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.

Fourteen Orders of Magnitude

81

References . . . . . . . . . . . . . . . . . . . . . . . . . . 91 8.

Axions What are Axions . . . . . . . . . . . Microwave Search for Cosmic Axions Birefringence of the Vacuum . . . . Shining Light Through Walls . . . . References . . . . . . . . . . . . . . .

9.

93 . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

Photoinjectors

93 96 101 106 108 111

. . . .

. . . .

. . . .

. . . .

. . . .

Matter From Light Nonlinear QED . . . . . . . . . . . . . . . . The Experiment and its Logistics . . . . . . Results of the Experiment . . . . . . . . . . Breakdown of the Vacuum by Laser Fields . References . . . . . . . . . . . . . . . . . . .

11.

. . . . .

Lasers The Laboratory for Laser Energetics . . . . . . Radial Compression of Electromagnetic Fields . Visitor at LEP . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

10.

. . . . .

111 113 118 123 125

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

125 129 135 143 144 145

How it All Started . . . . . . . . . . . . . . . . . . . . . 145 The A0 Photoinjector and the Drive Laser . . . . . . . . 148

October 4, 2012

6:51

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

b1432-fm

Contents

ix

The Big Blunder . . . . . . . . . . . . . . . . . . . . . . 150 Electro-optic Sampling . . . . . . . . . . . . . . . . . . . 152 References . . . . . . . . . . . . . . . . . . . . . . . . . . 156 12.

The Earth Tides Gravitational Wave Laser Interferometers The Free-spectral-range (fsr) Channel . . Tidal Frequency Shift . . . . . . . . . . . The Isotropy of Space . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

157 . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

158 163 169 171 172

List of Figures

175

Attribution of Figures

187

Name Index

191

Subject Index

195

October 4, 2012

4:7

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

This page intentionally left blank

b1432-fm

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 1

Level Crossing

When I arrived at MIT in the summer of 1955, I was assigned as a research assistant in the group of Professor Francis Bitter, who was a kind and extremely supportive adviser to all his students. Bitter was well known as a leader in magnetism and in the mid 1930s established at MIT a facility where high magnetic fields were available for studying the susceptibility and other properties of materials. The magnets, of his own design, could reach 5 Tesla in a 4-inch diameter bore, by carrying large currents provided from a bank of submarine batteries [1]. After service with the US Navy during World War II, Bitter returned to MIT where he carried out a series of nuclear magnetic resonance experiments, and became interested in using the same principle in atomic systems. This led to the “double resonance” techniques which was first correctly formulated by A. Kastler of the College de France [2]. The first experiment was carried out by Jean Brossel, a former student of Kastler, and by then a doctoral student in Bitter’s lab [3]. When I joined the group it consisted of four fairly experienced graduate students, Bert Aubrey, Dick Lacey, Jack Stanley and Norman Adams, and a recent graduate, Paul Sagalyn, who would occasionally come to help us; there was also a close connection with the MIT atomic spectroscopy lab. The effort of the group was on the “double resonance” technique, to be explained below, in order to measure atomic hyperfine structure, and thereby extract from the 1

b1432-ch01

October 4, 2012

2

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

data precise values of nuclear magnetic moments. I “inherited” the working apparatus that Sagalyn had used in his thesis, and was therefore able to obtain new results in a short time, but without fully understanding the subtle details of the theoretical framework. For my thesis, I was to study Mercury-197 (197 Hg), a radioactive isotope of Mercury that also had an excited nuclear isomeric state, 197∗ Hg of sufficiently long lifetime such that it could be subjected to double resonance and to spectroscopic investigations. It was during this work that serendipity led me and our laboratory technician to consider “level crossing” in atomic systems. We made a lame attempt to detect the effect, but failed because we did not appreciate its full significance. However, to tell this story, we must first discuss in some detail the basics of double resonance experiments. Mercury has 80 electrons, two of which are outside of the closed shells and therefore determine its atomic spectrum. The spectral line that we studied was the well known UV line at λ = 253.7 nm, which is very strongly emitted in a Hg discharge, and is also easily excited by resonance radiation. It arises from the transition of the excited 6 3 P1 state to the 6 1 S0 ground state. The spectroscopic notation implies that the excited state has orbital angular momentum L = 1 (hence P state), total spin S = 1 (hence triplet), and total angular momentum J = 1 (subscript). The low lying excited states and spectral lines are shown in Fig. 1.1(a). Natural Hg contains several isotopes, and the size and the magnetic dipole (and electric quadrupole) moment of the nucleus, when different from zero, affect the energy of the spectral lines arising from the transition of the 3 P1 to the ground state. For isotopes with an even number of nucleons, the nuclear spin is zero, and the spectra differ only by their corresponding “isotope” shift. However 199 Hg and 201 Hg have nuclear spin I = 1/2 and I = 3/2 respectively, which are coupled to the total electronic angular momentum J, giving rise to a new quantum number F that labels the vectorial sum of I and J. For a given value of J, and when I < J, there are 2I + 1 F levels as a consequence of the coupling of the nuclear moments to the total electronic angular momentum; when J < I the number of levels is 2J + 1. This gives rise, in addition to the isotope shift, to the

b1432-ch01

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Level Crossing

b1432-ch01

3

Figure 1.1: (a) The spectrum of lines emitted in the transitions of the low-lying states of Hg. Wavelengths are given in ˚ A, where 1 ˚ A = 10−10 m. (b) The hyperfine structure of the 253.7 nm line, arising from the transition 3 P1 → 1S0 in natural Hg [3].

hyperfine structure of the spectral line. The structure of the 253.7 nm line of natural Hg is showna in Fig. 1.1(b). Energy differences are given in spectroscopic units of inverse length, κ = ∆E/hc = ∆ν/c, so that 1 cm−1 corresponds to ∆ν = 30 GHz. When an external magnetic field is applied to the sample, the degeneracy is lifted and the eigenstates acquire additional energy according to their m-projection quantum number, leading to a “splitting” of the energy levels (Zeeman effect). In the absence of nuclear spin, m = mJ = −J, −J + 1, . . . , J − 1, J, takes 2J + 1 values; in the presence of nuclear spin, and for weak magnetic field strength, each F -level is split into 2F + 1 sublevels labeled by their a

The notation for the Hg isotopes in the figures is based on the pre-1970 convention, where the atomic number A = Z + N , is shown in the upper right, rather than on the lower left of the chemical symbol of the element.

October 4, 2012

4

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

b1432-ch01

Reminiscences: A Journey Through Particle Physics

mF quantum number. For such weak fields the additional energy due to the magnetic field is ∆EJ = mJ gJ B

and ∆EF = mF gF B,

(1)

where gJ , gF are the corresponding “g-factors”, and B is the external magnetic field. For the even isotopes of Hg, I = 0, and the situation is simple. Since the excited atoms are in the J = 1 state, the level splits into three sublevels that grow as −B, 0, B, and since the ground state has J = 0, the spectral line for the 3 P1 → 1S0 transition depends similarly on B. For odd isotopes, every hyperfine level splits into 2F + 1 sublevels at low fields. However, when the interaction energy ∆E becomes comparable to the hyperfine splitting, the coupling to the electron’s angular momentum dominates, and the energy levels follow the mJ quantum number, but each such level is split further into 2I + 1 sublevels. This is shown in Fig. 1.2 for 199 Hg where I = 1/2. We are now able to describe the double optical resonance experiment. It is based on the concept of “optical pumping”, but at a time when tunable lasers were not available. In the absence of a laser, the exciting radiation was provided by an electrodeless discharge (to reduce the line width) of a single isotope of Hg, 198 Hg, placed in a strong magnetic field, the “scanning” field. A filter was used to select

Figure 1.2:

Zeeman effect of the hyperfine levels of the 3 P1 state of

199

Hg [3].

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Level Crossing

b1432-ch01

5

the 253.7 nm line, and by tuning the scanning field, the frequency of one Zeeman component of the emitted radiation could be set to a particular value. This “pump” radiation was incident on a Hg cell, (the sample), placed in a separate magnetic field, the “splitting” field. Any desired m-sublevel of the sample could be excited by adjusting the frequency and polarization of the “pump” radiation. The “resonance” radiation emitted from the sample was observed at 90 degrees relative to the incident excitation, and maintains the polarization of the pump. A schematic of the apparatus is shown in Fig. 1.3. Consider now a transition from the resonantly excited m-sublevel in the sample to another m-sublevel before the atom returns to the

Figure 1.3:

Schematic of the double resonance apparatus [4].

October 4, 2012

6

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

ground state. The decay from this new sublevel will, in general, alter the polarization of the resonance radiation. The sample is placed in a microwave cavity, at a location where the oscillatory magnetic field H1 is maximal. When the microwave frequency equals the energy difference between the two m-sublevels, the appropriate rotating component of H1 induces transitions. The transition is detected by the appearance of the orthogonal polarization in the emitted resonance radiation. In my experiment, we used S-band microwaves (war surplus equipment) at f ∼ 3 GHz which, for 198 Hg, resonate when the splitting field is of order ∼0.3 T. A typical resonance signal as a function of the strength of the splitting field is shown in Fig. 1.4.

Figure 1.4: Line shape of the signal intensity at fixed microwave frequency as a function of the “splitting” magnetic field [4].

b1432-ch01

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Level Crossing

b1432-ch01

7

The width of the line in Fig. 1.4 is ≈10 MHz, and it exceeds the natural width of the excited state, because the line is broadened by the microwave power. Measuring the transition frequency and the splitting magnetic field one can determine the hyperfine spacing and therefore the value of the nuclear moments. The nomenclature “double resonance” is now clear: the first resonance occurs when the pump radiation has been correctly selected to excite the desired msublevel of the sample, and the second resonance occurs when the microwave frequency coincides with the energy difference between the two m-sublevels, that depends on the strength of the splitting magnetic field. After working with natural Hg and completing an extensive study of the hyperfine structure and isotope shift [4], I turned my attention to my thesis topic, involving the investigation of radioactive 197 Hg. This metastable isotope has a lifetime of 65 hours, and, as already mentioned, there also exists an isomeric (excited nuclear) state with a lifetime of 23 hours that is labeled as 197∗ Hg. The nuclear decay scheme is shown in Fig. 1.5. The sample was prepared by bombarding gold with 15 MeV deuterons from the MIT cyclotron, using the reac80 tion 79 197 Au(d, 2n)197 Hg. The Hg was then boiled off the gold foil and sealed in a quartz cell. This latter process produced some anxious

Figure 1.5:

Nuclear decay scheme of radioactive

197

Hg and

197∗

Hg [5].

October 4, 2012

8

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

moments, when the glass vacuum distillation apparatus shattered in one of our attempts to extract the mercury from the gold (reverse alchemy!). In the end, about 1 mCurie of 197 Hg, which corresponds to ∼1013 atoms, was loaded into the cell. This was sufficient for obtaining good double resonance signals. The radioactive samples were used to carry out successful double resonance experiments on both 197 Hg and 197∗ Hg, but also to take high resolution spectra of the hyperfine structure of the 253.7 nm line of 197∗ Hg using the MIT grating spectrograph in 13th order and also crossed by a Fabry–Perot etalon. The spectroscopic work was done under the guidance of Dr. S. P. Davis, who went on to a distinguished career on the faculty at U. C. Berkeley, and with help from Professor L. C. Bradley. One of the more rewarding results was the measurement of the isotope shift of 197∗ Hg and its comparison to the shift in 197 Hg, which led to the quantitative establishment of the difference in the radius of an excited and ground state of an atomic nucleus. 197 Hg has nuclear spin I = 1/2 and the hyperfine structure is therefore very similar to that of the stable 199 Hg isotope, but shifted due to the different size of the nucleus. On the other hand 197∗ Hg has nuclear spin I = 13/2. Coupling of this spin to the, J = 1, total orbital angular momentum, gives rise to three hyperfine levels, with F = 11/2, F = 13/2, and F = 15/2. In the presence of an external magnetic field, these hyperfine levels split correspondingly, into 12, 14 and 16 Zeeman sublevels. To calculate the energy of the sublevels as a function of the magnetic field it is necessary to invert a 13 × 13 matrix. At that time, in 1957, this was a daunting task and I therefore asked the MIT computing center to do the calculation on their recently-acquired IBM 704 computer. The result, in Fig. 1.6, shows the multitude of mF sublevels at low field, while at high field the pattern converges towards the three mJ sublevels expected from the J = 1 total electronic angular momentum of the excited atomic state [5]. I was very proud of the plot and made an enlarged copy which I posted prominently in the lab. The lab was kept in running order by a gifted technician, Mr. E. Bardho. When he saw the plot, he was intrigued by the

b1432-ch01

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Level Crossing

Hg [5]. 197∗

Zeeman effect of the 3 P1 level of Figure 1.6:

9

b1432-ch01

October 4, 2012

10

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

convergence of most of the F = 13/2 sublevels when the splitting magnetic field was B ≈ 0.8 T, and reasonably asked what would happen when that field value was reached. We decided that transitions between sublevels would take place, as in the standard double resonance experiment, and would be observable even in the absence of microwave power. The conclusion was correct, and the effect indeed occurs whenever two m-sublevels cross, provided ∆m = 0 or 2 [7]. We set out to observe the effect by sweeping the “splitting” magnetic field near the calculated cross-over value, which was done by advancing with a stepping motor the contact point on a rheostat. The sweeping speed was adjusted to optimize the observable signal under normal operating conditions, that is, when the microwave power was on. We did not find a signal and soon gave up the search. The reason that we did not observe a signal was that in the absence of microwave power, the width of the transition is governed by the very narrow natural width of the excited atomic state, and given the integration time of our detection chain, the sweep was too fast to observe such a narrow line. About three months after I left MIT and was not any longer involved in atomic physics, a group at the University of Michigan led by Peter Franken published the first results on “level crossing” in atomic systems [6]. They used Helium, which has an electronic configuration similar to that of Hg, and observed the cross-over between mJ -sublevels with ∆mJ = 0 in a moderate magnetic field. As shown in Fig. 1.7, the crossing occurred for the sublevels belonging to the 23 P1 and 23 P2 levels of the first excited state of orthohelium. Soon after this announcement my former colleagues returned to my apparatus, and within half an hour located the level crossing in 199 Hg [8]. The cross-over of the F = 1/2, mF = +1/2 and F = 3/2, mF = −3/2 sublevels can be seen in Fig. 1.2. The advantage of this technique is that because the line is so narrow, the fine, or hyperfine, spacings can be obtained with greatly improved accuracy using the value of the magnetic field at which the cross-over occurs. The moral of this reminiscence is that, while intuition is valuable, it is not sufficient, and that it is important to understand beforehand what to expect and look for, when carrying out an experiment.

b1432-ch01

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Level Crossing

Figure 1.7:

b1432-ch01

11

Level crossing in Helium [6].

References 1. 2. 3. 4. 5.

“Magnets”, by F. Bitter, Doubleday Anchor Books, Garden City, NY 1959. J. Brossel and A. Kastler, Compt. Rend. 229, 1213 (1949). J. Brossel and F. Bitter, Phys. Rev. 86, 308 (1952). P. L. Sagalyn, A. C. Melissinos and F. Bitter, Phys. Rev. 109, 375 (1958). A. C. Melissinos, Phys. Rev. 11, 126 (1959); A. C. Melissinos and S. P. Davis, Phys. Rev. 115, 130 (1959). 6. F. D. Colgrove et al., Phys. Rev. Lett. 3, 420 (1959). 7. P. A. Franken, Phys. Rev. 121, 508 (1961). 8. H. R. Hirsch, Bull. Am. Phys. Soc. Series II, 5, 274 (1960).

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

This page intentionally left blank

b1432-ch01

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 2

Strange Particles

The Rochester Emulsion Group I came to Rochester in the Fall of 1958, having been hired by Mort Kaplon as a postdoc in the Nuclear Emulsion group that he headed. Mort was a gifted physicist who had completed his Ph.D. in particle theory under Robert Marshak. It seems that research with emulsions started at Rochester when the Austrian physicist Marietta Blau, who was then on the staff at Brookhaven Laboratory, donated a few exposed plates to Marshak, who in turn assigned Kaplon to analyze them. Nuclear emulsions were exposed to cosmic rays in stacks flown in baloons at high altitude. After the package was recovered, the emulsions were developed, mounted on glass slides and were then scanned under high magnification for interesting events. The observed tracks were digitized by a large staff of technicians, referred to as “the scanners”. A member of the emulsion group at that time was graduate student Masatoshi Koshiba, who in 2002 was awarded the Nobel prize in Physics for his contributions to the study of solar and atmospheric neutrinos. As particle accelerators reached higher energy, emulsions were also exposed to secondary particle beams, thereby guaranteeing that a large number of interactions of interest were recorded. Prior to my arrival, the Rochester group had exposed an emulsion “stack” 13

b1432-ch02

October 4, 2012

14

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

at the University of California Bevatron to a beam of low energy K − mesons. The K − would be generally captured on a nucleus in the emulsion, resulting at times in the production of Σ-hyperons through the reaction K − + N → Σ + π. We were interested in the decay Σ+ → p + π 0 , a process that could be easily identified in the emulsion. K-mesons, Λ and Σ hyperons had been discovered during the previous decade, and because of their peculiar properties were labeled “strange particles”. We wrote a paper [1] about our results on the lifetime of the Σ+ , but the interest of the group was moving rapidly to more elaborate techniques. One effort was to fly a spark chamber in a high-altitude balloon to search for γ-rays, that is, high energy photons. This project was led by Giovanni Fazio and Graeme Duthie. The other effort was participation in experiments at the large national accelerator facilities which attracted me. An opportunity arose when a physicist from Brookhaven National Laboratory (BNL) on Long Island, Seymour (Sam) Lindenbaum, who was participating in an experiment on the Rochester cyclotron, invited me to work with his group at the BNL Cosmotron. I started commuting to BNL in 1959 and took up residence in Long Island. While researchers from other Universities were active at the Cosmotron, one can plausibly argue that our group was the first resident University “user” group.

The Low Energy Λ − p Interaction Brookhaven National Laboratory was founded in 1947 on the site of (the former) Camp Upton, which had been the staging area for the transfer of US troops to Europe in World War I. It was to be a multidisciplinary laboratory, but with emphasis on particle physics. The first accelerator was the Cosmotron, a 3.3 GeV proton synchrotron completed in 1953, with sufficient energy to produce strange particles. When I arrived at BNL a higher energy machine, the 6.2 GeV Bevatron was operating at Berkeley, where the antiproton had been discovered a few years earlier. The Cosmotron had undergone a complete upgrade including the addition of a new expanded experimental area. Magnetic quadrupoles had just been introduced to focus

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

15

particle beams extracted from the accelerator. While Berkeley had the record in energy, construction of the Alternating Gradient Synchrotron (AGS) was well advanced at BNL, and came into operation in 1960, reaching an energy of 33 GeV. The senior member of the Brookhaven group that I worked with was L.C.L. (Luke) Yuan. Yuan had designed the radio frequency system for the Cosmotron and was both a learned and very kind individual. He was married to the distinguished nuclear physicist Chien Shiung Wu, a professor at Columbia University, and also happened to be one of the grand-sons of Yuan Shihkai the second president of China. The experiment on which my Rochester colleagues, Taiji Yamanouchi, Giovanni Fazio, graduate student Jim Reed and I were going to work, had been designed by Lindenbaum, who was interested in measuring the production spectra for π and K mesons in pp collisions. This was achieved by extracting the 2.9 GeV proton beam from the Cosmotron and directing it onto a liquid hydrogen target. To identify and differentiate between π, K mesons and protons, we used threshold Cerenkov counters. All of these features of the experiment, routine today, were novel at that time, and it was exciting to be on the Cosmotron floor. In designing the experiment Lindenbaum had laid out detection channels so that data could be recorded simultaneously at four production angles as can be seen in Fig. 2.1. Because the intense external proton beam was being transported to the target, extensive concrete shielding was used to protect personnel and detectors from stray radiation. A feature that was most beneficial for the experiment was the availability of a channel at 0◦ that allowed observation of particles produced in the forward direction. We first measured the π + and π − production spectra which were already known with less accuracy, and the results were published in 1961 [2]. No theory of strong interactions existed at that time (and this is still true for low energies) and phenomenological models were used instead, the most common being the “isobar” model, and a field theoretical calculation based on one pion exchange. Fits to the data using these models and taking into account of the available phase space were adequate.

October 4, 2012

16

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 2.1: Plan view of the external beam lines at the Cosmotron that were used to measure particle production [3].

More interesting were the K + spectra. To identify the K + , which were of relatively low velocity, we had to use high pressure Cerenkov counters that were constructed in the Rochester shops. The operating gas was CO2 , and because it was necessary to operate at high pressure, the supply tanks had to be heated with electrical tapes; this caused some anxious moments in the middle of one night when a tank overheated and the safety valve blew off releasing a cloud of gas. To obtain a spectrum we had to retune the spectrometer and the pair of quadrupole magnets, for each setting of momentum acceptance. As the momentum setting reached the maximum values at the edge of the allowed phase space, the yield did not drop as expected, but

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

17

Figure 2.2: The K + momentum spectrum, from pp collisions, at the edge of phase space, showing a strong enhancement in the Λ − p interaction. The data are compared to the prediction for different scattering lengths, α [4].

instead started rising again exhibiting a clear peak. This was most pronounced for the channel at 0◦ . The spectrum from Ref. [3], transformed to the pp center of mass (cm) system is shown in Fig. 2.2. A similar (but less pronounced) enhancement was also observed at θ = 17◦ as well as at lower incident proton kinetic energies, Tp . At the edge of phase space where the K + carries maximum momentum, the Λ and the p move collinearly and with the same velocity; there the “missing mass” from the reaction p+p → K + +Mmissing takes the value Mmissing ≈ mΛ +mp . Hence the observed enhancement could be due to a Λ − p resonant state, with

October 4, 2012

18

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

minimal Q-value. The quantum numbers would be (baryon number) B = 2 and (strangeness) S = −1, making it an “exotic” state, that is, one not accommodated in the standard quark model. The accepted interpretation for the enhancement is that there is a very strong interaction between the Λ and the p, at threshold i.e. when they are practically at rest with respect to each other. Other possibilities are discussed in [4]. This was the first, and thus far the only, observation of this unexpected feature. The Λ-nucleon interaction manifests itself also in the formation of hypernuclei which are nuclei with one neutron replaced by a Λ. Jim Reed, a key participant in this experiment, chose this research as the subject of his doctoral thesis.

Analysis of Bubble Chamber Film at Rochester While nuclear emulsions offer great spatial resolution, it is not possible to select, in time, what events will be recorded; i.e. time resolution is completely absent. Yet, it is highly desirable to be able to record only events of interest, and this was achieved with the invention of the “bubble chamber” by Donald Glaser. The chamber could be triggered to record events that occurred in a time window of milliseconds, often containing only a single interaction. Soon, Louis Alvarez at Berkeley, now LBL, demonstrated that pictures of the sensitive volume of an expansion bubble chamber filled with liquid hydrogen could be recorded, and that they provided high quality information about particle interactions. The hydrogen bubble chamber combined the function of target and detector for elementary particle interactions, and the Alvarez group carried out striking experiments at the Bevatron, discovering many resonant (excited) states of strange baryons and also of the mesons. Several universities, and all accelerator laboratories, followed LBL by building their own bubble chambers filled with hydrogen or heavy liquids. The events were recorded on spools of sensitive film, and measured on special projection tables, where the track coordinates were digitized and stored on computer punch cards. The information was processed by the large-frame computers of that time, using special reconstruction programs written by physicists. Since

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

19

the scanning and measuring of events was labor intensive, university groups were given film by the laboratories, did their own measurements and published the results, either independently or in collaboration with a laboratory team. This was the mode that was adopted for Rochester. By then the work on nuclear emulsions had ceased and most of the staff migrated over to the scanning and measuring of the new film. Measuring bubble-chamber film was less tiring than observing through a microscope, because the frames from the 70 mm film could be projected onto large tables, but nevertheless it still was tedious work. We had to build scanning machines, acquire measuring machines, and our first dedicated computer: a second hand PDP-1, from Digital Equipment Corporation. Attempts to automate the measurement process did not succeed until much later. The next issue was where to get some film to analyze. We were given about 20,000 frames from the Brookhaven 20-inch hydrogen bubble chamber exposed to 900 MeV π + , courtesy of Ralph Schutt, the leader of the BNL group. The film had been exposed some time ago, but not as yet analyzed, so there was no objection to having it “farmed” out. We looked at the reactions π + p → π + p and π + p → π + pπ 0 . The physics was to understand the pion production mechanism and to search for a possible low mass π + π 0 resonance, reported to exist. The analysis was carried out mainly by Wesley Metzger, and became the subject of his Ph.D. thesis. He was able to make quantitative comparisons [5] to various production models, and equally importantly, to set up a complete analysis system in Rochester. Tom Ferbel, fresh from his doctoral work at Yale, joined the Rochester faculty in 1965 to lead the group that would analyze data recorded by bubble chambers. Brookhaven had just completed construction of the 80-inch hydrogen bubble chamber, under the leadership of William Fowler; the chamber was installed at a location where beams from the AGS could be directed to it. An important new innovation was the use of beams with separated particles, which, given the high momentum, required radio frequency devices. This technique had just been introduced at Brookhaven, and beams of antiprotons,

October 4, 2012

20

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

of K − -mesons and to a lesser extent of K + -mesons were of interest. The Yale group, under Jack Sandweiss had led the construction of the electrostatic “beam separators”, and more recently of the rf variety, and was given first choice to analyze film from interactions in the 80-inch chamber. They chose an exposure with incident p. The Brookhaven group instead chose the K − exposure. This was a a lucky break for BNL as it led to the discovery of the Ω− baryon that was predicted in the 3-quark (eightfold way) model. A Rochester graduate student, Georges London, was part of the team that found the S = −3 baryon in the reaction K − + p → Ω− + K 0 + K + , followed by the weak decay Ω− → Ξ0 + π − [6]. The mass of the Ω− was found to be exactly as predicted by Muray Gell-Mann and our Rochester colleague Susumu Okubo. This historic experiment, led by Nick Samios, confirmed beyond doubt the validity of the 3-quark model. I still clearly remember when London phoned from the scanning room at Brookhaven to tell us about the discovery. Rochester was approved to study an exposure of the 80-inch chamber filled with hydrogen to a beam of 13 Gev K + . One of our first efforts was to analyze events of the type K + p → K + pπ + π − and → K 0 pπ + π 0 . An excited state of the K-meson, the K ∗ (890), was known to decay either into K + π 0 or into K 0 π + . However, there was less information on excited states of higher mass. Our data for the (Kππ)+ mode [7] is shown in Fig. 2.3. A pronounced peak is seen at a mass M ∼ 1320 MeV (more recent experiments relying on phase-shift analysis showed that there is more structure in that mass region). There is also a smaller peak at M ∼ 1780 MeV, which was confirmed by later work. By examining the angular distribution and correlations of the decay particles, the quantum numbers of the new resonances, as well as their production mechanism were studied. Graduate student Barbara Forman-Lasinski and postdoc Haruo Yuta, were important contributors to this work. These kinds of investigations continued for several more years including the first bubble chamber data from Fermilab. Eventually following a variety of measurements of inclusive cross sections in the early 1970s the members of the bubble chamber group chose to follow purely “electronic” experiments at Fermilab.

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

21

Figure 2.3: The (Kππ)+ effective mass spectrum from 13 GeV/c K + p collisions. Note the resonant peaks [7].

The Ratio of K + /K − Lifetimes; a Test of CP T Invariance The discovery of CP violation by Cronin, Fitch and collaborators [8], in 1964 was an important milestone in physics and was recognized through the award of the 1980 Nobel prize to the senior authors. Parity violation in weak interactions had been discovered seven years earlier, in 1957, and although parity, P , was violated maximally, yet the data indicated that the product of parity, P , and charge conjugation, C, was conserved. The violation of CP was found in the K 0 system and the effect is exceedingly small. An immediate question was raised whether the K mesons would exhibit violation of other symmetries, especially the CP T symmetry, where T stands for time reversal, and CP T conservation reflects a basic axiom of field theory that should be respected in all particle interactions. CP T conservation guarantees, for instance, that the mass of a particle is exactly that of its antiparticle and that the lifetime, i.e. the inverse of the total decay rate, of a particle and

October 4, 2012

22

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

its antiparticle are equal. Professors T.D. Lee and C.S. Wu carried out an extensive compilation of the properties and interactions of K mesons, but found that the decay rates were only poorly known. Given the experience of our group with K mesons, Luke Yuan suggested that we should make a careful comparison of the K + /K − lifetimes, and our proposal to make such a measurement at BNL, using partially separated K + and K − beams from the AGS, was quickly approved. On kinematic grounds, the largest sample, and thus the best precision can be obtained from decays at rest. However slowing down the K mesons would imply large corrections, especially for the K − , due to their interaction with the stopping material; also the identification of K mesons becomes difficult at low momentum. As for decays in flight, the higher the momentum, the longer is the needed flight path to observe a given number of decays. Nevertheless, high momentum beams can be well collimated and it is much easier to identify the different particles. We made measurements at momenta of 1.6 GeV/c and 2.0 GeV/c, where the relative composition of the separated beams was K/π ∼ 1/2. The K mesons were identified by differential Cerenkov counters that used cells of liquid radiator. After the beam was collimated and the K flux determined, the number of surviving mesons was measured at three stations spaced approximately 16 m one apart from the other, for a total decay distance of ∼50 m. For a beam of momentum 2 GeV/c this corresponds to three lifetimes. About 107 incident K-mesons were compared to the number surviving at each station. The main emphasis in the data analysis was in understanding and minimizing any possible systematic biases between the positive and negative beams. Knowledge of the beam momentum was essential for a determination of the absolute value of the lifetimes. The transmission of the beam line was verified by tuning the separators with protons and antiprotons, and corrections were applied for the small contamination of the sample by pions. The final result [9, 10], confirming the equality of the lifetimes was τ (K + )/τ (K − ) − 1 = −0.00090 ± 0.00078

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

23

We also measured with slightly less precision the ratio of π + /π − lifetimes and found it equal to unity within the uncertainty of the measurement. For the absolute value for the K + lifetime we cited τ (K + ) = (1.2272 ± 0.0036) × 10−8 s, a result, which differs by only 3σ from the presently accepted value [11], forty years later. While CP T invariance requires that the total decay rate of particle and antiparticle be equal, partial decay rates can differ if CP is violated.b A comparison of the rates for K ± → µ± + ν and K ± → π ± π + π − made at about the same time, showed no difference between particle and antiparticle rates [12] at a precision of ∼ 0.5%. The strictest test of CPT invariance comes from the equality of the 0 masses of the K 0 and K mesons. What is measured is the difference in the KS0 and KL0 mass, which has the value ∆m = 3.5 × 10−6 eV 0 0 eigenstates, [11]. The K 0 and K are linear combinations of the KS,L and therefore their mass difference must be less than ∆m. Namely the particle/antiparticle mass difference, for the K mesons, must be less than one part in 1014 . This provides a very strong endorsement for the CP T theorem in the Kaon system. Collaborators in the lifetime measurements were Fred Lobkowicz, a fellow faculty member, and dear friend with whom I also coauthored an introductory physics text; Yorikio Nagashima who had just joined our group as a postdoc and who eventually had a brilliant academic and research career in Japan, and graduate student Stuart Tewksbury, who also became a distinguished scientist and academic. John Fox, a Brookhaven staff physicist, was an essential participant in the experiment, by setting up the K + /K − beams.

K + /K − Backward Scattering and pp Annihilations The first of these two experiments was proposed by Arne Lundby, a Norwegian Physicist who was on the CERN staff. Arne was considering joining Brookhaven, and during an extended visit was making b

This was first shown by S.Okubo and was used by A.Sakharov in his proposal to explain the baryon asymmetry of the Universe.

October 4, 2012

24

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

plans for a new research project. The idea behind “backward” scattering was the observation that the π + p elastic cross section, while rapidly decreasing away from the forward direction, showed also a rapid increase in the backward direction and in fact exhibited a peak at 180◦ . The presence of such phenomena can be explained in terms of particle exchange and the minimal value of the corresponding momentum transfer, characterized by the Mandelstam variables, s, t, u. For instance, in the forward direction one considers the exchange of the “vacuum” and the momentum transfer t is between the incoming and outgoing π + . In the backward direction the exchanged virtual particle is the ∆0 baryon resonance, and the momentum transfer u is between the incoming pion and the outgoing proton; in this case one has also to consider direct channel effects, governed by s. The extension of the experiment to K mesons exploited the separated beams available at the AGS, with the key technical aspect being the use of wire spark chambers with magnetrostictive readout. These were novel detection chambers built by Joe Fisher of the Brookhaven Instrumentation Division, an old friend of Lundby’s from their graduate student days at the University of Chicago. Alan Carroll, a gifted Brookhaven physicist had joined the experiment but the group was still below critical mass, especially since Lundby eventually decided to return to CERN. This is where the Rochester group joined up. The second experiment too, has an interesting history to which we will return in due time. Other participants in the experiment were Bob Phillips from BNL, and from Rochester, physicists Fred Lobkowicz, Yori Nagashima, and graduate students Tony Smith and Stuart Tewksbury. The experimental setup [13] is shown in Fig. 2.4, and uses a dipole magnet to analyze the momentum of the forward going proton. The track of the backscattered meson is measured in a set of chambers set at 45◦ upstream of the target and these chambers also record the incident track. For elastic scattering, the direction of the incoming track and of the two outgoing tracks, as well as the momentum of the outgoing proton suffice to over-constrain the event. The wire chambers were read out using a magnetostritive line that encoded the

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

25

Figure 2.4: Plan view of the apparatus for measuring K ± p elastic backward scattering at the Brookhaven AGS [13].

delay between a fiducial signal and the signal from the spark(s) in the chamber. The high voltage was pulsed on the chambers following a trigger generated by signals from scintillation counter arrays, and the data recorded by a PDP-8 computer. Significant iron shielding was used to minimize the leakage of magnetic flux from the 48-inch aperture of the dipole magnet that would interfere with the magnetostrictive readout. Data were taken at several K + incident momenta between 1.0 and 2.5 GeV/c. A typical set of differential cross sections for K + p backward elastic scattering as a function of θcm is shown in Fig. 2.5 [13]. Indeed there is a distinct peak in the backward direction which grows steeper as the energy of the incident K + increases. This behavior is indicative of Λ, or Y 0 exchange in the u-channel. The K − p backward cross sections do not show a peak, but instead decrease as the scattered K − angle tends toward 180◦ [14]. This fits in with the picture of particle exchange in the u channel since no baryon with strangeness S = +1 exists to provide this contribution. The experiment also measured π ± backward scattering for incident momenta

October 4, 2012

26

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 2.5: The peaking of the differential cross section in the backward direction in K + p elastic scattering near θcm ∼ 180◦ , for different incident K + momenta [13].

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

27

Figure 2.6: The momentum dependence of the π + p elastic cross section at θcm ∼ 180◦ . The dashed curve is the π − p backward cross section multiplied by a factor of ten [15], where a factor of ∼9 is expected for ∆ exchange.

between 1.5 and 3.0 GeV/c. The form of the differential cross section depends strongly on incident energy. As an example we show in Fig. 2.6 the cross section for π + p scattering at θcm ∼ 180◦ as a function of momentum from our data [15] as well as from other experiments.

October 4, 2012

28

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

The next experiment, pp → π + π − , K + K − is the “crossed” configuration from the previous elastic scattering experiments. The study was proposed by Alvin Tollestrup, Barry Barish and Jerry Pine of CalTech, and approved to run when our experiment was finished. It would entail constructing a completely new experiment, but very similar to the apparatus that was already in place. It was thus decided to join forces, and use the existing set-up with the only “modification” being to tune the beam separators for antiprotons, instead of K or π mesons. The distinction between a produced π + π − and K + K − pair could be made unambiguously from the kinematics, that is, from the angles of the produced particles and the momentum of the forward track. With our enlarged research team and the contribution of two capable Caltech graduate students, John Yoh and Howard Nicholson, we were able to rapidly take data and also do the analysis at the same time. Because the pp system is an eigenstate of strong CP the distribution of the π + must be a reflection of the π − in the center of mass system. Thus the cross sections for backward angles can be combined (folded over) since dσ(θ) = dσ(π − θ). There is significant energy dependence, as can be seen in Fig. 2.7, in both π + π − and K + K − production [16]. Analogous structure is present in pp elastic scattering which was also part of our measurements. These data could be well fitted by an expansion in Legendre polynomials of up to sixth order. The fits are shown by the curves in Fig. 2.7, and can be attributed mainly to Alan Carroll’s efforts. The pp annihilation data was successfully compared to the results of our and other backward scattering measurements [17, 18]. Antiproton annihilations at low energy or at rest, were studied previously in bubble chambers and were first considered to be special because of the large number of pions that were produced, but they are no different than the scattering and interactions of other particles at equivalent energies. The advantage of pp is that the initial states are CP eigenstates with most quantum numbers equaling zero, and this can be helpful in some analyses. At high energies, however, it is the quarks and gluons that interact, and a pp collider differs little from its pp counterpart.

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Strange Particles

b1432-ch02

29

Figure 2.7: The folded differential cross section for pp annihilations to π + π − (left), and K + K − (right). The curves are Legendre polynomial fits [16].

October 4, 2012

30

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

M. F. Kaplon et al., Annals of Physics (NY) 9, 139 (1962). A. C. Melissinos et al., Phys. Rev. Lett. 7, 454 (1961). A. C. Melissinos et al., Phys. Rev. Lett. 14, 604 (1965). J. T. Reed et al., Phys. Rev. 168, 1495 (1968). W. J. Metzger et al., Phys. Rev. 164, 1680 (1967). V. E. Barnes et al., Phys. Rev. Lett. 12, 204 (1964). J. Berlinghieri et al., Phys. Rev. Lett. 24, 1087 (1967). J. H. Christenson et al., Phys. Rev. Lett. 13, 238 (1964). F. Lobkowicz et al., Phys. Rev. Lett. 17, 548 (1966). F. Lobkowicz et al., Phys. Rev. 185, 1676 (1969). K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010). W. T. Ford et al., Phys. Rev. Lett. 18, 1214 (1967). A. S. Carroll et al., Phys. Rev. Lett. 21, 1282 (1968). A. S. Carroll et al., Phys. Rev. Lett. 23, 887 (1969). A. S. Carroll et al., Phys. Rev. Lett. 20, 607 (1968). H. Nicholson et al., Phys. Rev. Lett. 23, 603 (1969). B. C. Barish et al., Phys. Rev. Lett. 23, 607 (1969). H. Nicholson et al., Phys. Rev. D7, 2572 (1973).

b1432-ch02

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 3

Deep Inelastic Scattering

Muon-Proton Scattering The experiments on deep inelastic scattering of electrons carried out in 1968 at the (then) recently completed two-mile-long linear accelerator at Stanford University (SLAC), had a profound impact on our understanding of the structure of matter [1]. The abundant yield of inelastic scattered electrons at large momentum transfer implied the presence of point-like constituents inside the proton and the neutron. The constituents were named “partons” by Feynman [2] and could be identified with the “quarks” that Gell-Mann [3], (or“aces” that Zweig [4]) had postulated to explain the symmetries of the newly discovered elementary particles. Surprisingly, the muon scattering experiments carried out at the Brookhaven AGS four years earlier [5] already showed evidence for deep inelastic scattering, but the significance of the effect was not appreciated. This was because the muon experiment did not have nearly the resolution of the SLAC electronscattering data, neither in the incident beam momentum, nor in the detection of the scattered electron. In later years, however, muons were used extensively to probe deep inelastic scattering at incident energies exceeding those attainable by electron beams [6, 7]. The muon scattering experiment was conceived by John Tinlot and designed to run at the Alternating Gradient Synchrotron (AGS), which was to come into operation at Brookhaven National Laboratory in 1960. It was a collaboration between Rochester (J. Tinlot), 31

b1432-ch03

October 4, 2012

32

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Columbia (L. Lederman) and Brookhaven (R. Cool) and was the 4th proposal submitted to the AGS. The motivation was to compare muon and electron scattering from a hydrogen target to uncover any difference in the interaction of muons, as compared to that of electrons, with protons. Previous experiments had used cosmic ray muons, scattered from iron plates, but given the low flux and the crude target, the results were not very enlightening. Tinlot’s bold idea (for the times) was to create a beam of muons from the decay of pions by introducing an absorber, in this case concrete, after a significant path length in which the initial pions had decayed. The pions and other hadrons interacted, while the muons emerged from the absorber only slightly degraded in energy. This is the same process through which the muon flux reaches the Earth’s surface, originating from the decay of pions formed by energetic cosmic rays in the upper atmosphere. A variant of this scheme is still used for producing neutrino beams [8]. The layout of the first muon beam at the AGS is shown in Fig. 3.1. The pion target was “internal”, located within the accelerator lattice, and a channel of quadrupole magnets provided some focusing to contain both the pion and muon beams. Following the decay region, steel collimators were used to reduce the off-beam (halo) muons, yielding a beam with a broad momentum spectrum centered at 3 GeV/c. The pion contamination was less than 1/104 . In the early 1960s, the study of elastic electron scattering from protons and nuclei was of great interest. Experiments were being carried out at the Stanford 1 GeV linac and at the Cornell 3 GeV synchrotron. Those experiments led to precise measurements of form factors, that is of the spatial distribution of charge in protons, neutrons and in nuclei [9]. The muon experiment was proposed with the same goal in mind, but because the incoming beam momentum was neither monochromatic nor known a priori, the experiment was designed so as to measure the angle and energy of the recoil proton. The kinematics of elastic scattering imply that the square of the (invariant) 4-momentum transfer q 2 = −2MT , where T is the kinetic energy of the recoil proton, and M the proton rest mass. The differential cross section for µ − p (or e − p) scattering is described

b1432-ch03

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Deep Inelastic Scattering

b1432-ch03

33

Figure 3.1: The muon beam line at the Brookhaven AGS. The target was placed after the G-9 dipole bending magnet of the accelerator ring, so that negative pions of 6–10 GeV/c momentum were deflected into a channel of quadrupole magnets by the G-10 bending magnet [5].

by the Rosenbluth formula [10], and depends primarily on q 2 , and weakly on the incident muon momentum E, 4πα q2 G(q 2 ) dσ + · · · ), = (1 − dq 2 q 4 1 + q 2 /4M 2 2M E

(2)

where q2 2 G , 4M 2 M with α being the fine structure constant, and GE and GM the proton electric and magnetic form factors. An innovation of the Brookhaven experiment was the use of optical spark chambers for the measurement of the angle and energy of the recoil protons. The energy was obtained from the range of the proton track in a large spark chamber built with 1-inch thick steel plates. Because of the nuclear interactions in the plates, the efficiency for the G(q 2 ) = G2E +

October 4, 2012

34

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 3.2: The layout of the target and detectors for the µ − p scattering experiment (a) top view, (b) side view [5].

range measurement varied from 80% to 40% for proton momenta of 600–1100 MeV/c. Given the small µ-p cross section it was necessary to use a long target. The hydrogen target was 30 cm in diameter and 1.8 m long, and as a consequence, the recoil detector had to cover a large area. The layout of the experiment is shown in Fig. 3.2, and the tracks in the spark chambers were recorded on film, which was later scanned and measured manually.

b1432-ch03

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Deep Inelastic Scattering

b1432-ch03

35

While the experiment was in preparation, the question of inelastic scattering was brought up. By analogy to the well known radiative processes, for which corrections are required in electron elastic scattering, the concern was about the possible emission of one or more π 0 mesons. To guard against such events it was decided to add a detector for the scattered muon as well. To cover the large range of angles into which the scattered muons emerge, four liquid scintillator tanks placed behind a concrete absorber were used to provide a trigger signal, and the angle of the scattered muon was recorded in two thick-plate spark chambers as can be seen in Fig. 3.2. The decision to detect the scattered muon as a complement to the measurement of the angle and energy of the recoil proton, saved the experiment! While the information on the recoil proton and scattered muon was not sufficient to fully characterize inelastic events, it provided for an unambiguous selection of elastic events. The event selection criteria were based on the coplanarity of the muon and recoil-proton tracks, and on the comparison of the momentum of the recoil proton to the value expected, for elastic events, from the opening angle between the recoil proton and the scattered muon. Knowing the scattered muon angle, as well as the proton angle and momentum made possible the reconstruction of the incident muon momentum. Furthermore, unless the two tracks formed a vertex within the target region the event was rejected as an accidental trigger. A plot of the correlation of the momentum calculated from the opening angle, and of the measured recoil momentum, is shown in Fig. 3.3 for the events that were selected as elastic, and for a Monte-Carlo sample. As the data accumulated, we were swamped by inelastic events and considered them as a big nuisance. For instance, in one of the runs, which emphasized events with high momentum transfer, we accumulated 50,000 triggers. After scanning the film, 2,000 events were kept for measurement and they yielded 150 elastic interactions, the majority of the remaining events being inelastic [5]. I remember being asked by W. J. (Bill) Willis how the experiment was progressing, and answering that things were fine, except for a staggering excess of inelastic events. The information on the inelastic events was incomplete, yet there was ample evidence for their abundance as compared to elastic events, in what was the first experiment

October 4, 2012

36

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 3.3: The relative difference between the measured recoil proton momentum and the value calculated from the measured angles, when the scattering is elastic, is plotted for selected events [5].

where high energy leptons were scattered from protons. Unfortunately, no one in our group appreciated the profound significance of these events. After the SLAC results were published [1] the interpretation of this “background” became evident. The results from the elastic scattering data are shown in Fig. 3.4 and agree well with the values expected by using the form factors determined from electron scattering. We now know that many other indicators, in particular the precise measurement of the g − 2 factor of the muon [11] confirm that, discounting their difference in mass, muons and electrons have the same interactions, as expected in the standard model. In addition to the senior members, Rod Cool, Leon Lederman and John Tinlot, two graduate students, Mike Tannebaum (Columbia) and Bob Ellsworth (Rochester) played a key role in the experiment; Al Mashke from BNL made important contributions as did Taiji Yamanouchi from Rochester.

b1432-ch03

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Deep Inelastic Scattering

b1432-ch03

37

Figure 3.4: The measured µ − p elastic scattering cross section is compared to the value expected from the electron data, represented by the solid line [5].

The Sequel: µ − p 2 The muon scattering experiment was repeated at Brookhaven in the early 1970s, but with the primary emphasis placed on deep inelastic scattering. Many of the same players were involved: Leon Lederman, Peter Limon and their group from Columbia, the Rochester group and our new postdoctoral fellow, Hans Jostlein, a group from Fermilab with Taiji Yamanouchi, John Sculli and Tom White, and a new group from Harvard led by Mike Tannenbaum and Tom Kirk, who had recently joined the faculty. The muon beam and the apparatus were much more sophisticated than in the earlier experiment.

October 4, 2012

38

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 3.5:

Layout of the detection apparatus for µ − p 2 [12].

Protons were extracted from the AGS and directed to an external target to produce a pion beam, which was allowed to decay into muons. The mean energy of the muons was 7.2 GeV, and the muons were magnetically analyzed so that both their momentum and position were tagged. A forward spectrometer recorded the scattered high energy muon, and another wide aperture spectrometer set on the opposite side at 45◦ accepted the hadronic recoils from the µ − p collision. Particle tracks were detected in wire chambers with magnetostrictive read out, and data acquired with a PDP-6 computer. A compressed view of the apparatus is shown in Fig. 3.5. Six graduate students were part of the team and completed their doctoral theses on different aspects of the construction, operation and analysis of the experiment: Alan Entenberg and Ioannis Kostoulas from Rochester, Pat Rapp and Morgan May from Columbia, Mike Murtaugh and Howard Gittelson from Harvard. K. Konigsman, I. J. Kim, and E. Aslanides participated in the later stages of µ − p 2. With a larger team, we undertook several measurements, including, what we called “deep” elastic scattering, which extended the momentum transfer to −q 2 = 3 GeV2 . The data again agreed well with the accepted parametrization of the proton form-factors [12]. We then measured deep inelastic scattering, and extracted νW2 , the

b1432-ch03

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Deep Inelastic Scattering

b1432-ch03

39

Figure 3.6: The measured nucleon structure function νW2 , obtained from the scattering of 7 GeV/c muons from a copper target as a function of x
= q 2 / (2mν + m2 ) [14].

structure function of the proton and deuteron at values of muon energy loss, ν, that ranged from 1 to 5 GeV [13]. Again no difference was found when compared to the electron data. We studied the hadrons produced in the collisions, and searched for an excited muon in the “missing mass” spectrum derived from the angle and energy of the recoiling protons. We also used nuclear targets where much higher collision rates could be achieved. One example is shown in Fig. 3.6, which gives the effective value of νW2 [14] as extracted from scattering off a Cu target. At small Bjorken x = q 2 /2mν, that corresponds to the forward direction, and for heavy targets, we observed that the cross section decreases below the value expected from the target’s composition, an effect referred to as “shadowing” of the target nucleus. Similar muon scattering experiments were later carried out at Fermilab [7] and at CERN [6], where the nucleon structure function could be measured at large incident energies, and thus at large ν, or equivalently at small x, over a wide range of momentum transfer q 2 . While in general the structure functions “scale”, that is they depend

October 4, 2012

40

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

only on x, q 2 -dependent scaling violations, as expected from calculations in quantum chromodynamics (QCD), are clearly observed [7].

References 1. E. D. Bloom et al., Phys. Rev. Lett. 23, 930 (1969). 2. R. P. Feynman, “Photon Hadron Interactions”, W. A. Benjamin (1972). 3. M. Gell-Mann, “The Eightfold Way” California Institute of Technology Report, CTSL-20 (1960); Phys. Letters 8, 214 (1964). 4. G. Zweig, CERN Rep. 8182/Th. 401, 8419/Th 412, (1964). 5. R. Cool et al., Phys. Rev. Lett. 14, 724 (1965); R. W. Ellsworth et al., Phys. Rev. 165, 1449 (1968). 6. J. J. Aubert et al., Physics Letters 94B, 96 (1980). 7. P. D. Meyers et al., Phys. Rev. D 34, 1265 (1986). 8. See for instance, S. E. Kopp, arXiv:physics/0508001. 9. R. Hofstadter, Rev. Mod. Phys. 28, 214 (1956). 10. M. N. Rosenbluth, Phys. Rev. 79, 615 (1950). 11. G. W. Bennett et. al. Phys. Rev. D 73, 072003 (2006). 12. I. Kostoulas et al., Phys. Rev. Lett. 32, 489 (1974). 13. A. Entenberg et al., Phys. Rev. Lett. 32, 486 (1974). 14. M. May et al., Phys. Rev. Lett. 35, 407 (1975).

b1432-ch03

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 4

The Rising pp Total Cross Section

The US-USSR Gas Jet Experiment With the successful operation in late 1959 of the 28 GeV Proton Synchrotron (PS) at CERN in Geneva Switzerland, and a few months later of the 30 GeV Alternating Gradient Synchrotron (AGS) at Brookhaven, these two accelerators dominated the field of high energy physics for the next decade. There were plans for higher energy machines, a 70 GeV synchrotron to be built in Serpukhov in the USSR, and a 200 GeV machine in the US. It was presumed that the US machine would be located in California, to continue the Berkeley tradition, now that the energy of the 8 GeV “Bevatron” had been surpassed. However, given the considerable cost of constructing and operating such a facility, it was inevitable that a Nationwide competition would take place before Congress approved the project. Senator Everett Dirksen of Illinois was instrumental in securing the funding for the new “National Accelerator Laboratory” (NAL), and for locating it near Chicago. This turned out to be a wise choice, because adequate land could be acquired, and the laboratory was in close proximity of large local airports. Robert Rathbun Wilson, a person of immense vision and remarkable originality, was chosen as the director of the new Laboratory [1]. At the time, Wilson was on the faculty at Cornell University, where he had built three electron synchrotrons of progressively higher energy, and had extensive experience in accelerator design. Wilson 41

b1432-ch04

October 4, 2012

42

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

had not been part of the initial group of Berkeley and Brookhaven physicists who had designed the 200 GeV machine, based on conservative standards. In contrast, by exploiting the latest technological developments and by not being deterred from taking risks, Wilson proposed a design that could reach 400 GeV (twice the original energy), at the same cost as the original proposal. Ground breaking for the NAL accelerator took place in 1968, and the first beam circulated in 1972. At its dedication in 1974, the laboratory was renamed the Fermi National Accelerator Laboratory (FNAL), or “Fermilab”, in honor of physicist Enrico Fermi. By now the era of “big physics” had arrived, and experiments were proposed, approved, and prepared well in advance of their execution, which stretched over years. Most experiments made use of particle beams that were extracted from the accelerator, and transported to the experimental areas. Looking for a more modest setup we were intrigued by a recent experiment at the 10 GeV accelerator in Dubna in the Soviet Union. A thin polyethylene (CH2 ) foil was introduced in the vacuum region of the accelerator in the path of the circulating beam, and served as a target that was struck by the protons in the beam. In the case of small-angle scattering, the very low energy recoil protons emerge from the foil at large angles (near 90◦ ) and are detected with solid state detectors. The angle and energy of the recoil protons suffices to characterize an elastic scattering event, and from the event rate one can determine the differential pp cross section in the forward direction. The Dubna group had subsequently improved their technique by using a hydrogen gas jet inside the accelerator, thus having a target of free protons without the contribution from the carbon in the CH2 foil. This greatly reduced the background and multiple scattering in the target, and made possible the extension of the cross section measurements to much smaller pp scattering angles. A group from Rochester, from Rockefeller University (Rod Cool), and from NAL (Taiji Yamanouchi), proposed to carry out a similar experiment at Fermilab. Simultaneously, the Dubna group (Vladimir Nikitin) in collaboration with Fermilab physicists (Ernest Malamud), also proposed the same experiment. Eventually the competing groups

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Rising pp Total Cross Section

b1432-ch04

43

were merged into a single team with balanced participation from the USSR and the US. Seven Soviet physicists would be in residence at Fermilab for two years. That was an exceptional step in the cold-war era, and yet, the collaboration worked out smoothly and produced significant results. The Soviet group supplied the gas-jet target, while the US groups provided the infrastructure, particle detectors and data analysis. The experiment was installed in the C0 straight section of the accelerator, and eventually, two other experiments used the same target to investigate particles produced in the forward direction. The technical difficulty in using a gas target within an accelerator, is the need to efficiently pump away the gas, so as to maintain the integrity of the vacuum in the beam pipe. This was achieved by “cryopumping”, that is, solidifying the hydrogen as it was captured in a “trap”, a surface cooled to liquid helium temperature and located below the beam line; after several hours of operation the solid hydrogen was isolated from the beam pipe, warmed to its gaseous state and pumped out, in preparation for the next cycle. Operation of the hydrogen trap involved use of remotely controlled cryogenic technology, which was novel at that time. The FNAL accelerator group was assigned the responsibility for the cryogenics, but this was also part of the director’s plan to increase expertise in handling liquid helium, which was a necessary step if superconducting magnets were to be used at some time to double the accelerator energy. This goal was achieved by 1983 under Wilson’s successor. In advance of the arrival and installation of the hydrogen gas-jet target, the US group used a CH2 foil as a target, but with a significant innovation introduced by Hans Jostlein, who had the idea of spinning a finger-like segment of 3-µm thick polyethylene foil attached to the end of a rapidly rotating shaft. The centrifugal force keeps the foil stretched, and the rotation exposes the foil to the beam at a low duty cycle, thereby preventing damage from the interaction with the intense proton beam. The recoil protons, with energies 0.8 to 80 MeV, were emitted at angles between 90◦ and 70◦ , and were recorded by solid state detectors, spaced 4 cm apart from each other, on a radius of 2.50 m from the target.

October 4, 2012

44

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 4.1: Schematic layout of the detection apparatus in the C0 straight section of the Fermilab accelerator, for the measurement of the small angle pp scattering cross section [2].

A schematic of the target and detector arrangement is shown in Fig. 4.1 [2]. Since the angle and energy of a recoil proton depends on the momentum transfer and only very weakly on the incident energy, data can be taken continuously, as the beam energy is ramped up during the acceleration cycle. Hence the energy dependence of the pp differential cross section, and of the parameters that characterize

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Rising pp Total Cross Section

b1432-ch04

45

it, can be established in a single data run. This was a very useful feature of using an internal target. In the early days of Fermilab’s operation our experiment took data using the rotating foil as that demanded only modest requirements from the intensity and quality of the proton beam. Elastic scatters were readily separated from background, and the elastic differential cross section was obtained at several energies. The distinguishing feature of the cross section in the forward direction is the exponential dependence on the square of the invariant momentum transfer −t = −(pµ1 − pµ2 )2 = 2mT . The differential cross section as measured at the incident energy of 132 GeV, is shown in Fig. 4.2. By invoking the analogy with optical diffraction theory, the slope b in

Figure 4.2: The differential cross section, dσ/dt, for pp elastic scattering in the region 0.005 < |t| < 0.015 (Gev/c)2 for incident energy E = 132 GeV. The rise at very small |t| is due to Coulomb scattering [2].

October 4, 2012

46

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 4.3: The slope, b, of the diffraction peak for |t| < 0.2 (GeV/c)2 , as a function of s, the square of the energy in the pp center of mass [2].

the exponential form of the cross section, dσ/dt ∝ e−b|t| , provides a measure of the proton’s size. We confirmed that the slope increases steadily with energy [3] as can be seen in Fig. 4.3, which is a compilation of data from several experiments. The “shrinking” diffraction peak implies that the proton size continues to grow with increasing collision energy. Extrapolating the differential cross section to the forward direction (t = 0), makes it possible to obtain the total pp cross section by use of the optical theorem which relates the total cross section, σT , to the imaginary part of the forward elastic scattering amplitude. In an early publication, where data was limited to energies below 200 GeV [4], we used this method to conclude that the total cross section remained constant in that energy interval, while in fact the cross section rises by 2.5%, but this was less than the uncertainties in our measurement. The gas jet was shipped from Dubna, installed at C0 and operated successfully in 1974. It’s use extended the reach in momentum

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Rising pp Total Cross Section

b1432-ch04

47

transfer for elastic events to very small values of |t|, of order |t| ≈ 0.001(GeV/c)2 which is past the region where the nuclear and Coulomb amplitudes interfere. At small |t| the Coulomb amplitude grows as t−2 becoming equal to the nuclear amplitude at |t| ∼ 0.005 (GeV/c)2 , and at lower values of |t| the Coulomb amplitude dominates the cross section as is evident in Fig. 4.2. The nuclear amplitude is mainly imaginary with only a small real part, in contrast to the Coulomb amplitude which is mainly real and negative (because the two protons repel each other electromagnetically). Fitting the data to the two components yields the ratio of the real to the imaginary part of the nuclear amplitude ρ = Re[A]/Im[A] at t = 0, which to our great surprise changed sign at a laboratory energy E ∼ 250 GeV.

Figure 4.4: Fits to the measured differential cross sections dσ/dt after subtracting the square of the Coulomb amplitude, for different incident proton energies. The contribution of the interference of the nuclear and Coulomb amplitudes which at low energies is positive, becomes negative above Elab ∼ 250 GeV [2].

October 4, 2012

48

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

This can be seen in the plots of Fig. 4.4, which are based on the analysis of the data made by Dan Gross in his Ph.D. thesis [2]. In Fig. 4.4 is plotted the differential cross section dσ/dt for pp scattering at small angles, after subtracting the square of the Coulomb amplitude, for several different incident proton energies, from 50 to 400 GeV. Therefore the subtracted data, and the corresponding fits, are the the sum of the square of the nuclear amplitude and of the interference term which dominates the plot as |t| → 0. At Elab ∼ 250 GeV the interference term changes from positive to negative indicating that the real part of the nuclear amplitude has become positive. A compilation of the ratio ρ, as a function of laboratory energy, is shown in Fig. 4.5. The dashed line corresponds to the expected behavior of ρ based on the then existing data, which had been interpreted to indicate that asymptotically, that is, at very high energies,

Figure 4.5: The real part of the pp elastic nuclear scattering at t = 0 as a function of the energy of the incident protons. The dashed curve is the result of a dispersion relation calculation assuming that the total cross section tends to an essentially constant asymptotic value above 50 GeV [2].

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Rising pp Total Cross Section

b1432-ch04

49

the real part of the nuclear amplitude would tend to zero. This in turn would imply that the total cross section tended to a constant value as was widely believed at that time. This belief originated from a misunderstanding of the Pomeranchuk theorem, which states that asymptotically both the pp and pp cross sections tend to the same value, but this limiting value need not be a constant. The real part of the nuclear amplitude is related to the total cross section through a dispersion relation (it involves an integral over all energies), and therefore a positive real part implies a rising total cross section. Our Soviet colleagues were adamant that the real part could not become positive and that consequently there must have been an error in the analysis of the data. This “caution” delayed the publication of our results [5]. Steve Olsen and I-Hung Chiang attempted to extract the total cross section directly from our data by using the optical theorem, as discussed previously. Unfortunately the uncertainties were too large to make a definitive statement. Conclusive evidence for the rise of the pp total cross section came from two experiments that were being carried out at the same time at the Intersecting Storage Rings (ISR), at CERN. The ISR was the first p−p collider ever built, the energy of the counter-circulating beams being limited to EB < 30 GeV. However, √ the cm collision energy s = 2EB ≈ 60 GeV, which corresponds to a fixed target experiment of incident energy E0 ≈ 2,000 GeV, was much higher than the cm energy that could be reached in pp collisions at Fermilab.c One of the ISR experiments [6] integrated the differential cross section, while the other [7] measured the real part and used the optical theorem. The ISR results are included in Fig. 4.6, which summarizes the pp and pp total cross section measurements as a function of the momentum of the incident particle that were available in the mid 1970s. Figure 4.7, shows a more recent comparison of the pp and pp total cross sections [8], and confirms the Pomeranchuk theorem. Indeed, the total cross sections, for both pp and pp, continue to rise with c

The relation between E0 , the laboratory energy in a fixed target pp experiment and s, when E0  mp , is s = 2mp E0 .

October 4, 2012

50

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 4.6: The total cross section for pp and pp interactions as a function of momentum of the incident particles [2].

√ increasing collision energy, and at plab ≈ 108 GeV, or s ∼ 104 GeV, σT (pp) reaches 100 mb. The data obtained from the gas-jet experiment have stood the test of time and provided insight into the fundamental process of pp collisions. The experiment (E36) went on to study inelastic scattering in the forward direction, so called diffractive scattering, for both pp and pd collisions, and remains one of the early examples of successful US–Soviet scientific collaboration.

The Flying Wire Our experiment (E36), pp elastic scattering at very low momentum transfer, was the first experiment carried out at Fermilab (NAL at that time), and started taking data even before the accelerator had reached its design energy. To quote, friend and colleague Steve Olsen, it was a period of trauma and excitement. Trauma, because many things went wrong, in particular, bending magnets in the main ring

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

b1432-ch04

Cross section (mb)

The Rising pp Total Cross Section

10

51

2

10

Plab GeV/c 10

-1

1

10

10

2

10

3

10

4

10

5

10

6

10

7

10

8

√s GeV

Cross section (mb)

1.9

10

2

102

10

103

104

2

10

Plab GeV/c 10

-1

1

10

10

2

10

3

10

4

10

5

10

6

10

7

10

8

Figure 4.7: Recent compilation of the total cross section for pp and pp interactions, as a function of the incident laboratory momentum. The points at the highest energies, for pp, are inferred from cosmic ray data; the points for pp, up √ to s = 2 TeV, are from the Tevatron. Note that both cross sections rise and tend, asymptotically, to the same value [8].

kept shorting out, and had to be removed, reconditioned and reinstalled. The reason was that to reduce costs, plaster of Paris instead of epoxy had been used to bind the windings of the magnet coils. Since immediately after completion of the construction work, the

October 4, 2012

52

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

humidity in the accelerator tunnel was very high, water was absorbed by the plaster, resulting in shorts across the coil windings. Excitement surged, as the beam made complete turns around the ring and was accelerated, opening a new domain to experiment. In one of the darkest periods, Boyce McDaniel took a six month leave from Cornell to help commission the accelerator. At these early stages of accelerator operation, beam diagnostics were primitive, and the rotating CH2 target was used as a primary diagnostic tool. The rotating target was designed by Rochester engineer Tom Haelen, and constructed in the University machine shop. A schematic of the target and of its location relative to the proton beam in the CO area is shown in Fig. 4.8. The upper sketch is a

Figure 4.8: Schematic layout of the rotating target in the CO area of FNAL. The upper sketch is a “beam eye” view of the rotating foil, while the lower sketch is a plan view.

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Rising pp Total Cross Section

b1432-ch04

53

“beam eye” view of the rotating foil, while the lower sketch is in the horizontal plane; the plane of rotation of the foil is inclined at 70◦ with respect to the beam line, so that the recoil protons could exit the foil without traversing excessive material. The target had to operate in the accelerator vacuum, and was retracted out of the beam line when not in use. A stepping motor provided a high resolution adjustment of the vertical position of the foil. Even at low beam current, the forward detectors in the CO area would register a burst of counts when the foil swept across the beam. Adjusting the vertical position of the tip of the foil, located the position of the beam so that the vertical profile of the beam could be mapped out. The angular position of the foil (rotating at 60 Hz), at which the beam was intercepted, is a measure of the horizontal position and horizontal extent of the beam. The angular position was inferred by timing the appearance of scattered particles in the detectors with respect to a trigger generated by the motor driving the foil. While this diagnostic was available only at one position in the machine circumference, it proved essential in the early tuning of the beam and was used routinely once the target controls and the counts from the forward scintillators were made available in the main control room of the accelerator. As the beam intensity increased, the interactions in the foil target became excessive and the counters would saturate. It was again Haelen who had the idea of replacing the foil with a 20 µm carbon filament; such filaments were a novelty and were just then becoming commercially available. The carbon filament would now be swept through the beam, and the time dependence of the observed counts directly reflected the horizontal profile of the circulating proton beam. Thereby, the rotating target became, serendipitously, a new and precise technique for measuring beam profiles. The technique involving fibers is now used in most high energy accelerators, to measure beam profiles and is referred to as the “flying wire” method [9]. Instead of a rotating fiber, the fiber is stretched horizontally (or vertically) and swept, at some fixed speed, through the beam in the orthogonal direction. Forward scintillation counters detect the spray of particles produced in the interaction of the beam with the fiber and, as in the rotating case, the time dependence of the

October 4, 2012

54

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

scintillator signal gives the corresponding beam profile. The speed of the sweep and the thickness of the “flying wire” are chosen so as to optimize the signal under different beam conditions. In electron accelerators, and in cases where higher resolution is desired, a “laser wire” is used instead of a carbon fiber. We never published the results of these technical studies because there was always some more pressing “physics” issues to deal with. Nevertheless, the usefulness of the rotating target as a way to measure beam profiler has endured, and the flying wire remains a common diagnostic tool, forty years later.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

http://en.wikipedia.org/wiki/Robert R. Wilson; A. Melissinos and S. L. Olsen, Physics Repots 17C, 77 (1975). V. Bartenev et al., Phys. Rev. Lett. 31, 1088 (1973). V. Bartenev et al., Phys. Rev. Lett. 29, 1755 (1972). V. Bartenev et al., Phys. Rev. Lett. 31, 1387 (1973). S. R. Amendolia et al., Phys. Letters 44B, 119 (1973). U. Amaldi et al., Phys. Letters 44B, 112 (1973). K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010). M. Hu et al. “Beam Profile Measurement with Flying Wires at the Fermilab Recycler Ring” 21st IEEE Particle Accelerator Conference, Knoxville, TN, USA, 16–20 May 2005, pp.2182.

b1432-ch04

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 5

Dimuons and Trimuons

The Landscape In the late 1960s, the 3-quark model was accepted by the majority of particle physicists. Kwan Lai, a gifted physicist from Brookhaven, suggested to me that collisions of π − mesons with protons had a better chance of leading to new particles with zero baryonic number, since the u quark could annihilate one of the dominant u quarks in the proton. Another hot topic was the production of neutral vector mesons, that is, mesons with quantum numbers J PC = 1−− . The primary example was the ρ0 -meson, mρ = 776 MeV, that decayed to π + π − . Such mesons have the same quantum numbers as the photon and were often invoked as virtual particles that could be exchanged in electromagnetic interactions. Given their quantum numbers, they would also be expected to decay into e+ e− or µ+ µ− pairs. Nobel laureate T.D. Lee believed, as it soon turned out to be, that a spectrum of such vector mesons existed, and he urged the experimenters to look for them. In considering future experiments with my colleague Taiji Yamanouchi of Fermilab, we mulled over these ideas. Taiji proposed that pp collisions would lead to qq annihilations even more abundantly, and we went to Brookhaven to look whether such a beam could be built and what yields to expect. As we were wandering on 55

b1432-ch05

.

October 4, 2012

56

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

the floor, we ran into Leon Lederman, our mentor from the µp experiment. Lederman was at Columbia and a very close friend of T.D. Lee, and he too, was scouting out Brookhaven, but for a beam that would be suitable for a search for vector mesons. He decided to pull out all the stops, and use as much particle power as could be mustered. Of course, Leon had the status and standing to make big demands on the laboratory. The entire proton beam would be extracted from the AGS, and dumped onto a Uranium target to maximize the interaction rate as much as possible. The vector mesons would be detected through their decay into µ+ µ− pairs that would penetrate through all the shielding, and thereby be identified, in spite of the huge background. The layout of the experiment [1, 2] is shown in Fig. 5.1, and in principle, it was straightforward. The detectors were subject to very high counting rates, with about a million muons emerging for every AGS pulse. The muons originated mostly from π and K decays, yet it was possible, by suitable coincidence techniques, to identify the dimuons in spite of this large background of single muons. The momentum of each muon was inferred, with a resolution ranging from 8% to 15%, from its range in a set of iron absorbers, after all the hadronic products had already been

Figure 5.1: Plan view of the apparatus for the Brookhaven dimuon experiment [2], showing the external proton beam, the Uranium target location, and the iron absorbers interspersed with scintillator hodoscopes, used to identify the muons.

b1432-ch05

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

b1432-ch05

57

Figure 5.2: The differential cross section for production of µ+ µ− pairs as a function of the effective mass mµ+ µ− , for an incident proton energy of 29.5 GeV. Note the shoulder at ≈ 3 GeV [2].

stopped in the shielding surrounding the target. The muon direction was established by assuming that they originated in the target and by their hit in a set of scintillator hodoscopes, but was degraded due to multiple scattering in the shield and absorbers. The resulting cross section dσ/dm is shown in Fig. 5.2 as a function of the effective mass mµµ of the two muons. The fact that the cross section showed a broad shoulder around ≈ 3 GeV caused lots of excitement [1, 2], and nearly a hundred theoretical papers were published in 1971–72 to explain the effect.

October 4, 2012

58

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

In reality, however, nature was sending a different message. In fact, a huge resonant peak was present in the dimuon spectrum centered at mµµ = 3.106 GeV, the now famous J/ψ state. The poor mass resolution in the experiment of Lederman and his colleagues, had turned the peak into a shoulder. The experiment was terminated when the Uranium target caught fire due to the large amount of heat generated, as the entire external proton beam was absorbed in a small volume of the target. The J/ψ was discovered a few years later, by Sam Ting and his collaborators at Brookhaven, but now using a thin Beryllium target and forming the effective mass of the e+ e− pairs, with particular attention to the momentum resolution of the detector. The mass plot is shown in Fig. 5.3, and there is no

Figure 5.3: Event yield for production of e+ e− pairs as a function of the effective mass me+ e− showing the discovery of the J/ψ in p-Be collisions. The shaded region refers to data taken with the setting of the spectrometer magnets reduced [3].

b1432-ch05

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

b1432-ch05

59

doubt about the existence of the state [3]. Almost simultaneously, the J/ψ state was observed in e+ e− collisions, by Burt Richter and his group at SLAC at the recently completed SPEAR storage ring. I remember being a member of the Brookhaven program advisory committee when the proposal by Sam Ting and his group, that led to the discovery, was approved. The outcome of the dimuon experiment was an embarrassment for Lederman, but he had his revenge, when in 1977 his group discovered at Fermilab the next vector meson, the Υ [4], by its decay into µ+ µ− . Lederman was a towering figure in experimental particle physics with many major contributions to his credit, and was quickly forgiven for having missed the J/ψ. He received the Nobel prize in 1986 for his work on neutrinos, and was director of Fermilab in the crucial period from 1979 to 1989. We will return to the Υ family in the next chapter where I discuss results from the Cornell “B-factory.”

The Rochester–Brookhaven Collaboration By 1974, the J/ψ had just been discovered putting the field in a completely new light. Work on an improved muon beam was progressing at Fermilab, which was now in full operation. A proposal by Rochester and Fermilab collaborators, to use this new beam was not accepted, in favor of a Princeton-Berkeley proposal. We proposed instead, in collaboration with a Brookhaven group led by George Kalbfleisch, to measure the dimuon spectrum in π − nucleon collisions. Part of our motivation was the availability of the C-1 beam line from the AGS, a beam that had been constructed for the µp − 2 experiment, but that could deliver both charged pions and muons. Rochester was to construct the requisite proportional wire chambers in our university shop, and re-use the hodoscopes from µp − 2. The detector configuration was that of an open spectrometer, using a large aperture (120 × 36-inches) magnet, as shown in Fig. 5.4. The electronics for the experiment were designed and assembled by Brian Wormington and Dave McCumber from Rochester. Data taking began in 1976 and continued for about two years. The

October 4, 2012

60

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 5.4: Layout of the apparatus of the Rochester–Brookhaven dimuon experiment. To set the scale, the opening of the large spectrometer magnet is 120 inches. The dashed lines represent the proportional wire chambers (PWC’s), while “S” refers to scintillation counter arrays. To identify muons, iron absorbers were placed downstream of the wire chambers; muons penetrate through the absorber, while other particles are strongly attenuated [13].

Brookhaven group included physicists Dick Strand, Josh Alspector and Jim Sharanguivel, while from Rochester postdoc Bill Metcalf, and three dedicated graduate students, Martin Miller, Dennis McCal and Joe LeBritton, shouldered much of the construction of the experiment and the analysis of the data. We had occasional help from Al Abashian and from Sam Borenstein, and technical support from both institutions, in particular from our own Jack Sanders and from Marti Van Lith of Brookhaven. The trigger for data acquisition demanded two hits after the muon filter and was quite efficient. The first result from the analysis [5] was the clean observation of a J/ψ signal with π − of 16 GeV/c and 22 GeV/c momentum incident on a copper target, as shown in Fig. 5.5. We found that the production cross section was peaked for J/ψ moving in the forward direction, that is with longitudinal

b1432-ch05

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

Figure 5.5: signal [5].

b1432-ch05

61

Dimuon mass spectrum in π − -Cu collisions showing a clear J/ψ

momentum along the incident π − , as shown in Fig. 5.6. This is not surprising because the c antiquark in the “sea” of the π − parton distribution function (pdf), carries a larger fraction of momentum than the c quark in the “sea” of the proton or neutron. Thus the cc, that materializes as the J/Ψ, has a net momentum in the π − direction. The forward motion is not seen at higher energies where J/ψ production proceeds mainly through gluon fusion. Results from similar experiments [6, 7] at higher energy were becoming available at the same time, and are included in Fig. 5.6. Our results, combined with the higher energy data, provided a satisfactory picture for the energy dependence of J/ψ production in meson-nucleon collisions. Equally significant was the production of µ+ µ− in the continuum. This electromagnetic process qq → l+ l− , (proceeding through the exchange of a virtual photon), is named after Drell and Yan who first proposed it [8]. The observed yield of µ+ µ− pairs is shown in Fig. 5.7

October 4, 2012

62

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 5.6: Distribution of J/ψ production as a function of Feynman xF = p∗long. /p∗max , where p∗ refers to the momentum in the collision c.m. frame. Data from other experiments are included [5].

where the contribution from the vector mesons (ρ0 and J/ψ) has been subtracted [9]. In Fig. 5.7 the continuum µ+ µ− data are plotted as a function of the scaled variable τ = M 2 /s and therefore can be directly compared at different energies. The differential cross section dσ/dM is multiplied by M 3 to make it independent of kinematic variables and only a function of the pdfs. At small values of τ , our low-energy data exceed by far the yield of the higher energy data from CERN and Fermilab. This is because at low energies, the parton

b1432-ch05

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

b1432-ch05

63

Figure 5.7: Drell-Yan production of µ+ µ− pairs as a function of the scaled variable τ = M 2 /s = x1 x2 . Note the difference by two orders of magnitude, in the yield with incident π − , as compared to that with incident protons [9].

distribution (or structure) functions vary with momentum transfer, a phenomenon commonly referred to as “scale breaking”; see Fig. 5.8 below. Most striking in Fig. 5.7, is the difference between the yield of Drell–Yan pairs produced by π − as compared to the production by protons. The data provide a direct confirmation that the q content of the proton is much smaller, especially at large τ , therefore large Feynman x, than that of the π − . In the Drell–Yan model, the yield of lepton pairs (of opposite charge), depends on the center of mass energy of the two colliding

October 4, 2012

64

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 5.8: The pion structure function (pdf) as a function of x1 , the fractional momentum of the leading antiquark, and its evolution with energy [10]. The curve through the 22 GeV/c data is a a properly parameterized fit to the data, while the curve through the 225 GeV/c data uses the same parametrization but evolved from the lower to the higher energy according to the QCD model of Ref. [11].

hadrons, and on an integral over the product of their pdf. Hence a measurement of dilepton production can be used to extract pdfs from the data. One necessary assumption is that the process can be factorized, that is, it depends only on the product of the nucleon and pion pdf. This turns out to be valid in perturbative QCD, and with the well-known pdf of the proton and of the neutron, can be used to extract from our data the pdf of the pion [10]. In Fig. 5.8 we show the pdf of the pion, as a function of x, (the fraction of the longitudinal momentum of the pion carried by the quarks). The pdf extracted from our data at an incident energy of Eπ = 22, is compared to the pdf extracted from data at Eπ = 225 GeV [6]. The curve through the 22 GeV/c data is a a properly parameterized fit, while

b1432-ch05

October 4, 2012

4:5

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Dimuons and Trimuons

b1432-ch05

65

the curve through the 225 GeV/c data uses the same parametrization but evolved from the lower to the higher energy according to the QCD model of Ref. [11]. A byproduct of the analysis of this low-energy data, was the determination of the intrinsic transverse momentum of the quarks in hadrons [12]. It was known for a long time, that to match the experimental data for particle production as a function of transverse momentum, an intrinsic transverse momentum had to be added to the parton distributions. This effect is much more pronounced at low energies and could be clearly measured in our data. At the end of the experiment the beam was modified from π − to µ− , with a mean muon energy of 10.5 GeV and a contamination from pions of 1, the scattered photon energy can exceed the K-N limit, which is obtained by setting n = 1 in the equation. It follows that for multiphoton scatters the limit on the energy of the forward-going electrons is lower than in the n = 1 case. This kinematic feature was exploited in the experiment, because the spectrometer disperses the scattered electrons away from the beam line making it possible to measure their momentum spectrum. On the other hand, the detection of pair production is fairly unambiguous, because, in principle, measurement of the positron momentum suffices to determine the kinematics of the event. The difficulty, in that case, is that to produce an e+ e− pair, it is necessary to absorb at least five photons from the field, and this probability is low in the perturbative regime. In fact, pairs are produced through two steps e− + ω → e− + γ

followed by

γ + nω → e+ + e− .

(6)

The first step involves the production of a high energy backscattered γ, and in the second step, the high energy γ interacts within the laser focal area (in which it was produced) and absorbs several photons from the field to create the e+ e− pair. Assuming that Eγ ≈ 28 GeV, which is the K-N upper limit for the scatterred γ-ray when the incident photons are in the green (λ = 527 nm), the cm energy squared in the second collision is s = 4Eγ nω, and this must equal or exceed 4m2e /c4 . One finds that n > 4, and since we must also account for the photon that was absorbed from the field to produce the initial high energy γ, at least five photons must be absorbed from the field, a characteristic “multiphoton” process. Often, the high energy γ is produced in a n > 1 process and so its energy exceeds 28 GeV, with fewer photons needed in the second step to produce the pair, however the total number of photons absorbed from the field must still exceed n = 5. These simple numerical relations are borne out by the data, as discussed below.

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

129

Backward scattering of laser photons from 20 GeV electrons was pioneered at SLAC in the early 1970s as a tool for producing high energy photons to study their interactions in a bubble chamber [10]. The achievable laser intensities at that time were many orders of magnitude weaker than what is needed to reach the nonlinear regime. Polarization effects, are important, both as far as the polarization of the laser beam as well as the polarization of the outgoing γ’s, but they were not investigated in this experiment. Whereas pair production from γ-rays striking a nucleus is quite a common occurrence, referred to as the Bethe-Heitler process [11], the results of our SLAC experiment provide the first observation of pair production involving real photons in the complete absence of matter. It is the first observation of multiphoton-photon scattering, albeit inelastic. Finally, the SLAC experiment is also the first experimental observation of the “breakdown of the vacuum” by an EM field.

The Experiment and its Logistics I had read Kirk McDonald’s proposal [1] with care, and was considering the possibility of a nonlinear QED experiment in 1989–90, while at CERN. At that time, there was a flurry of excitement about a possible e+ e− bound state observed in heavy ion collisions at Darmstadt [12] and if, as speculated, the effect was due to the strong electric fields present in heavy ion collisions, that state should also be observable in the nonlinear QED experiment. McDonald was interested in carrying out a similar study, and this led to our collaboration. It was clear that the experiment should be done at SLAC, where our first contact was Jim Spencer, also keen about nonlinear QED. Rochester would provide the laser and Princeton the detectors for the backscattered γ’s. To reach the nonlinear regime we planned for a laser at λ = 1054 nm, that would deliver ∼1 J in 2 ps, so that when focused to an area of 20 µm2 the electric field at the focus would approach √ E = I Z0 = 2 × 1010 V/cm. At SLAC, the electron beam energy could reach Ee ≈ 50 GeV, or γ ≈ 105 ; therefore in the rest frame of the electron the electric field at the laser focus would be of order Ecrit /3. To achieve this power level it was necessary to use a special

October 4, 2012

4:6

130

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

“slab” laser designed by Milt Shoup of the Rochester Laboratory for Laser Energetics (LLE). David Meyerhofer, a fellow Rochester faculty member, joined the effort from the start and provided much expertise in the area of high power lasers. Postdoc Charlie Bamber, laser expert Todd Blalock, and two Rochester graduate students, Theo Kotseroglou and Steve Boege moved to Stanford early on. Graduate students Thomas Koffas and David Reis, as well as engineer Wolfram Ragg joined the experiment soon thereafter. At SLAC the experiment would be run in the Final Focus Test Beam (FFTB) designed and built by David Burke and his group. SLAC staff members who were part of the experiment, in addition to David Burke and Jim Spencer, were Clive Field, Dieter Walz and graduate student Glenn Horton-Smith. Their contributions were essential in setting up the beam and the interaction region and, of course, they participated in the running and analysis. From Princeton, in addition to McDonald, Christian Bula was in charge of the online data acquisition system and also led the off-line analysis, while Eric Prebys assumed responsibility for the pair spectrometer and the analysis of the data from the forward-going γ’s. A proposal was submitted to SLAC in 1991, and approved by the director, Burt Richter, soon thereafter. In that version of the proposal we had suggested two interaction regions, with half of the laser beam to be directed to each region. In the first region, the electron beam would interact with half of the laser flux and produce a beam of high energy photons, which, after the electrons had been diverted from the original beam line, would scatter, at the second interaction region, from the remainder of the laser flux. The process at the second interaction region would be pure photon–photon scattering. It turned out that this second region was not needed, because the photon density at the laser focus is so high, that the high energy γ’s produced by Compton scattering, scatter again from the laser photons before exiting the focal area. In retrospect, this was fortunate, as it would have been quite difficult to set up the alignment and timing at the second interaction region. At this point, a fortuitus, but crucial event was the addition to the experiment of the University of Tennessee (UT) group under William Bugg. The UT group had

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

131

Figure 10.2: Schematic of the experimental setup: The laser pulse traverses through the electron beam at the interaction point, IP1, where the laser beam is focussed. This is schematically indicated by the large arrows on the left of the figure. The scattered electrons are deflected by the “dump” magnets into the electron calorimeter (ECAL). Positrons are deflected into the positron calorimeter (PCAL). The scattered photons are detected in a calorimeter at the end of the beam line (GCAL); they could also be converted to e+ e− pairs that were analyzed by the pair spectrometer [13].

developed compact Silicon-Tungsten calorimeters for eventual use at the SSC, and these instruments were ideal for the detection of the recoil electrons and positrons, and they became the principal detectors for the experiment. The UT group included, in addition to Bill, engineer Steve Berridge, physicist Achim Weidemann and graduate student Kostya Shmakov. The final layout of the experiment [13] is shown in Fig. 10.2. A set of permanent dipole magnets divert the primary electron beam to the ground where it is absorbed, in accelerator jargon the beam was “dumped”. The magnets also disperse the recoil electrons and any positrons produced by the beam. The Silicon-Tungsten calorimeters that detect the electrons and positrons, and the forward γ’s, are indicated as ECAL, PCAL and GCAL. A small fraction of the forwardgoing high-energy photons could be converted in a thin absorber and their energy measured in a magnetic pair-spectrometer with CCD readout. One major technical concern of the experiment was the synchronization of the laser pulse with the electron pulses, both being only few picoseconds long. To achieve this, the 476 MHz signal from the master oscillator of the SLAC linac was transported to our experimental area. The 4th subharmonic, i.e. 119 MHz, was used to drive

October 4, 2012

4:6

132

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 10.3: Timing curve showing the number of electrons scattered into the top row of the electron calorimeter (n = 1 scatters) as a function of delay of the optical pulse. The fitted gaussian has standard deviation σ = 4.3 ps as expected from the convolution of the length of the electron and laser pulses [13].

the mode-locker of the Nd:YLF laser oscillatorl which was thereby, synchronized with the linac frequency. By using an optical delay, the timing of the laser pulse could be adjusted to the electron pulse train [14]. The apparent fluctuations in laser pulse timing were ≈ 2 ps, and the time jitter of the electron pulses was of the same order. Direct confirmation of the synchronization is shown in Fig. 10.3 which gives l

Nd:YLF, Neodymium doped Yttrium Lithium Fluoride has larger bandwidth than Nd:YAG and is used when shorter pulses are desired.

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

133

Figure 10.4: Schematic of the chirped pulse amplificaton laser system used in the experiment [13].

the number of linearly scattered electrons as a function of the relative delay of the laser pulse. A gaussian fit yields a standard deviation of 4.3 ps, as expected from the length of the electron and laser pulse lengths. To assure the exact spatial overlap of the electron and laser beams, the entire scattering chamber could be moved in elevation and in the horizontal direction transverse to the beam, and positioned to ≈1 µm resolution. For details of the interaction region see Ref. [13]. The laser was based on chirped pulse amplification [15] and all components including the slab amplifier were built “in-house”, by the Rochester group. The laser system is shown in Fig. 10.4. The first component is the Nd:YLF mode-locked oscillator, which delivers ≈ 60 ps wide pulses. The pulses are stretched to ∼1 ns, and a single pulse is selected and amplified in a Nd:glass regenerative amplifier; it is further amplified in a double-pass configuration and shaped to match the slab aperture. After the slab, the pulse has reached ≈ 2 J, and it is spatially filtered and compressed in a pair of large gratings. It can be doubled to the green when so desired. After the pulse was

October 4, 2012

4:6

134

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

sent to the interaction region, it was returned to were the laser room, where its intensity, focal properties and duration were diagnosed. While the laser pulses were reproducible, the peak electric field at the focus fluctuated. This is because the peak field is a convolution of the energy, duration and focal spot of the laser, the latter two being more difficult to control. To avoid excess heating of the slab, the system was operated at a rate of 0.5 Hz. The FFTB electron beam consisted of ≈ 1 ps short pulses, each containing about 5 × 109 electrons, at a repetition rate of 11 Hz. The laser system performed to specifications but needed extended attention during the run. As mentioned previously, the Si-Tugsten calorimeters were the primary detectors used in the experiment. Figure 10.5 shows the trajectories of negatively and positively charged particles of different momenta emerging from the interaction region, traversing the

Figure 10.5: Calculated trajectories of electrons and positrons through the magnetic spectrometer, as a function of their momentum in GeV/c. The location of the electron calorimeter and of the positron calorimeter is indicated by the vertical bars [13].

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

135

magnetic spectrometer and reaching respectively, either the electron or positron Si-Tungsten calorimeters. The calorimeters were calibrated in an external beam, and were also tested in their actual location by introducing a thin radiator (wire) at the interaction point. In these tests it was easy to identify the number of electrons striking the calorimeter in a single pulse. In the actual experiment, the momentum was determined by the calorimeter row that was hit, based on a map of the dispersion imposed by the spectrometer, see Fig. 10.5, and could be compared to the energy deposited in the calorimeter. The calorimeters could be moved vertically to select the desired momentum acceptance and prevent them from saturating.

Results of the Experiment The calculated spectrum of the recoil electrons, including the effects of nonlinear scattering, from the infrared laser pulse, λ = 1054 nm, at the design intensity, is shown in Fig. 10.6, where the contribution from the absorption of n = 1 to 4 photons is indicated. The effect of plural scattering, that is the repeated occurrence of the linear process as the backscattered photon re-scatters within the laser focus, is included. The lowest recoil electron energy for n = 1 scattering from the IR is Er = 25.6 GeV. To cover the large dynamic range in the rate of recoil electrons and to avoid saturation, it was necessary to reposition the electron calorimeter during the run. The measured electron spectra for different laser intensities are shown in Fig. 10.7, where the data have been assigned an error, and the predictions include the experimental resolution. As the laser intensity increases, the lower limit on the electron momentum continues to decrease, as is expected because of the absorption of more photons from the field. The observed momenta are well below the n = 1 kinematic limit, thereby confirming that the scattering is nonlinear. The dashed curves in the figure indicate the contribution of plural scattering, which, in general, is suppressed as compared to the yield from nonlinear events. The observed scattering rate is indicated by the solid circles and is in good agreement with the calculation, shown by the squares. This also implies that the laser intensity was

October 4, 2012

4:6

136

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 10.6: The calculated rate of scattered electrons for linear, n = 1, nonlinear, n = 2 − 4, and plural Compton scattering for the infrared laser and for the electron beam parameters given in the text. The solid line is the sum of all possible processes. The rates for n = 2, 3, and n = 4 nonlinear processes are also shown as separate contributions [13].

measured correctly on a shot by shot basis, and even at the lowest intensities, the n = 2 plateau can be clearly identified. An alternative presentation of the data is shown in Fig. 10.8. Here, the yield of recoil electrons, at a given momentum, is plotted as a function of laser intensity, for different number of absorbed photons. The shaded regions are the predicted rates, with the bands indicating the uncertainty on laser intensity. The slope of these curves follows closely η 2n , where the intensity parameter η, was defined in Eq. 1, and n is the number of absorbed photons. Such dependence is typical of multiphoton processes in the perturbative regime. The complementary measurement of the forward-going (backscattered) photons proved to be more difficult, because of the very high flux of photons (γ’s) incident on the pair spectrometer. Nevertheless, a sufficient number of events with Eγ > 30 GeV, corresponding to n = 2 scattering, were identified, to confirm that nonlinear scattering was observed in this channel as well, at the expected rate.

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

137

Figure 10.7: The yield of electrons, nonlinearly scattered from the circularly polarized, IR laser, (1/Nγ )(dN/dp) as a function of electron momentum p, for six different laser intensities. The data are the solid circles with vertical error bars that correspond to the statistical and reconstruction uncertainties added in quadrature. The open squares reflect the simulation, with estimates of uncertainties due to resolution, indicated by the horizontal and vertical bars. The dashed lines are the simulation of just the n = m plural, linear scatters [13].

October 4, 2012

4:6

138

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 10.8: The yield of electrons (1/Nγ )(dN/dp) as a function of green laser intensity, for four representative values of electron momentum. Data for which the n = 2, n = 3 and n = 4 nonlinear Compton processes dominate, are compared to the simulation, shown as bands, that indicate the uncertainty in laser intensity when combining data obtained under differing laser power [13].

Before turning to pair production, we discuss a technical point related to establishing the collision of the electron and laser beams. It is quite possible to have a large yield of n = 1 Compton scatters, even if the electron beam crosses the laser beam away from the laser focus, that is, the point in space and time where the electric field has its largest value. This can be seen in Fig. 10.9(a), where the electron beam is in the same horizontal (x-z) plane as the laser beam, but

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

139

Figure 10.9: (a) Crossing of the laser pulse and electron beam in the x-z plane for two collision topologies, both yielding approximately the same linear Compton-scattering rate but drastically different nonlinear Compton rates. (b) Linear (n = 1) Compton event rate as a function of transverse beam displacement and relative timing. (c) As for (b), but for the rate of n = 2 nonlinearscattering events [13].

in the upper sketch, the electron beam is displaced by ∆x from the focus. By adjusting the relative timing between electron and laser pulses, collisions will still occur, in fact over a longer path length than for the electron beam crossing the focus, as in the lower sketch of Fig. 10.9(a). Figure 10.9(b) shows the n = 1 (linear) yield for a raster scan of the time delay ∆t, vs the horizontal displacement ∆x, of the electron beam. Indeed, the yield appears to be uniform

October 4, 2012

4:6

140

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

when the displacement is compensated by the appropriate delay, for the 17◦ crossing angle. On the other hand, Fig. 10.9(c) shows the yield for the n = 2 (nonlinear) process, for the same raster scan as in (b). It indicates that the nonlinear scattering rate is significant mainly when the electrons cross the laser focus. These results provide direct evidence that the n = 2 yield is a nonlinear process, but they also offer a necessary diagnostic for aligning the laser and electron beams. This is extremely important in observing pair production, as that process is highly nonlinear, involving the absorption of at least n = 5 photons. While the results on nonlinear Compton scattering were quite convincing, pair production is more difficult to observe, as it involves higher order nonlinear effects and is a more complex process. Because the number of photons that must be absorbed from the field of the green laser is approximately one half of what would be required with infrared light, the green laser was used throughout these studies. Only the positron from the e+ e− pair was detected, since the corresponding electron could not be distinguished from the thousandfold higher yield of recoil electrons from Compton scatters. The momentum of the positron was obtained from its vertical position in the calorimeter, while its energy was determined by the total charge collected in the calorimeter. These measurements of energy and momentum uniquely identified the positrons. The background was established by counting positrons in the absence of a laser pulse. Fig. 10.10(a) shows the positron spectra with the laser on and off and Fig. 10.10(b) shows their difference. The predicted spectrum for e+ production is included in Fig. 10.10(b), for comparison with the data, and as expected, peaks at half of the momentum of the high energy γ that initiates the pair production process described by Eq. 6. The rate of production of e+ e− pairs per laser shot, Re+ as a function of the laser intensity at the focus, is shown in Fig. 10.11, where the laser intensity is expressed by the corresponding multiphoton parameter η. The rate increases exponentially as a function of η, and a power-law fit to the data, in the form Re+ ∝ η 2n yields n = 5.1 ± 0.2. This is in exact agreement with a perturbative treatment when, on the average, five photons are absorbed from the laser field. An argument is often heard that since a laser field is not

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

141

Figure 10.10: (a) Distribution of the number of positron candidates (Ne+ ), as a function of momentum for the same number of electron pulses when the laser was on and when the laser was off. (b) Spectrum of signal positrons obtained by subtracting the two distributions in (a). The curve shows the expected momentum spectrum [13].

Figure 10.11: The dependence of the positron rate per laser pulse, Re+ , on laser field intensity, expressed by the parameter η. The line shows a power law fit to the data. The shaded distribution is the 95% confidence limit on the residual background from showers of beam particles, after subtracting the positron rate with the laser-off [13].

an eigenstate of the photon-number operator, it is not relevant to speak about the number of photons in the field. However, this experiment establishes only the difference in the number of photons in the laser field, before and following the interaction.

October 4, 2012

4:6

142

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

b1432-ch10

Reminiscences: A Journey Through Particle Physics

We now briefly digress to discuss a key technical point that made the analysis of the pair production data possible. The e+ e− production rate depends on the tenth power of η, and any small errors in η could therefore greatly distort the presentation of the results plotted in Fig. 10.11. It is simply not possible to determine η to the required accuracy from a direct measurement of the laser parameters, especially for a single shot. However, in the same laser shot that the pair was produced, several nonlinear scatters did also occur. From the observed rate of these nonlinear Compton events, that also depend on η but at a lower power, we could extract η, for the given laser shot, with much smaller uncertainty. As already mentioned, pair production can also be expressed through Schwinger’s result given by Eq. 4, where the e+ e− yield is expected to depend exponentially on the parameter Υ, rather than to follow a power law, which is typical of the growth in the perturbative regime. Because the pair is produced by a high energy photon interacting with the field, we must redefine Υ as Υγ =

2Eγ Erms , mc2 Ecrit

(7)

where, when Eγ  mc2 , Υγ ≈ Υe , with Υe as defined by Eq. 3. The pair production data is plotted in Fig. 10.12 as a function of 1/Υγ , and includes also data points obtained at the higher incident electron beam energy of 49.1 GeV. A fit to the form Re+ ∝ exp(−A/Υγ ) yields A = 1.27 ± 0.25 which agrees with the expected value of A = 1.1. We therefore conclude that at the laser intensity of our experiment, the nonlinear interaction of photons with electrons can be described by either the perturbative or the non-perturbative formulations [13]. In concluding I want to say that the experiment worked not only, because of the dedication and contributions of many participants but also because of the fortuitous turn of events. For instance, the fact that a single interaction region was adequate to observe photon– photon scattering had not been anticipated. The careful alignment of the laser and electron beams, using the raster scan of the n = 2 scatters, was crucial to observing pair production. The stability of the relative timing between the electron and laser beams was unknown

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Matter From Light

b1432-ch10

143

Figure 10.12: Number of positrons per laser shot as a function of 1/Υγ . The circles are the 46.6 GeV data whereas the squares are the 49.1 GeV data. The solid line is a fit to the data of the form ∝ exp(−A/Υγ ) [13].

until the beginning of the experiment. The availability and excellent performance of the Silicon-Tungsten calorimeters was a key feature, and finally, the survival of the laser system, the laser pulse shaping and the efficient doubling to the green could easily have failed. Theoretical modeling and data analysis by many, made sense of the rather confusing early results. Delivery of the electron beam took operator skill as well as participation from the experimenters.

Breakdown of the Vacuum by Laser Fields The process of “Vacuum Breakdown” has been a sort of holy grail for physicists interested in building high power lasers, and it still remains a topic of interest. The aim is to build a pulsed laser that can deliver, when suitably focused, a critical field intensity. To me, there are several caveats to this goal. First of all, a wave field, such as exists at a laser focus can not lead to pair production because the invariant E 2 − B 2 = 0, whereas the invariant mass of an e+ e− pair is positive definite, Me+ e− > 4m2e . It is now accepted that to observe spontaneous pair production from the vacuum, one must bring into collision two counter-propagating laser beams. This is equivalent to

October 4, 2012

4:6

144

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

establishing a standing wave pattern, at the nodes of which the field is purely electric. In either case, this involves photon–photon scattering, as first calculated by Breit and Wheeler [7], and can be viewed as such, rather than as the breakdown of the vacuum. Multiphoton effects will be dominant in such interactions. Another problem with attempting to deliver a pulse with critical field strength in the laboratory frame, is that the pulse must be focused. In particular, even before the field reaches critical strength, enough pairs will be produced, and the photons will scatter from them and loose energy [16]. Nevertheless, an e+ e− plasma can be created when a high intensity laser is incident on a material target. Finally, I mention the interesting application of the Schwinger formula to the breakdown of the vacuum by a strong gravitational field. This analogy, while not rigorous, leads to the equation for Hawking radiation [17].

References 1. K. T. McDonald, Proposal for Experimental Studies of Nonlinear Quantum Electrodynamics, Princeton Univ. Report DOE/ER/3072-32 (Sept. 1986). 2. H. R. Reiss, J. Math. Phys. 3, 59 (1962); Phys. Rev. Lett. 26, 1072 (1971). 3. A. I. Nikishov and V. I. Ritus, Sov. Phys. JETP 19, 529 (1964); 19, 1191 (1964); 20, 757 (1965); 25, 1135 (1967); N. B. Narozhnyi, A. I. Nikishov, and V. I. Ritus, Sov. Phys. JETP 20, 622 (1965). 4. J. D. Jackson Classical Electrodynamics, J. Wiley, New York. 5. O. Klein and Y. Nishina, Z. Phys. 52, 250 (1935). 6. V. B. Berestetskii, E. M. Lifshitz, and L. P. Pitaevskii, Quantum Electrodynamics, 2nd ed. (Pergamon Press, New York, 1982), Secs. 40 and 101. 7. G. Breit and J. A. Wheeler, Phys. Rev. 46, 1087 (1934). 8. J. Schwinger, Phys. Rev. 82, 664 (1951). 9. E. Brezin and C. Itzykson, Phys. Rev. D2, 1191 (1970). 10. R. H. Milburn, Phys. Rev. Lett. 10, 75 (1963). 11. H. Bethe and W. Heitler, Proc. Roy. Soc. London, A146, 83 (1934). 12. T. Cowan et al., Phys. Rev. Lett. 56, 444 (1986). 13. C. Bamber et al., Phys. Rev. D60, 092004 (1999). 14. T. Kotseroglou et al., Nucl. Instrum. Methods in Phys. Res. A A383, 309 (1996). 15. D. Strickland and G. Mourou, Opt. Commun. 55, 447 (1985); M. Pessot, P. Maine, and G. Mourou, Opt. Commun. 62, 419 (1987). 16. A. M. Fedotov, et al., Phys. Rev. Lett. 105, 080402 (2010). 17. A. C. Melissinos, Int. J. Mod. Phys. E17, 891 (2008).

b1432-ch10

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Chapter 11

Photoinjectors

How it All Started Perhaps an appropriate description for Rochester’s involvement in photoinjector research would be: “Have Laser, Will Travel”. More seriously, since our group had gained expertise in short pulse lasers and in chirped pulse amplification (CPA), we engaged in photoemission studies from different materials. At that time, the late 1980s and early 1990s, there was considerable interest in low emittance electron beams for use in Free Electron Lasers (FEL) and in linear colliders. Sheffield [1] had shown that a cathode placed in an RF cavity could be excited by a laser to deliver adequate charge, and that the electrons were quickly accelerated by the high field, preventing the emittance of the bunch from growing. Today, all low emittance electron beams use such photoinjectors with small variations depending on the cathode material that is used. For instance, a photoemitting cathode can be inserted in the cavity, or the copper surface of the cavity can be directly used, or to achieve higher fields with less RF power, superconducting cavities have been considered. In the meantime, in preparation for the SLAC experiment we had accumulated a substantial amount of laser gear, and we wanted to maintain a presence in Rochester even after moving to SLAC. So we set ourselves the ambitious task of building a superconducting RF 145

b1432-ch11

October 4, 2012

4:6

146

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

gun and of determining how much current could be extracted. We sought help from the real experts at Cornell, and soon realized that the project exceeded our capabilities, which left us searching for a good thesis problem for graduate student Alan Fry. Hassan Padamsee of Cornell suggested that Helen Edwards at Fermilab was trying to set up a photoinjector and would be glad for a collaboration in the laser area. It sounded like a match made in heaven, and indeed our equipment, even if second hand, and our expertise, fitted well with the needs of the Fermilab effort. A second graduate student, Michael Fitch joined the group, and this was the start of the photoinjector project at Fermilab. The background of the Fermilab photoinjector is rather interesting. In the late 1980s a group of electron accelerator physicists launched an effort to build a superconducting electron Linac in the TeV range, which was distinct from the ongoing work at SLAC to design and construct the next e+ e− collider using normal conducting, i.e. copper, cavities. The acronym TESLA, for “TeV Electron Superconducting Linear Accelerator” was adopted, and the project was initially led by Maury Tigner of Cornell. DESY was among the original participants, but soon became the driving force in TESLA, by providing space and resources, and establishing a “Test Facility” (TTF). Helen Edwards was also an early participant in TESLA and was able to convince Fermilab to join the project, and provide to DESY the modulators for the TTF klystrons. It was felt that if Fermilab was to benefit from its investments in TESLA, it should at least operate a photoinjector on its own site. Costs were always an issue, and therefore Rochester’s contribution of the drive laser, and the opportunity to train two graduate students in accelerator physics, was an important factor in the approval of the photoinjector project and its location in the A0 area. The A0 lab was in operation from 1995 to 2011. It served as a training ground for a dozen doctoral students in accelerator physics, and as a testbed for new beam configurations and diagnostics. As the project evolved, other Universities joined the collaboration, Northern Illinois University, the University of Chicago and UCLA. The advantage of A0 was that it was small enough so that it could be

b1432-ch11

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Photoinjectors

b1432-ch11

147

operated by the students, and there was no pressure on beam delivery to restrict the time devoted to testing new experimental schemes. Still, the technology was sophisticated: the capture cavity had to be kept at liquid Helium temperature and the gas returned to the compressors; adequate shielding had to be installed and safety standards had to be adhered to, which all necessitated significant infrastructure support on behalf of Fermilab. Typically, the electron beam exited the RF gun with an energy of 3.8 MeV, and after the capture cavity the energy reached 16.6 MeV. The TTF at DESY took a different path: it became a fully fledged low emittance electron beam and reached 1 GeV in 2004. It was the first Self Amplified Spontaneous Emission (SASE) free electron laser and operates as such [2]. The original TESLA concept of building an e+ e− linear collider in the TeV range had to be reconsidered in view of the international nature of such a facility and the high cost. Instead, the beam energy of the DESY linac will be increased to 20 GeV and will drive an FEL in the X-ray regime. The effort on a TeV electron linear collider (ILC) is now international, and it has been agreed that it will be based on superconducting rf. Fermilab is participating in the R&D for this new machine, using in part, the experience developed at A0. Coincidentally, our group also contributed to the prototype of the photoinjector for the SLAC free electron laser. Long range planning for the SLAC accelerator complex included construction of an energetic free electron laser X-ray source, the “Linear Coherent Light Source” (LCLS). A low emittance beam would be accelerated in a section of the electron linac and traverse through an undulator of sufficient length to spontaneously lase coherently, at X-ray energy in the range from 0.8 to 8 keV. The first step in this program was to build a laser driven rf photoinjector, the “Gun Test Facility” (GTF). At a conference in UCLA in 1994, Herman Winick mentioned the plans for the GTF, which had not as yet been funded. Since we had just completed the nonlinear QED experiment at SLAC, we offered some of our laser equipment which was then set up at the GTF by Rochester graduate student David Reis under the direction of David Meyerhofer. This work became the topic of Reis’s doctoral

October 4, 2012

4:6

148

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

dissertation. The LCLS came into operation in 2008, and David, who is now on the Stanford faculty, is using the facility to carry out photon science experiments.

The A0 Photoinjector and the Drive Laser The A0 photoinjector was to be identical to the one that was being assembled at DESY as the front end of the TTF beam. An engineering drawing of the A0 beamline is shown in Fig. 11.1. In the beginning only the RF gun and a set of diagnostic hardware were installed. The six-cell superconducting rf (srf) accelerating cavity, focusing quadrupole magnets, and the magnetic chicane were added later. The time structure of the beam was determined by the duty cycle of the srf cavities, the pulses being spaced 1 µs apart, a train of 800 pulses constituting one macropulse; the ∼1 ms long macropulses

Figure 11.1:

Engineering drawing of the A0 Beamline [4].

b1432-ch11

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Photoinjectors

b1432-ch11

149

were to be repeated at 10 Hz. Every pulse was to contain a charge Q = 10 nC, the key to delivering adequate charge being the photocathode efficiency. Metallic cathodes have very poor efficiency, less than 10−4 electrons being emitted per incident UV photon. At A0, Cs2 Te cathodes, which have efficiency of 1–2% were used, but the vacuum had to be maintained below 200 Hz. Another

October 4, 2012

4:6

160

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

unconventional feature of the GWI’s used in LIGO is the recycling of the light returning from the arms, as a result of which, the power incident on the beam splitter is increased by a factor of ∼40 over the power supplied by the laser source. This in turn reduces the level of the shot noise, and improves the signal-to-noise ratio. The light used in the interferometer is generated by a chain of Nd:YAG lasers, hence the operating, or carrier, wavelength is λ = 1064 nm. The seed laser is a non-planar-ring-oscillator (NPRO) followed by several amplification stages. In a back-of-the-envelope calculation of the sensitivity of the LIGO GWI, it would be assumed that the energy incident on the beam splitter is 10 J, so that the shot-noise-limited (due to photon statistics) measurement of the phase shift would be ∼10−10 of a fringe, corresponding to a displacement ∆x = 10−10 λ ≈ 10−16 m. If further the light makes B = 100 round trips in the arms, leading to an effective arm length of L = 4 × 105 m, the measured strain would be h = ∆L/L = 2.5 × 10−22 , which is of the correct order of magnitude. A clear and adequately detailed description of the LIGO interferometers is given in Ref. [5]. If the interferometer is set to a dark fringe in the absence of a gw, then when the gw induces a phase shift ∆φ, the detected power at the dark port will be proportional to ∆φ2 , which has the disadvantage that the signal is quadratic in the small quantity ∆φ. To obtain a signal linear in ∆φ the electric field returning from the arms must interfere with a reference field of large amplitude. When the reference field is shifted in frequency from the carrier, the signal appears at the difference frequency and we speak of “heterodyne” detection.m The reference field is already available in the interferometer because of the presence of sidebands offset by 25 MHz from the carrier, that are introduced by frequency modulating a fraction of the carrier power. Such sidebands are necessary for sensing and correcting the motion of the optics using the Pound–Drever–Hall (PDH) locking technique [6]. While the optics are free to move along the arms, their position can

m

Or one could operate far off the dark fringe condition, but this introduces unacceptable noise due to the fluctuations of the carrier power.

b1432-ch12

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Earth Tides

b1432-ch12

161

be controlled by driving current through coils, that act on four permanent magnets glued on the back side of the mirrors. As anyone who has run an interferometer knows, keeping the interferometer in lock is a heroic act, best left to a computer. Any small perturbation must be quickly compensated before the interferometer moves out of lock beyond the limits where the control system can return it to its desired operating point. This is achieved by feedback loops with gain tailored to have specific frequency response. The LIGO interferometers have numerous such loops, interconnected under computer control. The purpose of the control system is to keep the interferometer in lock, and one may wonder how the signal is detected. This information is encoded in the control signals. A gw will change the length difference between the two arms and this in turn generates a signal at the dark, or antisymmetric port (AS) in the orthogonal quadrature (Q), hence the name “ASQ channel”. The signal in this channel, reflects the changes in the arm length difference. If the change occurs at low frequency, then the GWI remains on the dark fringe, and the information is contained in the control signal. In the opposite case, of a high frequency change in the difference of arm lengths, the GWI temporarily moves off the dark fringe giving rise to a detectable error signal. We have been describing the differential arm length degree of freedom, DARM, which is directly affected by a gw. However, for the GWI to stay in lock many other conditions must be met. The Fabry-Perot (FP) cavities in the arms must stay on resonance, which implies that the common arm length, CARM, should remain an integer number of wavelengths; this is achieved by a combination of mirror motion and fine tuning of the laser frequency. To preserve the dark fringe condition, the Michelson configuration must be controlled, and finally the recycling cavity must also be kept on resonance. Correction signals for these channels are derived from photodiodes at other locations in the GWI. A complete description of the control system can be found in Ref. [7]. Of course, there are many disturbances that would take the GWI out of lock. For instance, if the laser beams were not propagating in vacuum the fluctuations introduced by scattering in the residual air would immediately unlock the interferometer. Vibrations in the

October 4, 2012

4:6

162

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 12.2: Strain sensitivity density for the LIGO 4 km Interferometers. http://www.ligo-wa.caltech.edu/ligo-science/G060293-00.jpg.

optics are particularly pronounced at low frequencies, and must be suppressed as much as possible; therefore the gain of the feedback loops is increased significantly at low frequency. The performance of the GWI is usually presented by a sensitivity curve, such √ as shown in Fig. 12.2, which gives the strain density (∆L/L = h)/ Hz that could be detected at a particular frequency. Above f ∼ 300 Hz the sensitivity is limited by the shot noise of the laser. The loss of sensitivity at higher frequencies is due to the self-cancelation of the phase shift induced by the high frequency gw’s; at low frequency the loss of sensitivity is dominated by seismic noise. The sharp lines in the spectrum are due to resonances in the mechanical modes at which the suspension of the optics vibrate. The GWI is calibrated by driving sinusoidally, and at fixed frequency, one of the end mirrors, and comparing the interferometer signal to the excitation amplitude. Three sharp lines are injected above the noise level and are continuously monitored.

b1432-ch12

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Earth Tides

b1432-ch12

163

For a table-top interferometer with rigid arms one can assume that the physical length of the arms remains fixed. This is far from the case for LIGO, where the mirrors are free to move and in addition the mirrors are mounted on the non-rigid surface of the earth. The earth’s surface is deformed with fixed periodicity by the tidal forces of the sun and the moon. While the tidal forces can be precisely calculated as a function of date, time and location, the deformation can not be predicted, even to micrometer accuracy, because the elastic coefficients of the earth are not known to such precision and vary at different locations [8]. The deformation of the length of the LIGO arms due to the earth tides, has both a common mode component, whereby both arms are extended (or shortened), and a differential mode where one arm is extended while the other arm is shortened, and which mimics a gw signal. For the 4 km arms, the differential mode can reach 200 µm which, by far exceeds the dynamic range of the control system. Thus the tidal effects must be corrected by a predictive feed-forward system based on a calculation of the tidal effects, to an accuracy sufficient to allow the control system to keep the GWI locked. The LIGO Scientific Collaboration (LSC) operates two laboratories, one at the Hanford site near Richland, in the state of Washington, and one in Livingston parish in Louisiana, near Baton Rouge, each laboratory being supported by a sizable and dedicated staff. Rochester graduate student Bill Butler moved to the Hanford site in 2001, and participated in the early commissioning of the interferometer under the direction of Fred Raab, the head of the observatory. This led to further involvement of Rochester with LIGO, in particular searching for a high frequency stochastic background. Rochester joined the LSC officially in 2004, and continued to be a member of the collaboration until 2009.

The Free-spectral-range (fsr) Channel The Fabry–Perot cavities in the arms of the GWI have a spectrum of equally spaced levels separated by ν = c/2L ≡ νf sr which is referred to as the “free spectral range” (fsr) frequency, and for L = 4 km,

October 4, 2012

4:6

164

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

we find νf sr = 37.5 kHz. The width of the levels is determined by the reflectivity of the mirrors, i.e. the finesse of the cavity, and is typically ∆ν ∼ 200 Hz. The GWI operates on a single level, and since the carrier wavelength is λ0 ≈ 1 µm, the mode number, n, at which the light resonates in the arms, is of order n = 2L/λ0 ≈ 8 × 109 . The carrier frequency bandwidth is stabilized by the control system to 1–2 Hz, yet even slight frequency noise on the carrier can excite the adjacent levels, which can remain populated because light at ν± = ν0 ± νf sr resonates in the Fabry–Perot arm cavities. We will refer to the frequencies displaced from the carrier by ±νf sr , as the “fsr” channel, and what makes this channel special is that under normal data taking conditions, where the carrier is locked on the dark fringe, the fsr channel is off the dark fringe. This is so because the lengths of the two arms are not exactly equal, but differ by a small macroscopic amount, ∆L ∼ 2 cm. The phase shift ∆φ± at the fsr channel, ν± , is 2B 2B ∆φ± =± (Lx − Ly )ν± = [(Lx − Ly )ν0 ± (Lx − Ly )νf sr ] 2π c c Here B is the effective number of round trips that the light travels in the arms. When the carrier is locked, (2B/c)(Lx −Ly )ν0 is an integer, so ∆φ0 , modulo 2π, is zero and it follows that ∆φ± /2π = B (∆L/L). For ∆L = 2 cm, L = 4×103 m and B = 100, one finds ∆φ± /2π = 5× 10−4 . This is a very large phase shift as compared to the carrier phase shift, when the interferometer is locked, typically ∆φ0 /2π ∼ 10−11 , but it does not affect the operation of the GWI because the field at ν± is weaker than at ν0 by many orders of magnitude. A similar condition holds for the “Schnupp” asymmetry that directs the RF sidebands to the AS port, while keeping the carrier dark. The data at LIGO is acquired by sampling at 16,384 Hz, which therefore limits the information that can be obtained to frequencies ν < 8 kHz. To extract information at ν± , the data from the ASQ channel was digitized at 262,144 Hz and then heterodyned and filtered to cover the frequency range 37,504 ± 1,024 Hz. For calibration purposes, another heterodyned channel at 1,024 ± 1,024 Hz, as well as the original channel at 262,144 Hz were also recorded during the S5 run. The fast data acquisition system was designed by Daniel

b1432-ch12

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Earth Tides

b1432-ch12

165

Figure 12.3: Uncalibrated amplitude spectral density for the H1 interferometer in the region of its free spectral range averaged over one month [11].

Sigg, while Rochester supplied the fast ADC’s to digitize the signals and participated in the commissioning. The sampling rate was set to 2,048 Hz and the data were written as 64 s long frames. In processing the data, 8 s long intervals were Fourier analyzed, and the eight spectra within each frame were averaged. In Fig. 12.3 is shown the spectrum in the fsr region at bandwidth, BW = 0.125 Hz, and further averaged over one month of data. The excited level at ν± is clearly present, and the narrow lines arise from the thermal excitation of internal resonances in the mirrors, the “test masses”. The spectra of the signal at the ASQ port in the region of ν± , averaged over 64 s, were correlated between the H1 and H2 interferometers to search for a stochastic background. The absence of a correlation yielded upper limits on a stochastic background at ν = 37.52 kHz [9]. The limits on the stochastic amplitude that were obtained are √ of the same order as at lower frequencies, h ≤ 10−23 / Hz, however the limit on the energy density in gw’s is much weaker because, as already mentioned, for given h, the energy density grows as f 2 .

October 4, 2012

4:6

166

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

This work was the subject of the doctoral dissertation of Rochester student Stefanos Giampanis, with important contributions from his fellow graduate student Tobin Fricke. GWI’s have some weak directionality so that if a source of gw in the 37.52 kHz range is at a fixed location in the sky, due to the Earth’s rotation, the source would appear to sweep through the direction of maximal interferometer acceptance, generating a periodic signal. Long-term Fourier analysis would reveal such a component if it is present. To explore this possibility we integrated the power spectral density, PSD, in the region ν± ± 200 Hz, for each frame, to obtain a time series of the power at ν± sampled every 64 s. This is shown in Fig. 12.4 for the 16 month period April 2006 to July 2007. To our surprise the integrated power is modulated at the daily and twice daily frequencies as can be seen in the inset in Fig. 12.4.

Figure 12.4: Integrated power in the free spectral range (fsr) region as a function of time, April 2006 to July 2007. The data are for the H1 interferometer and are sampled every 64 s. Note the daily and twice-daily modulation that can be seen in the inset. The vertical lines indicate monthly intervals [11].

b1432-ch12

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Earth Tides Table 12.1. Symbol

b1432-ch12

167

Observed and known frequencies of the tidal components (Hz). Measured

Predicted

6.536 × 10−8

6.338 × 10−8

S declinational

Diurnal O1 P1 S1 m K1 , s K1

1.07601 × 10−5 1.15384 × 10−5 1.15741 × 10−5 1.16216 × 10−5

1.07585 × 10−5 1.15424 × 10−5 1.15741 × 10−5 1.16058 × 10−5

L principal lunar wave S solar principal wave S elliptic wave of s K1 L, S declinational waves

Twice-daily N2 M2 S2 m K2 , s K2

2.19240 × 10−5 2.23639 × 10−5 2.31482 × 10−5 2.31957 × 10−5

2.19442 × 10−5 2.23643 × 10−5 2.31481 × 10−5 2.32115 × 10−5

L major elliptic wave of M2 L principal wave S principal wave L, S declinational waves

Long period Ssa

Origin, L = lunar; S = solar

Spectral (Fourier) analysisn revealed that the modulation contains all the tidal frequencies. The observed and predicted tidal frequencies are compared in Table 12.1. The resolution on the observed frequencies is ∆ν = 6 × 10−9 Hz and if this value is adopted as the measurement error, comparison of the measured and predicted frequencies yields χ2 /d.f. = 1.86. The spectra at the daily and twice daily frequencies are shown in Figs. 12.5, 12.6. Similar signals were found in the data from both the Hanford and the Livingston LIGO interferometers. The other channels of the GWI, such as ASQ, DARMCTRL, show some degree of tidal modulation but not at the exact frequencies. These data were presented at the 12th Marcel Grossman conference in 2009 [11], and more details can be found in Ref. [12]. The analysis of the long-term data was carried out by Rochester graduate student Chad Forrest and was the subject of his MS thesis. It is evident from Figs. 12.5, 12.6 that in the fsr channel very low frequency periodic perturbations acting on the interferometer, can be identified, in spite of the seismic and vibrational noise. This n

Because there are large breaks in the data that covers 16 months, we use the Lomb–Scargle algorithm [10].

October 4, 2012

4:6

168

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

Reminiscences: A Journey Through Particle Physics

Figure 12.5: region [11].

Frequency spectrum of the integrated fsr power in the diurnal

Figure 12.6: Frequency spectrum of the integrated fsr power in the the twicedaily region [11].

is a consequence of the long integration time available, since the data recorded and analyzed, for H1 spans 4.2 × 107 s. The question is what causes such a precise modulation in the ν± channel, and there are two possibilities: either the phase ∆φ± or the amplitude of the electric field at ν± are modulated. If we assume that in

b1432-ch12

October 4, 2012

4:6

SPI-B1432

9in x 6in

Reminiscences: A Journey Through Particle Physics

The Earth Tides

b1432-ch12

169

addition to the static biasing phase arising from the arm length difference, ∆φbias = B(∆l/L), there is also present a time dependent phase, ∆φt , where ∆φt