Fall 1995, Vol

S T A N F O R D L I N E A R A CCELERATOR CENTER

Fall 1995, Vol. 25, No. 3 A PERIODICAL OF PARTICLE PHYSICS

FALL 1995 VOL. 25, NUMBER 3

Editors RENE DONALDSON, BILL KIRK

Editorial Advisory Board JAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN, JOEL PRIMACK, NATALIE ROE, ROBERT SIEMANN, HERMAN WINICK

Illustrations page 4 TERRY A NDERSON

Distribution CRYSTAL TILGHMAN

The Beam Line is published quarterly by the Stanford Linear Accelerator Center, PO Box 4349, Stanford, CA 94309. Telephone: (415) 926-2585 pag INTERNET: [email protected] BITNET: beamline@slacvm FAX: (415) 926-4500 Issues of the Beam Line are accessible electronically on the World Wide Web from the SLAC home page at http://www.slac.stanford.edu. SLAC is operated by Stanford University under contract with the U.S. Department of Energy. The opinions of the authors do not necessarily reﬂect the policy of the Stanford Linear Accelerator Center.

Cover: Display of a top quark candidate event with an electron (the large magenta arrow), a muon (green tube), two jets (gray and pink arrows), and the missing trans- verse momentum carried by neutrinos (large light arrow). The arrow widths are proportional to the energy carried by the object. The lattice work shows the cells of the calorimeter showing signiﬁcant energy deposit. The spheres represent the amount of energy deposited in the calorimeter cells (orange and white for electromagnetic deposits and blue for hadronic deposits). The faint shad- ings represent the edges of the sub-detectors. The same dilepton event is shown as a lego plot on page 12. More such displays can be found on the World Wide Web at http://d0sgi0.fnal.gov/art_souvenir/art_souvenir.html.

Printed on recycled paper CONTENTS

FEATURES

4 DISCOVERY OF THE TOP QUARK Two leaders of the Fermilab experiments that isolated the top quark tell the adventure of its discovery. Bill Carithers & Paul Grannis

17 IN WATER OR ICE? Physicists are building four huge particle-detector arrays to do high energy neutrino astronomy. John Learned & Michael Riordan

25 OSTEOPOROSIS—NEW INSIGHTS Synchrotron radiation is giving us new insights into causes and treatments for osteoporosis. page25 John Kinney

31 LASER-ELECTRON INTERACTIONS High power lasers combined with SLAC’s unique high energy electron beam are opening a new ﬁeld of research. Adrian Melissinos

Ti:Sapphire Lasers DEPARTMENTS Bunch 1

Combiner 2 GUEST EDITORIAL Bunch 2 Circular James Bjorken Polarizer Left or Right Circularly Polarized Light

CONTRIBUTORS 38 Unpolarized Gun

40 DATES TO REMEMBER Mirror Box Linearly Polarized Light

e–

Load Lock

Polarized e– Gun Bunchers

page 31 Accelerator GUEST EDITORIAL

James Bjorken (Bj), who played a central role in the HEFEATURE ARTICLE in the summer development of the quark-parton model at SLAC from issue of the Beam Line was the 1963–1979, spent ten years at Fermilab in 1979–1989, serendipitous discovery of X rays 100 including a period as Associate Director of Physics. years ago by Wilhelm Roentgen. Upon He is presently co-spokesperson of the Fermilab T observing ﬂuorescence occurring some distance test/experiment T864 in the Tevatron collider which away from a Crookes tube he was operating, he he describes as being “closer to Roentgen in style set aside his duties as university rector and than to CDF or to DØ.” Since returning to SLAC in teacher and retreated into his laboratory to work 1989, he has been active in theoretical and experi- alone, sharing nothing with colleagues. He mental activities. reemerged six weeks later with his discoveries ready for publication and for the subsequent press conferences. In this issue is featured the particle-physics discovery of the year, that of the top quark. It is at an opposite extreme, a programmed discovery under way for most of a decade at Fermilab, fea- turing two competing experimental groups, the CDF and DØ collaborations, each consisting of 400 to 500 physicists, plus countless engineers, technicians, and support personnel, especially the many members of the Fermilab Accelerator Division who provide the essential ingredient of the colliding beams. This article is written by the spokespersons of the two competing collaborations and tells much of how scientiﬁc discovery is done in such an extreme social environment. Each collaboration is a community in itself, with three branches of governance, robust bureaucracy and organization charts, and a considerable level of specialization. It happens that over the last year or two I have spent a lot of time at Fermilab, much of it precisely halfway between these behemoth detectors. So I watched with interest and bemuse- ment the excitement of the chase, along with the frustrations, the impatience, the fatigue, the

2 FALL 1995 anxieties, and some of the inevitable politics. I purposes; I have never seen this competitiveness witnessed the awkward ﬁrst stage of CDF evi- to be corrosive. The evidence is in these pages for dence, when four short draft papers were by com- the reader to see, in the very fact of co- promise merged into a very long paper. I even authorship and in the nature of the interactions promised CDF collaborators to read the thing and between the collaborations as described in the ar- did, carefully, beginning to end, and found it a ticle. This piece of competition has been a class classic—a masterful exhibit of how science re- act. sults should (but seldom do) get reported. Not only has this been true between the col- Throughout I maintained a somewhat dour pos- laborations, but it seems also to have been the ture toward those CDF/DØ crowds, baiting them case within them. This is no mean feat, since regarding their obsessive pursuit of top to the ap- harmony within a big group of strong individual- parent neglect of all else (not quite true of istic physicists of great talent and often even course), including that hidden piece of New greater ego is not easy to maintain. I can do no Physics Beyond the Standard Model lurking un- better than quote here what is found near the end detected in their data. But when the top quark re- of the article, and I do this without regrets for sults eventually bubbled up to the surface, I creating some redundancy: could not do anything but be bouyed by them as well, joyful in the occasion and deeply respectful In the end, the chief necessity for the conver- of the magnitude of the accomplishment. gence on the top discovery was the willingness of a collaboration to abide by a majority view. The history of physics is full of near- Securing this willingness requires extensive at- simultaneous discoveries by separate individuals tention to the process—of being sure that all or groups, and with that often has come acrimo- shades of opinion, reservations, and alternate ny and controversy, from Newton and Leibnitz to viewpoints are fully heard and understood. It is Richter and Ting, and down to the present time. more important perhaps that each point of view There has been competition between CDF and is carefully listened to than that it be heeded. A ﬁne line in resolving these viewpoints must DØ as well. In fact, it was built in from the be- be drawn between autocracy and grass-roots ginning by then-director Leon Lederman, who democracy. The process must have the confi- visited CERN’s big collaborations, UA1 and UA2, dence of the collaboration, or its general while they were discovering intermediate bosons effectiveness can diminish rapidly. W and Z and searching for the top quark. At CERN, it was vital to have two collaborations as These are not mere words, but an account of checks and balances, and Lederman upon his re- successful actions. In this increasingly fractious turn strongly encouraged the creation of the pre- world of ours, it should be read and taken to sent DØ collaboration, something which was not heart by all those who despair of progress being in the works prior to that. And the ensuing made through reasoning and consensus. CDF/DØ competition has served for constructive James Bjorken

BEAMLINE3 DISCOVERY of the TOP QUARK by BILL CARITHERS & PAUL GRANNIS

Two leaders of the Fermilab experiments that isolated the top quark tell the adventure of its discovery. s e c i v r e S a i d e M l a u s i V b a l i m r e F

4 FALL 1995 ANKIND has sought the elementary building blocks of matter ever since the days of the Greek philosophers. Over Mtime, the quest has been successively reﬁned from the original notion of indivisible “atoms” as the fundamental elements to the present idea that objects called quarks lie at the heart of all matter. So the recent news from Fermilab that the sixth—and possibly the last—of these quarks has ﬁnally been found may signal the end of one of our longest searches. s e c i v r e S a i d e M l a u s i V b a l i m r e F

BEAMLINE5 But the properties of this funda- combinations of three quarks or a CDF Collaboration mental constituent of matter are quark-antiquark pair. Quarks also bizarre and raise new questions. In seemed to form a counterpart to the Argonne National Laboratory particular, the mass of the top quark other class of elementary particles, Istituto Nazionale di Fisica Nucleare, is about forty times that of any the leptons, which then included the University of Bologna Brandeis University other—a fact which suggests that per- electron (e) and muon (µ) (both with University of California at Los Angeles haps it plays a fundamental role in unit charge) and their companion University of Chicago the question of how the mass of any chargeless neutrinos, νe and νµ. The Duke University object arises. leptons do not feel the strong inter- Fermi National Accelerator Laboratory action, but they do participate in the Laboratori Nazionali di Frascati, Istituto N 1964 Murray Gell-Mann and electromagnetic interactions and the Nazionale di Fisica Nucleare George Zweig proposed the quark weak interaction responsible for ra- Harvard University Hiroshima University Ihypothesis to account for the ex- dioactive decays. They have the University of Illinois plosion of subatomic particles dis- same spin as the quarks and also Institute of Particle Physics, McGill covered in accelerator and cosmic- have no discernible size or internal University and University of Toronto ray experiments during the 1950s and structure. The Johns Hopkins University early 1960s. Over a hundred new par- But most physicists were initially National Laboratory for High Energy ticles, most of them strongly inter- reluctant to believe that quarks were Physics (KEK) Lawrence Berkeley Laboratory acting and very short-lived, had been anything more than convenient ab- Massachusetts Institute of Technology observed. These particles, called stractions aiding particle classiﬁca- University of Michigan hadrons, are not elementary; they tion. The fractional electric charges Michigan State University possess a definite size and internal seemed bizarre, and experiments re- University of New Mexico structure, and most can be trans- peatedly failed to turn up any indi- Osaka City University formed from one state into another. vidual free quarks. And—as became Università di Padova, Instituto Nazionale di Fisica Nucleare The quark hypothesis suggested that apparent from studies of fundamen- University of Pennsylvania different combinations of three tal theories of quarks and leptons— University of Pittsburgh quarks—the up (u), down (d), and major conceptual problems arise if Istituto Nazionale di Fisica Nucleare, strange (s) quarks—and their an- the numbers of quarks and leptons University and Scuola Normale tiparticles could account for all of the are not the same. Superiore of Pisa hadrons then known. Each quark has Two major developments estab- Purdue University an intrinsic spin of 1/2 unit and is lished the reality of quarks during University of Rochester Rockefeller University presumed to be elementary, like the the 1970s. Fixed-target experiments Rutgers University electron. So far, quarks appear to directing high energy leptons at pro- Academia Sinica have no size or internal structure and tons and neutrons showed that these Texas A&M University thus represent the smallest known hadrons contain point-like internal Texas Tech University constituents of matter. To explain constituents whose charges and University of Tsukuba the observed spectrum of hadrons, spins are just what the quark mod- Tufts University University of Wisconsin quarks had to have electric charges el had predicted. And in 1974 ex- Yale University that are fractions of the electron periments at Brookhaven National charge. The u quark has charge 2/3 Laboratory in New York and Stan- while the d and s quarks have charges ford Linear Accelerator Center −1/3 (in units where the electron (SLAC) in California discovered a charge is −1). striking new hadron at the then very The observed hadron spectrum large mass of 3.1 GeV—over three agreed remarkably well with the times that of the proton. This hadron expected states formed from (called the J/ψ after its separate

6 FALL 1995 Electric First Second Third Charge Family Family Family DØ Collaboration +2/3 up (u) charm (c) top (t) QUARKS Universidad de los Andes -1/3 down (d) strange (s) bottom (b) University of Arizona -1 electron (e) muon (µ) tau (τ) Brookhaven National Laboratory LEPTONS 0 electron neutrino (νe)muon neutrino (νµ) tau neutrino (ντ) Brown University University of California, Davis names in the two experiments) was chart above). Ordinary matter is com- University of California, Irvine found to be a bound state of a new posed entirely of ﬁrst-generation par- University of California, Riverside kind of quark, called char m or c, with ticles, namely the u and d quarks, LAFEX, Centro Brasileiro de Pesquisas its antiquark. The c quark has a plus the electron and its neutrino. Físicas much greater mass than the first But the third-generation quark dou- Centro de Investigacion y de Estudios Avanzados three, and its charge is 2/3. With two blet seemed to be missing its charge Columbia University quarks of each possible charge, a +2/3 member, whose existence was Delhi University symmetry could be established be- inferred from the existing pattern. In Fermi National Accelerator Laboratory tween the quarks and the leptons. advance of its sighting, physicists Florida State University Two pairs of each were then known: named it the top (t) quark. Thus be- University of Hawaii (u,d) and (c,s) for quarks and (e,ν ) and gan a search that lasted almost University of Illinois, Chicago e Indiana University (µ, νµ) for leptons, satisfying theo- twenty years. Iowa State University retical constraints. Korea University But this symmetry was quickly SING THE RATIOS of the ob- Kyungsung University broken by unexpected discoveries. In served quark masses, some Lawrence Berkeley Laboratory 1976 experiments at SLAC turned up U physicists naively suggested University of Maryland a third charged lepton, the tau lep- that the t might be about three times University of Michigan Michigan State University ton or τ. A year later at Fermi as heavy as the b, and thus expect- Moscow State University National Accelerator Laboratory in ed that the top would appear as a – University of Nebraska Illinois a new hadron was discovered heavy new hadron containing a tt New York University called the upsilon or ϒ, at the huge pair, at a mass around 30 GeV. The Northeastern University mass of about 10 GeV; like theJ/ψ, it electron-positron colliders then un- Northern Illinois University was soon found to be the bound state der construction (PEP at SLAC and Northwestern University University of Notre Dame of yet another new quark—the bot- PETRA at DESY) raced to capture the University of Panjab tom or b quark—and its antiparticle. prize, but they found no hint of the Institute for High Energy Physics, Experiments at DESY in Germany top quark. Protvino and Cornell in New York showed In the early 1980s a new class of Purdue University that the b quark has spin 1/2 and a accelerator came into operation at Rice University charge of −1/3, just like the d and s CERN in Switzerland, in which University of Rochester quarks. counter-rotating beams of protons Commissariat à l’Energie Atomique, Saclay With these discoveries, and and antiprotons collided with an en- Seoul National University through the development of the Stan- ergy of about 600 GeV. The protons State University of New York, dard Model, physicists now under- and antiprotons brought their con- Stony Brook stood that matter comes in two stituent quarks and antiquarks into Superconducting Super Collider parallel but distinct classes—quarks collision with typical energies of 50 Laboratory and leptons. They occur in “gener- to 100 GeV, so the top quark search Tata Institute of Fundamental Research University of Texas, Arlington ations” of two related pairs with dif- could be extended considerably. Be- Texas A&M University fering electric charge—(+2/3, −1/3) sides the important discovery of the for quarks and (−1, 0) for leptons (see W and Z bosons that act as carriers

BEAMLINE7 of the uniﬁed electroweak force, the rely on the production of separate t Jet Production CERN experiments demonstrated an- and t¯ quarks from annihilation of in- other aspect of quarks. Though coming quarks and antiquarks in the

π+ quarks had continued to elude direct proton and antiproton, with subse- u – detection, they can be violently scat- quent decays into observable parti- d d π– tered in high energy collisions. The cles (see box on the right). The design u– Jet high energy quarks emerging from of DØ stressed recognition of the tell- + u K the collision region are subject to the tale leptons and jets over as large a s– strong interaction as they leave the solid angle as possible. Meanwhile p s K– – scene of the collision, creating ad- CDF had installed a new vertex de- u d p– d ditional quark-antiquark pairs from tector of silicon microstrips near the u u– g the available collision energy (us- beams intended to detect short-lived d u– 2 – ing E = mc ). The quarks and anti- particles that survive long enough to d quarks so created combine into travel a millimeter or so from the in- ordinary hadrons that the experiment teraction point. This detector was COLLIDING PROTON (p) and can detect. These hadrons tend to particularly good at sensing the pres- − antiproton (p ) bring together cluster along the direction of the orig- ence of the b quarks characteristic of quarks uud and uud. In this inal quark, and are thus recorded as top decay. Another method of tagging A − example, a u and u scatter by a “jet” of rather collinear particles. b quarks, by detecting their decays exchanging a gluon (g), the carrier Such quark jets, previously sensed at into energetic leptons, was used by of the strong nuclear force. Like all SLAC and DESY, were clearly ob- both experiments. Thus the two ex- quarks, the scattered u quark pos- served at CERN and became a key in- periments, while searching for the sesses the strong interaction “charge” called color. The strong gredient in the next round of top same basic decay sequence, had attraction of the u quark color to the quark searches. rather complementary approaches. other quark color charges prevents With the advent of the CERN col- With the data from this 1992–93 it from escaping freely. Energy from lider, and in 1988 the more powerful run, progress accelerated. First, DØ the collision region is converted to 1800 GeV collider at Fermilab, the published a new lower limit of matter in the form of quark- search for the top quark turned to 131 GeV on the possible top mass antiquark pairs. The antiquark new avenues. At the large masses from the absence of events with the (with anti-color) joins with a quark – to produce a colorless meson which now accessible, the tt bound state characteristic dilepton or single lep- is free to escape. was unlikely to form and isolated top ton signatures. This paper turned out In this example, the primary quarks were expected. For masses be- to be the end of the line in excluding scattered u quark is accompanied low that of the W boson, W decay top masses. Up to then, with the − − − − – by (dd ), (uu ), (ss ), and (dd ) crea- into a t and b¯ could predominate. absence of an excess of candidate ted pairs, leading to the formation of Some indication of this process was events, the analyses had tended to re- π+, π−, K +, and K − mesons, all trav- reported in 1984 by the CERN UA1 strict the event sample, so as to set eling in similar directions and forming a “jet” of particles that may be experiment, but it was later ruled out as high a limit on the mass as pos- detected in the experiment. (The re- by the CERN UA2 and Fermilab CDF sible. But by summer of 1993, CDF maining d quark will be joined to experiments. By 1990 CDF had ex- noticed that the top mass limits were one of the other spectator quarks in tended the top mass limit to 91 GeV, not improving with additional data, the collision.) thus eliminating the possibility for due to a growing handful of events W decay to top. that passed all preassigned criteria. Naive (in retrospect) estimates of N 1992, the DØ detector joined their statistical signiﬁcance triggered CDF as a long Tevatron run began. intense activity to review the results IFurther searches would have to and prepare a draft publication with

8 FALL 1995 Top Production and Decay

ORTOPQUARK MASSES above MW + Mb, top production Fproceeds mainly through annihilation of incoming quarks and antiquarks from the beam particles.

q t CDF the goal of publishing the results by , a few physicists felt that no gluon the October 1993 pp− Workshop in publication should occur until the

Japan. As the analysis was too com- collaboration consensus was for “ob- – – plex for a single journal letter, CDF servation.” The other end of the spec- q t planned a series of four papers in trum felt that the data were already − Physical Review Letters (PRL)—two in that category. Finally, CDF settled The q and q fuse briefly into a gluon, devoted to the counting experiments on a conservative interpretation and the carrier of the strong force, and − in the two search topologies; one de- was careful to disclose even those re- then rematerialize as t and t quarks scribing the kinematics of the events sults that contra-indicated top. traveling in roughly opposite direc- and giving an estimate of the top The pressure within CDF to pub- tions. The top quarks have a very −24 mass; and a fourth to bring it all to- lish increased enormously as the short lifetime (about 10 seconds) and decay almost always into a W gether with the overall significance, winter 1994 conferences approached. boson and a b quark (t → W +b ; − − conclusions, and top production In addition, pirated copies of the t → W −b). The b quarks evolve into cross section. In a heated argument Physical Review D manuscript were jets seen in the detectors. The W + at the October collaboration meet- circulating widely and rumors (some and W − have several decay possi- ing, it became clear both that the astoundingly accurate and others bilities: mass and kinematics sections need- comically off-base) were flying about ± ± ed more work and that the “four the globe. The rumor mill churned W → e ν (fraction = 1/9) → µ±ν (fraction = 1/9) PRL” format was not working well. once again when CDF withdrew its → τ±ν (fraction = 1/9) CDF decided instead to prepare a sin- top talks at the LaThuile and → qq− (fraction = 2/3). gle long Physical Review article, to Moriond Conferences. An April CDF − concentrate on the remaining holes Collaboration meeting was the wa- The q and q from W decays appear in the analysis, and to forego any pub- tershed event, as the group ham- as jets. The final states containing lic discussion of new results. mered out the final wordings. After τ’s are difficult to isolate and were By the next CDF collaboration a virtually unanimous agreement on not sought in the experiments. The meeting in late January, the count- the final draft, the collaboration final state with both W ’s decaying into qq−, though relatively copious, ing experiments were complete, the spokespersons finally notified Fer- are buried in huge strong interaction mass and kinematics analyses had milab Director John Peoples of their backgrounds. The remaining de- made real progress, and the single intent to publish. cays, studied by both CDF and DØ, long draft was in reasonable shape. This CDF analysis found 12 can- can be categorized by the number The big remaining issues were the didate events that could not be eas- leptons (l = e or µ) from the W de- wordings of the title and conclusions ily explained on the basis of known cays: Dilepton channel: section and the “between the lines” backgrounds (which were predicted − − − tt →W +bW −b → l +l −ννbb message that they sent to the com- to be about six events). The odds for Single lepton channel: − − − − munity. Traditionally, scientific pub- background fluctuations to yield the tt →W +bW −b → l+νqq bb − − − lications about new phenomena use observed events were low—about 1 or l −ν qq bb certain key phrases: “Observation in 400—but not small enough to war- of” indicates a discovery when the rant the conclusions that the top had The dilepton channels are relatively case is unassailable; “Evidence for” been found. Under the hypothesis free from background but comprise − − means the authors believe they are that the data did include some tt only 5 percent of all tt decays. The seeing a signal but the case is not events, the mass of the top was es- single lepton channels give larger yields (30 percent of all decays) but iron-clad; and “Search for” implies timated to be 175 GeV±20 GeV. The − have substantial backgrounds from the evidence is weak or non-existent. cross section for tt production was W + jets production processes which Within every collaboration, there is determined to be about 13.9 pico- must be reduced. a wide spectrum of comfort levels in barns, larger than the theoretical pre- claiming a new result. On one end of diction of 3–7 picobarns.

BEAMLINE9 DØ used the period while CDF was tuning each of the separate elements Top Mass Distribution preparing its publication to re- of the process, but until summer 6 optimize its own earlier analysis, 1994 the intensity of the collider was DO focusing on higher mass top. By April disappointing. During a brief mid- ) 2 1994, DØ was also in a position to summer break, however, one of the c /

V 4 show some of its new results, which Tevatron magnets was found to have e

G were subsequently updated in the been inadvertently rotated. With this 0 2 (

/ summer Glasgow conference and problem ﬁxed, beam intensities rose s t

n 2 published in Physical Review Let- immediately by a factor of 2. With e v

E ters. DØ also had a small excess of the now good understanding of the top-like events but with smaller sta- accelerator, a further doubling of the 0 tistical signiﬁcance; its analysis was event rate was accomplished by 100 200 based upon the dilepton and single- spring 1995. In a very real sense, the Fitted Mass (GeV/c2) lepton channels with a combination final success of CDF and DØ in dis- 5 of lepton tags and topological sup- covering top rested upon the superb CDF pression techniques to reduce back- achievements of the Fermilab Ac-

) 4 2 c

/ ground. Nine events were observed, celerator Division. The improved op- V e compared with an expected back- erations meant that the data samples

G 3 0

1 ground of about four, giving odds for accumulated by early 1995 were ap- ( / s

t 2 a background fluctuation of about proximately three times larger than n e

v 1 in 40. The excess events corre- those used in the previous analyses, E 1 sponded to a cross section of 8.2±5.1 and both experiments were now − picobarns. The expected yield of tt poised to capitalize on the increase. 0 80 120 160 200 240 280 events was virtually the same for DØ By December, both collaborations Reconstructed Mass (GeV/c2) and CDF. Taken together, these re- realized that the data now on tape CDF DØ The mass of the top quark can be sults from and were not suf- should be enough for a discovery, if reconstructed from the energies and ficient to establish conclusively the the earlier event excess had been ap- directions of its decay products as existence of the top quark. proximately correct. In fact, the ex- measured in the detectors using the The final chapter in finding the periments do not keep daily tallies conservation laws for energy and momentum. Since the top quark has top quark began with the resumption of the nu mber of events in their sam- a unique mass, the data (indicated of the collider run in late summer ples. The physicists prefer to refine by the black histograms) should 1994. The performance of the Teva- their analysis techniques and selec- show a “peak” in the reconstructed tron was the key to the success. The tion parameters in order to optimize distribution. The non-top background (the red dashed curve for Tevatron involves a collection of sev- the analysis on simulated events be- DØ and the red dotted curve for en separate accelerators with a com- fore “peeking” at the data. This ret- CDF) has very different shapes. A plex web of connecting beam lines. icence to check too often on the real simulation is required to provide the Many technical gymnastics are re- data stems from the desire to avoid correspondence between the measured jet energies and the parent quired to accelerate protons, produce biasing the analysis by the idiosyn- quark momenta. The red dotted secondary beams of antiprotons from crasies of the few events actually curve for DØ shows the expected an external target, accumulate and found. At the beginning of Jan uary, contribution from top for the best fit store the intense antiproton beams, DØ showed a partial update in the value of the top mass. The solid red curve shows what a simulated top and finally inject the counter-rotat- Aspen Conference using some new quark mass distribution would look ing beams of protons and antiprotons data but retaining previous selection like when added to the background, into the Tevatron for acceleration to criteria; these results had increased and these curves should be com- 900 GeV. Enormous effort had been significance, with only a 1 in 150 pared to the actual data. poured into understanding and chance of background fluctuations.

10 FALL 1995 Its simulations showed that for the before. (Los Angeles Times reporter, large-mass top now being sought, K. C. Cole, called a distinguished Fer- World Wide Web Statistics even better signiﬁcance could be ob- milab physicist to get an explanation tained. Recognizing that the statis- of statistical evidence presented in OTH THE CDF and DØ World tics were nearly sufficient, the col- the O.J. Simpson trial. The physicist Wide Web pages can be laboration began working in high used, as illustration, “the recent sta- Baccessed from the Fermilab gear to finalize the selection crite- tistical evidence on the top quark home page at http://www.fnal.gov/ ria and obtain the last slug of data be- from CDF and DØ,” and Cole swift- (click there for top quark discovery). fore a late-January Tevatron shut- ly picked up the chase.) Since the announcement of the dis- down. In mid-January, CDF updated In its paper, CDF reported ﬁnding covery of the top quark last March, all of its data; with all of the pieces six dilepton events plus 43 single- the CDF and DØ home pages have assembled side by side, it became lepton events (see box on the next received over 100,000 hits each. clear that the collaboration had the page for more details); it concluded There were 2279 hits on the CDF necessary significance in hand for that the odds were only one in a mil- page alone on the day after the claiming a discovery. The CDF lion that background fluctuations announcement. Six months after the process was now considerably could account for these events. DØ, discovery, the Fermilab top quark streamlined by the experience of pro- in its paper, observed three dilepton page is receiving about 200 hits a ducing the earlier top quark papers. events plus 14 single-lepton events day. By February, both CDFand DØ rec- and concluded that the odds were ognized that convergence was im- two in a million that these could minent. Both worked around the have been caused by backgrounds. clock to complete all aspects of the The top quark masses reported by the analyses. Even though each collab- two experiments were 176±13 GeV oration knew that the other was clos- for CDF and 199±30 GeV for DØ. And ing fast and in the process of writing both experiments gave consistent re- its paper, there were still no formal sults, although somewhat below − exchanges of results, nor of intend- CDF’s earlier value for the tt pro- ed timing. Some copies did leak duction cross section (see box on the out—for example, drafts left on print- next page for more detailed infor- ers in institutions with physicists on mation). both collaborations and computer The top quark appears to be a hacking from one collaboration to point-like particle; it has no internal another. The one avenue of cross- structure that we can discern. As one fertilization that seems not to have of the six fundamental constituents operated was through couples with of matter, it has properties very sim- one person in each group! ilar to the up and charm quarks, with On February 24, CDF and DØ the exception of its remarkable mas- “Observation” papers were submit- siveness and its very short lifetime. ted to Physical Review Letters(PRL); The top quark is about 200 times public seminars were scheduled at more massive than the proton, about Fermilab for March 2. By agreement, forty times that of the second heav- the news of the discovery was not to iest quark (the b), and roughly the be made available to the physics same as the entire mass of the gold community or news media until the nucleus! Surely this striking obesity day of the seminars. In spite of all ef- holds an important clue about how forts, word did leak out a few days mass originates.

BEAM LINE 11 CDF AND DØ RESULTS

HE RESULTS FROM THE TWO COLLABORATIONS were remarkably similar. CDF found 6 dilepton events Jet #2 CDF Twith a background of 1.3; 21 single-lepton events in Jet #3 which 27 cases of a b quark tag by the vertex detector (with 6.7 background tags expected); and 22 single-lepton events with 23 cases of a b tag through leptonic decay (with 15.4 background tags expected). DØ found 3 dilepton events (0.65 background events); 8 single-lepton events Jet #1 with topological tagging (1.9 background events); and 6 single-lepton events with b-to-lepton tags (1.2 background events). A particularly striking example of a dilepton event Jet #4 with very energetic electron, muon, and missing ET (due to the neutrinos), plus two jets, is shown below from the DØ + data. The plot shows the detector unfolded on to a plane, e with the energy of the various objects indicated by the height of the bars. This event has a very low probability to be explained by any known background. The probability 3 meters that background ﬂuctuations could explain the observed signal was one-in-a-million for CDF and two-in-a-million for DØ—sufﬁciently solid that each experiment was able to claim the observation of the top independently.

Missing ET 100 ) V e

Electron Muon G 10 ( Primary

T Secondary

JET E Vertex 1 Vertex JET .7 –3

0 5 millimeters 6.4 η by the need to identify the correct combination of jets with 3.2 7 3. parent quarks in the decay and to accommodate the ten- φ 0.1 dency of the strong interaction to generate additional jets. Additional studies helped to establish that the new signal The two experiments obtained consistent results for this was indeed the top quark. Both experiments were able to mass measurement: 176±13 GeV for CDF and 199±30 GeV show that the candidate events were consistent with the for DØ. − expected presence of b quarks. The single-lepton channel The rate for observing tt pairs is controlled by the strong- − events should have one W boson that decays to a qq final interaction production cross section. Each experiment evalu- state, and the data showed the expected enhancement in ated this cross section at their mean mass value; CDF +3.6 the di-jet mass. obtained 6.8-2.4 picobarns, and DØ obtained 6.4±2.2 pico- Finally, both experiments made a measurement of the barns. These values are consistent with the theoretical pre- top quark mass, using the single-lepton events. In this diction within the combined experimental and theoretical study, the missing neutrino from one W decay must be error, although the tendency for the measurements to fall inferred and the mass of the parent top quark deduced from above theory is interesting and if it persists could indicate a constrained-fit procedure. This mass fitting is complicated that new physics is at play.

12 FALL 1995 ODERN HIGH energy physics experiments are unique M institutions; though large enough to require formal organizations and governance procedures,

they remain voluntary collaborations s e c i v of individual scientists who have r e S a banded together to permit achieve- i d e ments that a few physicists could not M l a u s

accomplish by themselves. But lack- i V b

ing the usual structures of large a l i m r

organizations—the ability to hire and e ﬁre, or to rigorously assign jobs to in- F dividual workers—they rely to a very must be given to optimizing the Left to right: Giorgio Bellettini, Paul large degree on the existence of a event selection process (only ten in- Grannis, John Peoples, Hugh shared purpose and consensus for teractions out of every million can Montgomery, and Bill Carithers following their constructive operation. be kept for analysis) and to improv- the seminars announcing the simulta- Although about 430 physicists ing the software algorithms for se- neous discovery of the top quark at Fermilab by the DØ and CDF collabora- participate in each collaboration, the lecting particles in the data. Work tions. Grannis and Montgomery are the number active in the top studies is must continue to improve the over- co-spokesmen for the DØ Collaboration, much less—roughly 50 directly en- all computing and software environ- John Peoples is the Director of Fermilab, gaged at a significant level. This ment. New detector development and Carithers and Bellettini are the co- results from the broad scope of col- is conducted with an eye to future spokesmen for the CDF Collaboration. lider experiments; the range of tasks upgrades, and major design and build- The collaborations presented their needing attention is very large. The ing projects are underway for future results at seminars held at Fermilab on March 2, 1995. physics research being done by the runs. The experiments are so big that collaborations is widely dispersed reviews by the collaborations them- over five major areas (top search, selves, the Laboratory, and external bottom-quark physics, strong- committees continually occur. interaction studies, electroweak bo- Finally, with collaborations of these son measurements, and searches for sizes, group psychologists, ﬁnancial new phenomena). Within these analysts, and tea-leaf readers are physics groups there are perhaps 50 needed! active analyses under study at any The techniques for the top quark given time. Often these other analy- search and measurement of its mass ses contributed synergistic tech- developed over several years. For niques to aid the top search; for ex- most important aspects (and some ample, studies of jets by the minor ones) several different ap- strong-interaction group entered proaches were developed by rival sub- directly into measurements of the groups or individuals. This multi- top mass. plicity is valuable for cross-checking A diversity of tasks occupy the results and as a tool for natural se- members of the collaborations. In ad- lection of the strongest methods, but dition to running the experiment such competitiveness and rivalry can around the clock and attending to its also be debilitating. In some cases, maintenance, constant attention the winner in these rivalries can be

BEAM LINE 13 established on purely technical for internal use, proliferate and are felt the heat of competition; for each, grounds—but often the basis for de- disseminated electronically across no doubt, the dream of being first cision is clouded, as the alternatives the globe. Electronic mail is an es- to recognize the top was a powerful have both pros and cons. Adjudicat- sential component; colleagues work- incentive to be rather closed- ing among these contenders is a dif- ing a continent apart on related prob- mouthed about innovations and ficult and time-consuming problem. lems can conduct an almost on-line progress. In some cases, the choice is made dialog. Discussion and argument over The history of the top search over through a survival-of-the-fittest fine points of interpretation can gen- the final two years shows evidence process—the approach first brought erate a flurry of productive messages of this competitiveness and friend- to full maturity, with documented among five or six experts. At the oth- ly rivalry at work. As the more se- error analyses and cross-checks, is er end of the scale, there remains an nior collaboration, CDF had devel- chosen. Consensus plays an impor- essential role for formal presenta- oped its techniques over time to a tant role in rejecting some alter- tions of work to the full physics relatively refined state. It started the natives; experts not involved in a group and full collaboration at reg- race for top with the UA2 experiment particular question can wield con- ular intervals. Repetition is usually in 1988 and had been denied success siderable influence in rejecting some necessary to bring the non-experts to only by top’s extraordinary mas- weaker approaches. When the issues a reasonable degree of familiarity siveness. CDF would have been dis- have sufficiently crystallized, special with the issues and methods, and appointed to lose the race to the new- advisory panels drawn from the rest to build acceptance of new work. er DØ experiment. On the other of the collaboration can be very use- To what extent did CDF and DØ hand, the Tevatron performance in ful in helping to define the direction. interact during the time of the top the 1992–1994 era improved so much The last resort of turning the issue search and discovery? At a formal that the data in hand was almost all over to the “managers”—the spokes- level, very little. New results from acquired after DØ’s start-up, so the persons or physics group conveners— each collaboration were periodically two experiments could work on al- is successful primarily when the shown at conferences and public pre- most equivalent data sets. DØ, with rivals have agreed that they are un- sentations, so the broad outlines of its strengths of excellent lepton iden- able to resolve their differences. For- each group’s approach were known, tification and full solid angle cover- tunately, this rarely happens. but beyond that there were no direct age, was able to develop powerful Effective communication on interactions. The grapevine was of tools for suppressing unwanted back- many levels is an essential ingredi- course active, with lunch-table con- ground; it was of course determined ent of successful performance in the versations among individuals, but to make its mark as the brash new- large collaborations. For the top much of the information exchanged comer by converging on the top. But studies, weekly meetings of a half in this way was the anecdotal ac- the competition remained friendly dozen subgroups of the top analysis count of the day and was often out- and supportive throughout; it also discuss problems and status; as many moded by the next. There were prob- promoted a quality of work and depth additional meetings occur on relat- ably several reasons for this relative of understanding that would have ed analysis topics such as electron isolation of two groups working to- been difficult otherwise. And the ul- identification; and a more general ward a common goal. At one level timate scientific value of two inde- meeting brings together all the both groups realized that the independent results was enormous. threads. Video conferencing and elec- pendence had real scientific value. How do hundreds of authors agree tronically posted minutes of the Guarding against making a discov- on a single paper in a timely fashion? meetings help keep those not at Fer- ery by subconsciously shaping the This too is a sociological challenge! milab involved, although most ac- analysis is an ever-present worry; by The process for writing the final pa- tive participants travel there fre- keeping the walls between CDF and pers began in late January. DØ ap- quently. Informal documents pre- DØ fairly high, this possibility could pointed primary authors—one for a pared on sub-topics, intended only be minimized. Also, the two groups short letter paper and one for a longer

14 FALL 1995 paper ultimately not used. The top group designated a few individuals as its authorship committee, bring- ing the necessary range of expertise to the authors. The collaboration review was conducted in large part through an appointed editorial board of ten physicists including authors, drawn from widely diverse portions of the collaboration. The editorial

board spent days carefully reviewing s e c i v the backup documentation, probing r e S a the analysis for consistency and i d e

correctness. It also helped to revise M l a u s

the paper’s language to make it ac- i V b

cessible to the widest possible au- a l i m r

dience. The completed draft was dis- e tributed to the full collaboration by F electronic mail, and comments were the basic experimental numbers. Happy and relieved physicists who have solicited from all. Well over a hun- Others favored a more interpretative completed PhD theses using the DØ dred written comments were re- approach. Finalization of the paper experiment assemble in the DØ main control ceived (plus many more verbal ones), itself required the resolution of these room. Students and postdoctoral candidates are the lifeblood of the experiment and have all of which were addressed by the oft-conflicting outlooks on what made contributions to every aspect of it. board. The revised paper was again makes good science. Pictured are Brad Abbott, Purdue; Jeff Bantly, circulated for concurrence of the col- In the end, the chief necessity for Srini Rajagopalan, Northwestern; Dhiman laboration before submission to PRL. the convergence on the top discov- Chakraborty, Terry Heuring, Jim Cochran, Greg CDF appointed a chief author to be ery was the willingness of a collab- Landsberg, Marc Paterno, Scott Snyder, Joey assisted by a group advising on the oration to abide by a majority view. Thompson, Jaehoon Yu, Stony Brook; Regina sections of their expertise. Simulta- Securing this willingness requires ex- Demina, Northeastern; Daniel Elvira, Cecilia neously, a review committee (dubbed tensive attention to the process—of Gerber, University of Buenos Aires; Bob Hirosky, Gordon Watts (CDF), Rochester; Jon Kotcher, a “godparent” committee within being sure that all shades of opinion, New York University; Brent May, Arizona; Doug CDF) was appointed. The draft and reservations, and alternate view- Norman, Maryland; Myungyun Pang, Iowa State. responses to questions were always points are fully heard and under- Others who have completed theses on accessible to the collaboration elec- stood. It is more important perhaps DØ but not pictured are Rich Astur, Bo Pi, Sal tronically, both on the World Wide that each point of view is carefully Fahey, Michigan State; Balamurali V., Notre Dame; Web (with security) and via ordinary listened to than that it be heeded. Ties Behnke, John Jiang, Domenic Pizzuto, Paul files on the Fermilab computer. A fine line in resolving these view- Rubinov, Stony Brook; John Borders, Sarah Durston, Rochester; Geary Eppley, Rice; Fabrice In the final convergence to a re- points must be drawn between autoc- Feinstein, Alain Pluquet, University of Paris Sud; sult, it was necessary to come to grips racy and grass-roots democracy. The Terry Geld, Michigan; Mark Goforth, Robert with rather large differences of sci- process must have the confidence of Madden, Florida State; Ray Hall, Thorsten Huehn, entific style among the collaborators. the collaboration, or its general University of California, Riverside; Hossain Johari, Some could find in any result, no effectiveness can diminish rapidly. Northeastern; Guilherme Lima, LAFEX/CBPF; matter how thoroughly cross- What did the physicists of CDF Andrew Milder, Alex Smith, Arizona; Chris Murphy, checked, the need for further inves- and DØ feel on completion of the top Indiana; Haowei Xu, Brown; Qiang Zhu, New York University. tigation. Some were wary of any discovery experiments? Certainly re- interpretation beyond a statement of lief that the quest was complete. Also

BEAM LINE 15 pride that such an elusive quarry had celebrations, but by a week after the been trapped and that the two inde- public announcements it was back pendent experiments had drawn so to business, both on the top studies similar a profile of the top quark. Vir- and on the dozens of other analyses tually all in both collaborations could in progress. take justifiable pride for their per- sonal contributions to the discov- OES THE DISC OVERY of the ery—an effective electronics con- top quark close the chapter troller for the high-voltage system D on our understanding of the was as essential for the discovery as fundamental building blocks of mat- devising the selection criteria for the ter? Surely not—it is truly just the top quark events. Despite the many beginning. Though we now have strains introduced by the frenetic found the long-awaited sixth quark, pace, the process of discovering the with properties as predicted, much top was for most the scientific event more accurate measurements are of a lifetime. Graduate students, post- needed. Dozens of questions about docs, junior and senior faculty, and how the top is produced and how it scientists worked together on equal decays are clamoring for answers. footing, with the contributions of the Does the extraordinarily large top youngest as significant as those from quark mass hold clues to the nature the oldsters. Though for most scien- of symmetry breaking or to the ori- tists, solving the many small daily gin of mass? Do the regularities ob- problems gives sufficient satisfaction served for the lighter quarks hold for to keep them hooked on physics, the top? Are there signs of new par- reaching a milestone touted as a ma- ticles that decay into the top? Or are jor discovery was a special thrill not there exotic new objects to be found to be forgotten. in top decays? The very massive top After the public announcements is unique among quarks in that it of the discovery, a new challenge does not live long enough to be arose in interacting with the news bound into hadrons, so it provides media, representatives of member in- a testing ground for the study of a sin- stitutions, and other scientists to ex- gle “bare” quark. As happens so of- plain and comment on the results. ten in physics, the latest great dis- The media blitz caused strains of its covery should serve as the crucial own. As each physicist had con- tool for the next advance in under- tributed in some major way to the standing the composition of matter. discovery, most wanted to share in the public limelight. In the unfa- miliar world of public exposure, some strains and disappointments emerged, but the opportunity to explain the significance of the work to the general public was for most a rewarding and educaional experience. How long did the euphoria of the discovery last? There were indeed

16 FALL 1995 by JOHN LEARNED & MICHAEL RIORDAN In Water or Ice?

Physicists are building four huge

particle-detector arrays to do high

energy neutrino astronomy.

HE GHOSTLIEST of elementary particles, neutrinos have fascinated particle physicists for half a century and become the objects of intenseT research these past few decades. Because they interact very weakly with matter, these “little neutral ones” also provide scientists a unique way to peer inside stars and supernovae. Thermonuclear processes occurring within the cores of these celestial objects generate profuse streams of energetic neutrinos that easily penetrate the outer layers and travel enormous distances, to be recorded here on Earth by huge underground detectors.

BEAM LINE 17 Now, after twenty years of devel- the intensities and energies of the times as massive as the Sun; they opment, neutrino astronomy is about neutrinos spewing forth from these swallow up stars, gas and dust while to forge on to its next great challenge: objects, we have in essence been able belching back perhaps 10 percent of the study of high energy neutrinos to measure their internal tempera- their feast in wildly transformed from cosmological sources outside tures. And discrepancies between the ways. Such a tiny, massive object was the Milky Way. In four corners of the predicted and observed fluxes of so- recently shown to exist at the core globe, four teams of physicists are lar neutrinos arriving at the Earth’s of the elliptical galaxy M87, which racing to build the first full-fledged surface strongly suggest that we may is practically in our own cosmolog- high energy neutrino telescopes. And be witnessing the results of new and ical backyard. in a remarkable example of interna- fundamentally different physics. (See According to one recent scenario, tional scientific cooperation, these “What Have We Learned About Solar there is a standing shock wave out- teams are collaborating with one Neutrinos?” by John Bahcall, in the side the event horizon of the black another and with other interested Fall/Winter 1994 Beam Line, Vol. 24, hole, a bit like the standing waves in groups on the design of a truly No. 3.) a river that delight white water en- gargantuan neutrino telescope— For at least 20 years, however, thusiasts. Infalling matter carries encompassing a cubic kilometer in many physicists have recognized that magnetic fields, which are drastical- total volume—that they hope to the best place to do neutrino astron- ly compressed and intensified by the complete by early in the following omy is at extremely high energies tremendous force of gravity when the millennium. over a trillion electron volts (1 TeV)— matter slams into this shock wave. the realm of Fermilab energies and Charged particles are accelerated to HEASTROPHYSICAL neutri- beyond (see box on next page). Be- high energies as they ricochet back nos observed thus far by un- cause the probability of a neutrino’s and forth between magnetized re- T derground detectors arrived at interacting grows in proportion to its gions, like a ping pong ball between the Earth with energies of roughly energy, so does the ease with which a pair of converging paddles. Ener- a million electron volts (1 MeV) or we can detect it and determine where getic pions created in this process more. Much lower than this and they it came from. Thus, for neutrino as- rapidly decay into pairs of gamma become essentially impossible to de- tronomers TeV energies are truly the rays or into muons and neutrinos. tect—although they are perhaps the experimenters’ dream. The gamma rays initiate particle most plentiful particles in Nature, They are also the astrophysicists’ showers and generate a hellish pho- even more common than photons. and cosmologists’ dream, too, for TeV ton cloud from which only the neu- Physicists have good reason to be- neutrinos give us the only practical trinos can easily escape, streaming lieve that every single cubic cen- way to observe physical processes oc- away with perhaps half the total en- timeter of the Universe contains over curring at the very center of galax- ergy emitted by this bizarre object. six hundred neutrinos. With an ex- ies—particularly the active galactic This process can explain the in- ceedingly tiny mass equivalent to nuclei (AGN’s) that have lately be- tensities of X rays and ultraviolet ra- just a few electron volts, these ubiq- come a hot research topic. Nowadays diation emitted by some AGN’s, from uitous spooks would constitute the many scientists suspect that the which the flux of high energy neu- dominant portion of all the matter variety of highly energetic cosmic trinos can then be inferred. A num- in the Universe, its much sought- beasts distinguished by their various ber of theorists have now estimated after “dark matter” or “missing peculiar radiations are all in fact just this flux; they agree that a significant mass.” different manifestations of a galaxy (meaning measurable) flux exists up The observation of MeV neutrinos with an AGN, viewed in various per- to about 1–10 PeV (1,000–10,000 has allowed astrophysicists to ex- spectives and at various times in its TeV). Quite apart from the great in- amine the physical processes respon- evolution. Most believe that these terest in using such high energy neu- sible for stellar burning and super- AGN’s are the sites of galaxy-class trinos to observe AGN dynamics, the novae explosions. By determining black holes a million to a billion prospect of detecting them has

18 FALL 1995 naturally spurred particle physicists to build big neutrino telescopes. For Optimum Energies for Neutrino Astronomy the foreseeable future, we have no chance of producing such particles BECAUSE THEIRDETECTABILITY improves steeply with energy, neutrinos of in earthbound accelerators. TeV energies and above have long been viewed as the natural starting point Of course, this is only one partic- for neutrino astronomy. But since the ﬂux of neutrinos falls off with energy, an ular model that may in fact be wrong. optimum must exist. Over the decades we have dreamed Factors favoring higher energies include the increasing neutrino cross sec- of doing high energy neutrino as- tion (their probability of interacting with matter); the increasing range of the tronomy, many promising models muons produced in these interactions (which means the sensitive volume of a have come and gone. But the estab- detector grows with energy); the decreasing angle between such a muon and lished existence of extremely high its parent neutrino (and thus the point source direction, yielding better resolu- energy cosmic rays, with energies up tion); and the increasing signal-to-noise background ratio between astrophysical neutrinos and those produced in Earth’s atmosphere. to a hundred million TeV, means that The last factor is a bit speculative but has pretty good justiﬁcation. These TeV neutrinos must also be plentiful background neutrinos are produced by ordinary charged cosmic rays interact- in the Universe. Given what we ing with air molecules. As the energy increases, a smaller and smaller fraction k now about particle physics, it is in- of the secondary particles produced in these collisions can decay into neutri- conceivable that such tremendous nos before hitting the Earth. Such local, atmospheric neutrinos constitute the energy could be concentrated into background against which point sources of astrophysical neutrinos must be re- photons and protons without a sub- solved. Because the atmospheric neutrino spectrum is expected to fall off more stantial amount of it spilling over steeply than those of many astrophysical neutrino sources, higher energies are into the neutrino sector. clearly preferred. 4 There are a number of more “pro- While all of the above factors favor higher energies as something like E , saic” sources of high energy neutri- they begin to saturate at a point that is typically in the energy region 1–10 TeV. This turns out to be the optimum range to begin doing neutrino astronomy. nos coming from within the Milky The neutrino cross section is given as a function of energy in the graph. It Way galaxy itself. One source is the spans a range from 0.01 TeV (1010 eV) to 106 TeV (1018 eV, or 1 EeV); the shock waves generated by a super- lower end of this range corre- 8 nova, in a mechanism analogous to 10 sponds to the energies of neutri- ) that described above for AGN’s. An- 2 nos produced at earthbound par- m – – – other is the lighthouse-like particle c 6 σ (νe + e → W ) ticle accelerators, while the 8 10 3 beam thought to be emanating from – upper end approaches the high- 0 1 its pulsar remnant. X-ray binaries— ( est energies so far encountered 4 in which neutron stars blaze away, n 10 in cosmic rays. The shaded o i

t TOT curve peaking at 6.4 PeV repre- fueled by feeding off their compan- c σ νn e

S 2 sents the cross section for the ion stars—have also been suggested s 10 s interaction of electron antineutri- o to be a potent neutrino source. What- r

C nos with atomic electrons, lead- ever the case, TeV neutrinos from our − 100 ing to the production of the W own galaxy are absolutely guaran- 1 103 106 boson in what is known as the teed. The existing flux of high en- Neutrino Energy (TeV) “Glashow resonance.” Over the ergy cosmic rays, mostly protons energy range from 4 to10 PeV, this striking process dominates all the other wandering about the galaxy, trapped neutrino interactions. Because of this large cross section, such electron anti- by its magnetic fields, will lead to neutrinos will not penetrate through the Earth; instead, they must be absorbed collisions with dust and gas, gener- in such relatively short distances (depending on energy) as a few kilometers. ating neutrinos that come from the galactic plane. Observing these neutrinos may help us solve the still

BEAM LINE 19 High energy neutrino

astronomy can open

a surprising mysterious puzzle of the origin of will bring us a grand sense of wit- cosmic rays. new window nessing a new projection of the life A much more exotic potential that exists outside the murky cave source is the gamma-ray bursts re- on the Universe, in which we find ourselves. cently found to populate the sky uni- formly in all directions. If these weird with implications CIENTISTS WORKING on neu- objects are indeed occurring outside trino detection have realized for our galaxy, as most astronomers now Sdecades that the Cherenkov ra- believe, then a large fraction of a so- for distance scales diation generated by neutrino inter- lar mass is being converted into en- action products in water (and ice) pro- ergy in each event. The neutrino flux from the largest vides an inexpensive way to build that some think must accompany large-volume detectors. Charged par- these tremendous bursts of energy to the smallest. ticles speeding through transparent might well be seen by underground media at velocities near the speed of detectors now being built. Such an light throw a cone of blue light in the observation would of course help forward direction. Sensitive detec- with understanding the nature of tors can “see” these particles at dis- these enigmatic cosmic explosions. And with a large enough neutrino tances on the order of 100 meters in Finally, big neutrino telescopes telescope, we can study inner space deep, clear ocean water (which has a will be important aids in searching as well as outer space. By counting characteristic attenuation length of for a key component of dark matter— the flux of neutrinos and antineutri- 50 m). Large photomultiplier tubes the vast halo of weakly interacting nos hitting the detector from differ- sensitive to this blue light are avail- massive particles, or WIMPs, thought ent directions, one could measure able for a few thousand dollars to cluster about the Milky Way and their attenuation in the Earth and do apiece. The typical cost of such a most other galaxies. Often thought “Earth tomography”—much like photosensor module (including elec- to be supersymmetric particles left computer-aided tomography, or CAT tronics, cabling and housing) comes over from the Big Bang, these WIMPs scans, are done on people using to about $10,000 each. As each mod- should become concentrated inside X rays. With sufficient statistics, neu- ule covers an effective area of 100 the Earth’s core and that of the Sun trino telescopes will be able to meas- m2, the cost per square meter for due to glancing collisions with atom- ure the density of the Earth’s core, such detectors is roughly $100—far ic nuclei there. Occasionally a WIMP for example, a feat that is not possi- cheaper than typical accelerator- would encounter its antiparticle and ble by any other means. based detectors (but at the cost of annihilate it, however, producing a The possibility of completely poorer resolution). spasm of other particles. But only unanticipated discoveries is also a Whenever a muon neutrino (or an- neutrinos can escape these core re- potent reason for building big neu- tineutrino) interacts with an atomic gions and reach our detectors, which trino telescopes. The history of as- nucleus by the exchange of a W par- would then record fluxes of high en- tronomy has shown us time and ticle, it produces a muon that carries ergy neutrinos coming from the cen- again that the most important ob- off the bulk of the neutrino’s energy. ters of the Earth and Sun. The big servations by a new kind of device Because muons have a long lifetime neutrino telescopes now being built were not even predicted when the and do not interact very strongly or contemplated will be sensitive to first such instrument was being built. with matter, they can carry the news WIMP masses from 10 GeV to 1 TeV, High energy neutrino astronomy can of neutrino collisions a very long covering essentially the entire range and almost surely will open a sur- way—kilometers in the case of very of masses expected for the lightest prising new window on the Universe, high energy neutrinos. A muon pro- particles predicted by supersymmetic with implications for distance scales duced at PeV energies, for example, theories. from the largest to the smallest. It can travel 20 kilometers. It is no

20 FALL 1995 wonder that neutrino detectors it—unlike that for detecting muons, currently under construction have which increases as the area of the ar- focused upon muon detection in ray. At the scale of several kilome- order to attain the large target vol- ters, however, these two begin to umes needed. converge—a good thing because we The precision with which we can can learn a lot by detecting all types measure the direction of the muon— of neutrinos simultaneously. and that of the neutrino producing A unique and interesting phe- it—will depend largely on how well nomenon to watch for is the pro- we can determine the exact times at duction of W− particles by 6.4 PeV which its Cherenkov light strikes electron antineutrinos striking elec- the individual modules. Fortunate- trons within the target volume. The ly, the latest generation of large-area cross section for this interaction is photomultipliers has such good time much greater than for all other weak resolution (a few nanoseconds) that interaction processes at this ener- we can expect to attain an angular gy; if there happen to be lots of elec- resolution of about a degree for TeV tron antineutrinos in the mix, it muons that traverse the entire array. should lead to a nice spike in the dis- ~100 m Neutrino telescopes will not be able tribution of cascade energies. Aside to challenge optical or radio tele- from the aesthetic pleasure of find- A typical “double-bang” event that might scopes in this department, but their ing this so-far unobserved funda- be observed in a very large neutrino angular resolution improves gradu- mental process, it will give us a good telescope, produced by interaction of a ally with energy. And statistical tech- energy calibration and a handle on multi-PeV tau neutrino. It generates a niques can be used to reﬁne our mea- the fraction of electron neutrinos at huge shower, together with a tau lepton surements still further, so that we this energy. that travels around 100 meters before may eventually be able to pinpoint a With a large enough detector ar- decaying in a second shower with about neutrino source to perhaps a hun- ray we may be able to observe what twice the energy of the ﬁrst. dredth of a degree. we call “double-bang” events pro- Electron neutrinos and tau neu- duced by tau neutrinos of a few PeV. trinos do not produce prompt, high Upon striking a nucleus in our tar- energy muons—and neither do muon get volume, such a neutrino will of- neutrinos when they exchange a Z ten produce a tau lepton that travels particle with the struck nucleus. In about a hundred meters before de- these cases we have to observe the caying. There will be one cascade cascades of secondary particles at the point of collision and anoth- (mostly hadrons, few of which trav- er at the point of tau decay, con- el very far) produced by the recoiling nected by a softly ionizing particle quarks in the struck nuclei. For between the two. The first cascade Cherenkov detection of these large will contain about half the energy of cascades, we must position the mod- the second, on average; both should ules with typical spacings deter- be easily visible, generating about mined by the mean transparency of 100 billion photons apiece. With a our medium to blue light. Therefore sufficient sample of these double- the sensitive target volu me of our ar- bang events, we would be able to ray for detecting cascades grows only measure the percentage of tau neu- as the total volume surrounded by trinos at energies of a few PeV; by

BEAM LINE 21 Size of Array Estimated a degree. Since the early 1980s, a Depth Height Width No. No. Year of group of physicists from Europe, Expt Location (km)Medium(m) (m)Strings Modules Operation Japan, and the United States has been working from the University of AMANDA Antarctica 1–2 ice 390 110 16 478 2000 Hawaii, doing R&D on this project— a Baikal Siberia 1.1 lake 70 43 8 96 1997 acquiring valuable experience in how DUMAND Hawaii 4.8 ocean 230 106 9 216 1997 to apply the techniques of high en- NESTOR Greece 3.7 sea 220 32 7b 168 1997 ergy physics to the deep-ocean en- a1 module contains 2 photomultiplier tubes vironment. b 1 tower is equivalent to 7 strings. Each DUMAND string contains 24 photosensor modules spaced 10 m comparing this ratio with those of energies and to truly gargantuan vol- apart along its instrumented electron and muon neutrinos, we umes, but both will probably have to section—plus floats to provide the will be able to study neutrino mix- ride piggyback first on a Cherenkov- necessary tension, hydrophones, ing with unmatched sensitivity to light detector in order to demonstrate environmental sensors and an elec- mass differences. The observation of their feasibility. tronics package. It is tethered to the such events would also be strong ev- ocean bottom with a remotely com- idence for neutrino mass. OUR LARGE detectors of high mandable release to facilitate re- In addition to sampling the energy neutrinos are now in covery and allow reconfiguration of Cherenkov light produced by inter- Fvarious stages of design and the array if desired. The electronics acting high energy neutrinos, physi- construction. They sport the names unit communicates with all the sen- cists have considered the possibility DUMAND (in the Pacific Ocean off sors and sends a data stream to shore of detecting the radio and sound Hawaii), AMANDA (in ice at the (including optical, acoustic, envi- waves generated by their violent col- South Pole), Baikal (in Lake Baikal, ronmental and housekeeping data) lisions with matter. Small radio puls- Siberia) and NESTOR (in the Mediter- via fiber-optic cable through a comes occur because oppositely charged ranean Sea near Greece). All are at mon junction box. particles in a cascade generate op- least ten times larger than their low- In December 1993 the collabora- posing microwave signals that do not energy brethren—such as Japan’s tion successfully laid the shore ca- completely cancel (owing to the fact SuperKamiokande detector—in their ble along the ocean bottom from the that its electrons migrate further enclosed volumes and muon- experimental site 30 km west of the than its positrons). The best avail- catching area, and their threshold en- Big Island to the laboratory at Kea- able medium for detecting such puls- ergy sensitivity is typically a thou- hole Point. Attached to this junction es appears to be polar ice, which will sand times higher. box before deployment, the first de- transmit microwaves for kilometers Having begun life in a series of tector string worked well for a short at temperatures below –60°C. Acous- late-1970s workshops, DUMAND is time but its electronics package tic pulses occur because cascades de- the elderly sibling of the other pro- failed due to a leak. Data acquired posit heat energy instantaneously in jects. As currently envisioned, it will before this failure confirmed narrow cylinders up to about 10 me- consist of nine strings of photosen- expectations about the overall sys- ters long. Detecting the sound waves sor modules in an octagonal array tem performance and background generated (with typical frequencies (with one string at the center) sub- counting rates. After further ship- of 20 kHz) will be difficult, but it may merged over 4 km deep off the Big suspended tests in the winter of be possible in the very quiet deep Island of Hawaii. This array will en- 1995–96, placement of three com- ocean for neutrinos with energies close about 2 megatons of water and plete strings will occur in the sum- above a few PeV. Both techniques have an effective area of 20,000 m2 mer of 1996. show good promise for extending for the detection of energetic muons, Having begun South Pole opera- neutrino astronomy to ultrahigh with an angular resolution of about tions in 1991, AMANDA is the

22 FALL 1995 Right: Physicists deploying the ﬁrst DUMAND string from the deck of the research vessel T. G. Thompson in December of 1993. One of the optical photosensors hangs in midair as the central electronics package is readied for submersion. Bottom: Artist’s conception of the planned DUMAND array and its deep-sea location off the Big Island of Hawaii. The cable and junction box are now in position; another attempt to install the ﬁrst of nine strings of photosensors and other detectors will occur in 1996. d e n r a e L n h o J Floats Kauai University of Hawaii Oahu 3000 m

Photosensor Molokai Modules Maui Lanai

1000 m 2000 m Electronics Pacific Ocean Unit 440 m DUMAND youngest of the four projects but has Array made impressive progress. Its ap- Photosensor Modules proach is simple: use hot water to drill holes in the Antarctic ice pack, “Big Island” freeze photomultiplier tubes in Hawaii place, and use standard electronics Junction techniques to get a large neutrino de- Box tector built rapidly with a minimum Cable of new technology development. The ice is very cloudy near the surface Keahole Point but was expected to be spectacu- Laboratory larly clear at some depth beyond about 500 m. Led by the University of Wiscon- sin, the collaboration installed four of the planned six strings, each containing 20 modules, at depths of 800–1000 m during the summer of 4.8 km Cable 1993–94. Upon activating these strings, however, physicists found bubble densities in the ice sub- stantially higher than anticipated;

DUMAND Array scattering of light from these bubbles made accurate signal timing impos- 30 km sible. In effect, AMANDA has blurry

BEAM LINE 23 vision—as if it were looking for the recent addition of the DESY lab- muons through translucent glasses. oratory in Hamburg, Germany, to the The big question is whether the collaboration brings great hope of ice clarity improves enough with solving these problems. depth to permit a full-scale experi- Planned for a deep-sea site south- ment this austral summer (1995–96). west of Pylos, Greece, NESTOR is Drilling costs grow with depth, and closest in concept to DUMAND. At expensive fiber-optic techniques may a depth of 3.5 km the Mediterranean be needed to obtain good timing. is surprisingly clear; a good site has AMANDA will run out of ice at about been located and studied a mere 20 2500 m, although the team may give k m from shore. This site is also close up if the clarity is still poor at1500 m. to a line extended from CERN With the observed low light absorp- through Italy’s Gran Sasso Labora- tion in ice, however, the array might tory, so that a proposed neutrino still make an excellent calorimeter— beam might be extended to the and microwave detection techniques detector to permit a search for hold great promise in this medium, neutrino oscillations. The detector too. array will be a hexagonal cluster of A group of Russian and German seven towers, each standing 450 m physicists building the Baikal detec- tall and containing 12 umbrella-like tor includes some of the elders in this floors with 14 modules per floor. In- field, who began work about the stallation of the first tower is ex- same time as DUMAND. The deep- pected to occur in 1997. Led by the est (1.4 km) clear lake in the world, University of Athens, the NESTOR Lake Baikal was chosen in the hopes collaboration includes teams of of avoiding the major background physicists from France, Greece, Italy e

s ﬂux of light due to radioactivity, but and Russia—among them a Saclay r o

M there appears to be an obscure, unan- group that provides the support and b o

B ticipated source that varies with infrastructure of a major French na- Awell-cladphysiciststandsbesidethehot- time—making it about as noisy as tional laboratory. waterdrillusedtoborethefirstfourAMANDA the deep ocean. One advantage is holes1000metersintotheAntarcticicecapat Baikal’s ability to place and retrieve VER SINCE the late 1970s it has theSouthPole.Thisequipmentwillbeusedto instruments in winter, without use been clear to neutrino as- boreadditionalholestodepthsofabout1500 of ships. Its unique cable-laying tronomers that an array filling metersduringtheaustralsummerof1995–96. E method uses a sled with a huge saw an entire cubic kilometer is really to cut a slot in the ice, through which needed to begin serious work in the a following sled unreels the cable as field. Everyone recognizes that we both drive shoreward; the slot rapid- are really building demonstration ly freezes behind. So far 36 modules devices—perhaps big enough to find (each with two photomultipliers act- a few exciting things—as explorato- ing in coincidence) have been placed, ry stepping stones to the huge de- with data being recorded and ana- tector we really need to finish the job. lyzed at a shore station. This group Many independent calculations have has suffered from the lack of funding converged upon a cubic kilometer as and high-technology materials, but the appropriate volume. Such a

24 FALL 1995 Right: One of the Baikal detector modules being lowered through the ice together with cabling and electronics. Each module consists of two photosensors in two separate pressure-resistant glass spheres. Bottom: A prototype NESTOR “ﬂoor,” which consists of 14 photosensors in an umbrella-like array, being lowered into the Mediterranean Sea off Greece. Slated for initial deployment in 1997, each tower in this neutrino telescope will consist of 12 such ﬂoors, whose arms open radially once in the water. e s r o M b o B

device should function well in detecting neutrinos from point sources, the galactic plane, gamma-ray bursters, AGNs and WIMPs. In 1994 there were several meetings at which physicists discussed the idea of forming a world collaboration to design and build such a gargantuan detector. The AMANDA and DUMAND teams plus a few newcomers have been working with the Jet Propulsion Laboratory, a major national laboratory in Pasadena, California, with the big-project experience needed in such an effort, which is clearly beyond the capability of a single university. The Saclay group joined NESTOR partly in order to gain experience towards building such a facility. Many of us hope to merge these two efforts into a single international project. One thing we are all proud of in this emerging ﬁeld is the tremendously supportive interaction among the physicists involved. Although there is lots of competition, as all of us naturally want to be first with any important discovery, we realize that by helping each other we also help ourselves in the long run. All four teams have meanwhile got their work cut out for them. No cubic-kilometer detector will ever begin construction until we can get at least one (and hopefully more) of the current generation up and counting neutrinos. Only then will we have the information we need to s i n

a decide whether the next step for neutrino astronomy v s e

R should occur in water or ice. o e L

BEAM LINE 25 Osteoporosis–New Insights by JOHN KIN NEY

Synchrotron radiation is giving us new insights into causes and treatments for osteoporosis.

STEOPOROSIS IS ADISEASEcharac- terized by fragile bones that results in fractures. Half of women over seventy will have a fracture as a result of osteoporosis. For many of them, this will lead to a decline in their quality of life and independence. These fractures, which often occur with little or no injury,

26 FALL 1995 are usually a result of reduced bone BONE STRUCTURE mass. Accordingly, research on os- CHARACTERISTICS teoporosis has focused on those factors that affect bone mass, such as About a hundred years ago, an Eng- estrogen deprivation, age, and phys- lish reverend named Abbott pub- ical activity. Treatments to control lished a small book titledFlatland. It osteoporosis have been largely based was a story of a two-dimensional on trying to maintain or to increase world occupied by polygons. The pro- skeletal mass; however, significant tagonist was a square who, one day, overlap exists between the bone mass was visited by a sphere. The sphere of normal individuals and those with carried the square above Flatland, and An X-ray tomographic microscope osteoporosis. This overlap has re- allowed him to view his three- image of a thin section of leg bone in a sulted in investigation into factors dimensional counterpart, the cube. rat. The thick bone (A) surrounding the other than low bone mass, such as Upon returning to Flatland, the marrow cavity is the cortical bone, or structural properties and bone qual- square set about to enlighten his two- cortex. The thin, web-like bone (B) within the marrow cavity is the spongy bone. ity, to explain the higher frequency dimensional world about the exis- The earliest stages of osteoporosis of fractures among individuals with tence of a three-dimensional uni- involve the resorption of spongy bone osteoporosis. verse. Unfortunately, the square was and a breaking of its interconnections. Bone is the hard, marrow-filled never able to provide convincing calciﬁed tissue that forms the skele- proof of the third dimension to his ton. Near the joints a porous spongy- fellow inhabitants of Flatland and like bone fills much of the marrow was eventually labeled a heretic and space as seen in the illustration on spent his remaining days in prison. the right. This spongy bone aids in Though this book was intended as transferring the stresses of motion to a social satire, it points out the dif- the hard outer shell called the cor- ficulties describing three-dimen- tex. The spongy bone (frequently sional structures using only two- referred to as trabecular bone) is com- dimensional views. The science of posed of an interconnected structure reconstructing three-dimensional in- of curved plates and struts. One of formation from two-dimensional the age-related changes that accom- views is called stereology. With stere- panies bone loss is a decrease in the ological methods it is possible to es- amount of spongy bone. In normal timate volume fractions and surface people the volume occupied by this areas of particles or voids from just a spongy bone decreases with age. This few flat microscopic sections cut decrease in bone volume is appar- from an object. ently due to a loss in the number of Stereological methods are cur- struts and a severing of the inter- rently used to estimate three- connections, as opposed to a general dimensional properties of bone from uniform thinning of the elements. two-dimensional or planar sections. Many questions regarding the de- These methods allow calculation of velopment and treatment of osteo- the volume fraction and surface ar- porosis could be answered if the areas of the spongy bone. It is also pos- chitecture of the spongy bone—and sible to quantify other structural fea- how it changes with disease and tures of the bone required to explain age—were better understood. bone formation, resorption, and

BEAM LINE 27 structure. The application of stere- in a rigid holder, and a shallow cut is measures would be performed on the ological principles has provided im- made that just exposes the interior. same animal. The savings in portant clues as to how a bone’s The surface is polished, stained, and the number of animals becomes even structure affects its properties and is photomicrographed. Then another greater as the sophistication of the altered by disease and aging. Unfor- cut is made parallel to the original experiment increases. Hence, there tunately, as we understand better the one. This fresh surface is then pol- is a strong motivation for developing complex structures and properties of ished, stained, and photomicro- imaging methods that provide biological systems, we begin to ex- graphed. This procedure is repeated the same information as serial ceed the ability of stereological meth- until the entire specimen has been sectioning—but on living animals. ods to describe the critical aspects of examined. The micrographs from An alternative method for recon- the structure. In particular, conven- each section are then digitized, and structing three-dimensional images tional stereological methods begin to a volumetric image is created. In or- is X-ray computed tomography or CT collapse when questions arise re- der to quantify the interconnected- (see “Positron Emission Tomogra- garding how the spongy bone net- ness of the spongy bone in a small phy” in the Summer 1993 Beam works are connected. For example, animal, it would be necessary to Line, Vol. 23, No. 2). Because little one of the easiest questions to pose obtain 50 to 100 sections from each or no sample preparation is required about any structure is to ask how one. This explains why this method for CT, tomographic methods provide many objects are contained within is rarely used. a cost-effective alternative to serial it. Indeed, it is impossible to answer Another disadvantage with sec- sectioning, and can, in principle, be the basic question “how many?” tioning is that it can only be per- used on living animals. CT is most from a limited number of two- formed on dead animals. This re- frequently used as a medical diag- dimensional views or sections. A quires a large number of animals to nostic (higher resolution CT scan- complete three-dimensional visual- study at each time point in an ex- ners have been developed, but they ization is necessary. periment. For example, in an exper- have not been approved for use on Questions such as “how many” iment that tests the effectiveness of humans). Unfortunately, the present relate to the topological structure a bone-growth drug at a single dose, resolution is not high enough to of an object. For any three-dimen- it is necessary to have control ani- image spongy bone with the accu- sional structure, the topology can mals, estrogen-deficient animals (in- racy required for quantitative mea- be described with three variables: duced by removing the animal’s surements. To be useful as a substi- i) the number of separate particles ovaries), and treated animals. If six tute for conventional methods, the (“how many”); ii) the interconnect- animals are required at each time resolution of the CT method must be edness of the particles (number of point for statistical accuracy, and if improved a hundred-fold over the handles or pathways connecting dif- we examine bone loss in the ovariec- newest instruments. This requires a ferent parts of the object); andiii) the tomized animals at three time points, new type of X-ray source and im- number of enclosed surfaces (for ex- and then examine treated, controlled, proved detectors. ample, bubbles, voids, or entrapped and ovariectomized animals at three particles). These variables are ex- additional time points after treat- COMPUTED TOMOGRAPHY tremely valuable for describing the ment, a minimum of 78 animals WITH SYNCHROTRON RADIATION process of a disease like osteoporosis would be required for this simple and the mechanical behavior of cal- study. If, on the other hand, it were The attenuation of X rays through cified tissues. possible to examine living animals, a sample is a sensitive measure of In the past, quantifying these topo- then time-point sacrifice would no atomic composition and density. By logical variables required time- longer be necessary. In this case, only measuring the X-ray attenuation co- consuming, artifact-prone serial sec- 18 animals would be required, and efficient as a function of position tioning of entire specimens. In serial the results would have greater sta- within a sample with CT, a three- sectioning, a specimen is imbedded tistical significance because repeated dimensional image of the sample can

28 FALL 1995 be obtained. Subtle compositional EXPERIMEN TAL STUDY OF POST and structural changes from one po- MENOPAUSAL BONE LOSS sition to the next appear as differences in the X-ray attenuation. As In a recent study, the leg bones long as the spatial resolution is small (tibias) of living female rats were im- with respect to the features of in- aged with the XTM at SSRL. Imag- terest, the three-dimensional X-ray ing times with the nearly mono- images obtained from these mea- chromatic X-ray wiggler beam at surements will provide valuable 25 keV were less than 30 minutes per structural information. animal. Shuttering of the direct beam High spatial resolution depends reduced actual exposure times to less upon i) a sufficient X-ray intensity, than two-and-a-half minutes. so that enough X rays reach the de- The rats were anesthetized, and tector to provide good measurement while unconscious they were secured statistics; ii) a detector with suffi- to a rotating platform with their right cient spatial resolution to discrimi- hind limbs elevated into the X-ray nate between closely separated X-ray beam. On the day following the ini- paths; and iii) monochromatic (sin- tial scans, rats were chosen at ran- gle energy) radiation. Synchrotron ra- dom and their ovaries were removed. diation sources, because of their high Five weeks after removal of the brightness and natural collimation, ovaries, all animals were imaged a have allowed the development of CT final time with the same imaging systems that have spatial resolutions parameters. approaching 1 µm.For this study we Once the data were acquired, the modified the X-ray tomographic three-dimensional images of the microscope (XTM) developed at tibias were reconstructed on a com- Lawrence Livermore National Lab- puter workstation. The volumetric oratory in Livermore, California, for data were analyzed by the follow- use on beam line 10–2 at the Stan- ing method. Cluster analysis was per- Conventional X-ray tomography ford Synchrotron Radiation Labora- formed on the spongy bone struc- instruments do not have the resolving tory (SSRL) at Stanford Linear Accel- tures in the three-dimensional power required to detect structural erator Center in order to image the images; it identiﬁed all of the spongy features within the spongy bone and three-dimensional structure and bone that was continuously inter- cortex. Above is the highest resolution mineral density of bone in living an- connected, and also identified any image of a rat leg bone with specialized commercial instrumentation. Below is imals. We wanted to demonstrate isolated structures that were dis- the same bone imaged with the X-ray that the XTM with synchrotron ra- connected from the surrounding cor- tomographic microscope and diation can detect microscopic tical bone and spongy structure. synchrotron radiation. Variations in the changes in the spongy bone structure Cluster analysis provided a direct color of the bone correspond to small and connectivity in estrogen- measure of the topological variables differences in the calcium concentration. deficient rats, an important animal that quantify the number of isolated model for osteoporosis. The im- bone fragments and the number of proved resolution available with syn- imbedded pores. For the intercon- chrotron radiation over the highest nected cluster, the connectivity was resolution CT scanners is shown in calculated from the three-dimen- the two ﬁgures on the right. sional image.

BEAM LINE 29 Top: An X-ray tomographic microscope image of a small region of spongy bone in the leg bone of an osteoporotic rat. The normal plate and strut-like structure of the bone has been greatly eroded, and several isolated bone fragments can be seen suspended in the marrow. Though contributing to the total bone mass, these isolated fragments do not contribute to the bones’ strength and resistance to fracture.

Middle: The spongy bone in a rat leg at the earliest stages of osteporosis showing the beginning of perforations to the plate structures.

Three-dimensional images of the Also, intermittent parathyroid tibias just prior to removal of ovaries therapy both alone and in combina- and five weeks after surgery showed tion with estrogen therapy has been that a 60 percent loss in bone volu me reported to preserve spongy bone occurred in the five weeks following connectivity. Because the reports of estrogen loss (see “Biological Appli- connectivity have been based on cations of Synchrotron Radiation” in two-dimensional data, we have been the Fall/Winter 1994 Beam Line, using synchrotron radiation with X- Vol. 24, No. 3, page 21). In addition, ray tomographic microscopy to de- the three-dimensional images dem- termine if intermittent parathyroid onstrated a significant change from therapy actually does increase bone an interconnected plate- and strut- mass and connectivity. What we like structure to one that is mostly have found is that spongy bone vol- disconnected struts. Also, dangling ume and connectivity significantly (or dead-end) elements are seen only decreased after 8 and 12 weeks of es- in the estrogen-deficient animals. trogen loss. Parathyroid treatment These dangling elements, although appears to increase bone mass by still contributing to the total bone thickening existing bone, not by mass, probably do not contribute to forming new connections (see mid- the strength of the bone. dle and bottom figures on the left). A small region of spongy bone at From our results, we hypothesize higher magnification in a rat with that to re-establish connectivity in ovaries removed is shown in the top the spongy bone, treatment would figure on the left. Of particular in- have to be given before significant terest is the small bone fragment that bone loss has developed, or treatment is isolated from the surrounding bone with a bone-forming agent such as and supported only by marrow. We parathyroid would be less effective. have only observed bone fragments Further studies need to evaluate crit- such as this in animals with osteo- ical time points for the administra- porosis, where they account for about tion of bone-forming compounds and 1.5 percent of the total spongy bone for assessing the effects of these treat- volume. These isolated bone fragments on improving bone strength. ments, as well as the more signifi- New insights gained from these cant fraction of dangling bone, may studies using synchrotron radiation be responsible for the overlap in bone are allowing us to develop more rapid mass between individuals with os- screening of new clinical treatments teoporotic fractures and individu- for osteoporosis, a major public als without fractures. health problem responsible for over Bottom: The spongy bone of an osteo- one million fractures a year in the porotic rat given parathyroid hormone CAN WE REGROW LOST BO NE? United States alone. after osteoporosis has been established. New bone is formed, and the spongy Intermittent parathyroid hormone bone is significantly thicker than the original bone; however, the original plate therapy has been shown to increase and strut-like structure of the bone is not bone mass and improve biomechan- reestablished. ical strength in osteoporotic rats.

30 FALL 1995 High power lasers combined

with SLAC’s unique high energy electron beam are opening up a new ﬁeld of research.

LASER-ELECTRON INTERACTIONS by ADRIAN MELISSINOS

HE INTERACTIONOFLIGHT with matter is the most funda- T mental process of our physical world. It heats our planet, it supports life, and it makes objects visible to us. The invention of the laser thirty-ﬁve

years ago made possible beams of light of much higher intensity which possess

remarkable directionality and coherence compared to those of ordinary

sources. Lasers of all shapes and sizes have not only revolutionized technology

but have also opened new research possibilities in many ﬁelds of science. One

such new avenue is the interaction of photons with high energy electrons,

which lets us study the fundamental electron-photon interaction in the ab-

sence of other matter and in the range of extremely high photon densities.

BEAM LINE 31 Right: Backscattering of a photon from a High Energy Laser That collisions will occur when a high energy electron. Before Electron Photon beam of electrons crosses through a Below: The 82-in. bubble chamber at counter-propagating beam of photons SLAC in 1967. Top to bottom are may seem obvious to some, but it Wolfgang Panofsky, Joseph Ballam, could also be relegated to the domain Robert Watt, and Louis Alvarez. This Lower Energy High Energy of science ﬁction by others. The out- chamber was a modiﬁed version of the After Electron γ ray original 72-in. chamber built and come of such an experiment depends operated by the Alvarez group at the on the density of the beams and on Lawrence Radiation Laboratory. the duration of the crossing, as well as on the fundamental probability for the interaction to occur. If we com- pare the energy of an electron in the Stanford Linear Accelerator Center (SLAC) beam with that of a laser photon, a great disparity is evident. The electron energy is 50 billion electron volts, whereas the photon energy is typically 1 or 2 electron volts; the collision can thus be described by the proverbial analogy of a Mack truck colliding with a ping-pong ball. It is nevertheless true that under appropriate conditions the electron can transfer almost all its energy to the photon, as shown in the illustration above left. This process is referred to as Compton (or photon) backscattering.

ACKSCATTERING of a laser beam was first exploited at BSLAC in 1969 to create high energy photons or gamma rays. The gamma rays were directed into the 82-inch bubble chamber, where their interactions in liquid hydrogen (on protons) were recorded and studied. An important aspect of high energy electron-photon collisions is that they depend on the state of polarization of the colliding beams. Elec- trons have spin that can point in one of two directions: along or against the direction of motion; the same is true for the photons. By measuring the rate of scattered events one can

32 FALL 1995 Ti:Sapphire Lasers Bunch 1

Combiner Bunch 2 Circular Polarizer Left or Right Circularly Polarized Light

Unpolarized Gun

Mirror Box Linearly Polarized Light

e–

Load Lock

Polarized e– Gun Bunchers The polarized electron source for the SLAC linac. Accelerator determine the state of polarization deliver photons of a speciﬁc energy. length of 790 nanometers excites ru- of the electron beam, a technique This property in combination with bidium atoms, which in turn collide that was pioneered at SLAC on the the polarization dependence of the with atoms of the helium isotope electron-positron ring SPEAR in the laser-electron interaction has made 3He. This causes a significant po- late 1970s. possible the highly successful polar- larization of the 3He nuclei, and An extension of the polarization- ized source that is now in use at the therefore of the two protons and neu- measurement technique, first used SLAC linac (see “The Heart of a New tron that form the nucleus. at Novosibirsk in Russia, allows the Machine” in the Summer 1994 issue Incidentally, Compton backscat- precise determination of the energy of the Beam Line, Vol. 24, No. 2). In tering is not simply restricted to the of the electrons in a storage ring. this case it is the laser photons that laboratory but also plays an impor- Three years ago such measurements transfer energy to the electrons so tant role within our galaxy, and pre- revealed changes in the energy of the that they can emerge from the semi- sumably in all of intergalactic space LEP electron-positron collider at conductor cathode and be acceler- as well. Very high energy cosmic-ray CERN in Geneva, Switzerland, ow- ated in the electron gun and then in- protons scatter from the photons that ing to the gravitational pull of the jected into the linac. In the strained form the microwave background ra- moon. The tidal force of the moon gallium arsenide crystal cathode, diation that permeates all of space. distorts the lattice of the 27 kilo- atomic levels corresponding to dif- Although the density of the radiation meter ring of magnets, resulting in a ferent polarization states have dif- is about 400 photons/cm3, many or- change of the beam energy at fixed ferent energies and can be selective- ders of magnitude lower than that radio-frequency and magnetic field ly excited by the laser. This has within a laser beam, the distances values. What was ﬁrst considered a yielded polarizations as high as 85 over which the interaction can occur puzzle was eventually resolved by percent. The same principle is used are correspondingly enormous com- the late Gerhard Fisher, a long-time to achieve polarization in some of the pared to terrestrial standards, so col- SLAC accelerator physicist. targets for the electron-scattering ex- lisions do occur. In fact, it is believed The energy spread of the photons periments being carried out in End that the highest energy of the cosmic in a laser beam is usually very small, Station A at SLAC. Here a titanium rays is limited by their collision with and certain lasers can be tuned to sapphire laser operating at a wave- the microwave background photons.

BEAM LINE 33 The T3 laser facility used in SLAC Experiment 144.

Neodymium lases in the infrared at a wavelength of about 1060 nanometers; however, it is possible to double the frequency of the light with good efficiency to obtain beams in the green wavelength of about 530 nanometers. Typically the laser de- livers 1–2 joules of energy in a single pulse, which is only 1.2 picoseconds N EXPERIMENT currently in The interaction of electrons with (trillionths of a second) long. Thus progress at SLAC (E-144) ex- such strong electric fields has hith- the peak power is of the order of 1012 Aplores for the first time a dif- erto been inaccessible to laboratory watts, to be contrasted with a com- ferent dimension of the electron- experiments (similar phenomena are mon helium-neon laser that operates photon interaction—what happens thought to occur in very dense neu- at a level of about 10−3 watts. Final- if the photon density is extremely tron stars). Just as atoms are torn ly the laser pulse must be focused as high? The experiment became pos- apart in fields exceeding 109 V/cm, tightly as possible. This implies the sible because of recent advances in one can speculate that the electrons use of large beams and short focal laser technology that can produce themselves may not withstand the length (to achieve a small f-number) densities as high as 1028 photons/cm3 critical field. What would be the and the absence of aberrations; near- at the focus of a short laser pulse. To manifestation of such a disruptive ly diffraction-limited spots of suggest the scale of this nu mber, we interaction? Presumably electron- 20 mm2 area have been achieved for recall that the density of electrons in positron pairs would be created out green light using f = 6 optics. ordinary matter is in the range of of the vacuum, the energy being sup- To reach joule energies in a single 1024/cm3. High photon density cor- plied by the electric field. Experi- pulse, a chain of several laser am- responds to high electric field at the ment 144 has indeed observed plifiers must be used. Initially a laser focus, approaching E = 1011 positrons produced under such cir- mode-locked oscillator produces a volts/centimeter (V/cm). At such cumstances. Furthermore, the or- pulse train of short pulses, typical- field strengths, atoms become com- dinary electron-photon scattering in- ly 50 picoseconds long. This is pletely ionized. More importantly, teraction is modified in the presence achieved by placing an acousto-optic however, when an electron from the of strong electric fields by allowing switch that is driven by an external SLAC beam passes through the laser for the simultaneous absorption of frequency with period equal to twice focus, it feels in its own rest frame several laser photons. We will return the time for one round trip of the an electric field multiplied by the ra- to these experimental results. pulse between the laser mirrors in- tio γ = ε/mc2. This effect is a conse- The laser system for E-144 is side the laser cavity. As a result, the quence of the special theory of rela- based on the experience gained at the cavity can lase only for a short time tivity, which is applicable when the Laboratory for Laser Energetics of the when the switch is open and in phase particle velocity approaches the University of Rochester; it uses with the external radio-frequency. speed of light. For the SLAC beam, neodymium embedded in a yttrium- This frequency has been chosen to the energy ε is about 50 GeV; since lithium-fluoride crystal matrix be a sub multiple of the accelerator mc2 = 0.5 MeV, the factor γ is of the (Nd:YLF laser) followed by neo- drive frequency, and in this way the order of 105. Thus the electric field dymium in a glass matrix (Nd: glass pulses in the laser train are syn- seen by the electron is about E = 1016 laser). It has the advantage of being chronized with the electron pulses V/cm; this field strength is referred highly compact as compared to lasers in the linac. A single pulse is selected to as the critical field of quantum of similar power and is nicknamed out of the pulse train by a Pockels electrodynamics. T3, for Table-Top-Terawatt laser. cell, but the energy in the pulse is

34 FALL 1995 only a few nanojoules. The pulse is then stretched in time and chirped in frequency before being injected into a regenerative amplifier. After 100 round trips in the regenerative amplifier, the pulse energy is at the millijoule level, at which point the pulse is ejected from the regenerative amplifier and further amplified in two additional stages. The last stage is of special construction using a slab of lasing material instead of the usual rods to permit a repetition rate of 1 hertz. Finally, the pulse is compressed in time in a pair of diffraction gratings to achieve the desired short length. The experiment is installed in the of electron (positron) momenta. The The laser-electron interaction point and Final Focus Test Beam (FFTB) line high energy gam ma rays continue the analyzing spectrometer for (See “The Final Focus Test Beam,” in the forward direction and are de- Experiment 144 inside the Final Focus in the Spring 1995 issue of the Beam tected in a separate calorimeter 40 Test Beam cave. Line, Vol. 25, No. 1), just down- meters downstream from the inter- stream of the primary focus. This lo- action point. cation is particularly advantageous One of the concerns during the because the tight focal area and short planning of the experiment was duration of the laser pulse must be whether the synchronization of the matched by an electron pulse of cor- laser pulses with the electron beam responding quality. The laser beam could be established and maintained. is transported by high performance The length of the laser pulse is mirrors to the interaction point, 300–600 micrometers, and the elec- where it crosses the electron-beam tron pulse is only slightly longer; fur- line at a 17 degree angle. Immedi- thermore, the two beams cross at an ately after the interaction point, a angle. Thus the timing and the path string of six permanent magnets de- lengths traversed by these two pulses flects the electron beam into a beam originating from very different dump; these magnets also serve to sources must be kept to a tolerance disperse the electrons that have in- of a fraction of a millimeter. This teracted and the positrons that have was successfully accomplished with been produced in the interaction so a phase-locked loop between the that their momenta can be measured. laser mode locker and the linac ra- Electrons and positrons are detected dio-frequency, as well as by careful in silicon calorimeters placed below construction of the transport line. A and above the original beam line. computer-controlled delay inserted These detectors measure the total in the laser line was used to set and energy deposited in a narrow range maintain optimal timing.

BEAM LINE 35 O T EVERY BEAM electron absorptions by a factor of eight, and crosses through the laser fo- in general the rate of n-photon ab- –4

) 10 cus, but at high laser den- sorptions increases as the nth pow-

V N e sity every electron that does cross er of the laser energy. That this is

G 400 mJ /

1 η = 0.34 true can be seen in the bottom graph

( through the laser focus interacts, 10–5 27 mJ giving rise to a backscattered gamma on this page where the observed rate E

d η = 0.11 / ray with energy between 0 and for two-, three-, and four-photon ab- n d *

γ 21 GeV (for infrared light and for sorption is plotted as a function of N / –6 46 GeV incident electrons). Some laser energy. The nonlinear behavior 1 10 5×107 gammas were produced by for these processes is clearly evident. each electron pulse, corresponding The rate for the production of 10–7 to conversion of approximately 1 per- electron-positron pairs is relatively 8 12 16 20 24 cent of the incident electron ﬂux (as low at present laser intensities; ap- Recoil Electron Energy (GeV) expected from the relative size of the proximately one positron is detected two beams). Interactions that involve in 1000 laser shots. The physical the absorption of two or more pho- mechanism in this case is the fol- E = 20.75 GeV tons are much less common but still lowing: an electron enters the laser –4 17.75 n = 2 ) 10 copious. In this case it is the recoil focus and produces a high energy V e

G electrons that are being detected; if gamma ray; the gamma ray interacts / 1

( a single (infrared) photon is absorbed, simultaneously with several (at least –5 10 four) laser photons to produce the

E the recoil electron energy is always d / greater than 25 GeV. Thus electrons pair. This result is the ﬁrst demon- n d

* n = 3 14.75 with energy below 25 GeV are a mea- stration of light-by-light scattering γ –6

N 10 sure of interactions where two or involving physical photons. There / 1 more photons were absorbed (there was little doubt that this process, n = 4 12.75 is also a finite probability that the which has been amply verified for 10–7 electron scattered twice at different virtual photons, would occur; how- 1 2 3 10 10 10 positions within the laser focus). The ever, the stage has now been set for Laser Energy (mJ) measured spectrum of recoil elec- further precision experiments that trons for two different laser energies can probe quantum electrodynamics Top: The normalized cross section for is shown in the top graph on the left. in the critical field regime. multiphoton Compton scattering plotted Recoil electrons in the range between as a function of momentum for laser 18 and 25 GeV result from the ab- T SHOULD COME as no surprise energies of 27 and 400 millijoules. The sorption of two photons, between 13 that an understanding of the elec- laser wavelength was λ = 1054 nano- and 18 GeV from the absorption of tromagnetic interaction in high meters and the electron energy I three photons, and so on. electric fields is essential for the de- ε = 46 GeV. The absorption of a single photon sign of the next generation of lin- Bottom: The normalized cross sections is a linear process; this implies that ear colliders. To achieve the required for 2, 3,and 4 photon absorption as a if the laser intensity is doubled, the luminosity, the beams at the colli- function of laser energy (λ = 1054 nano- scattering rate also doubles. In con- sion point of these new machines meters and ε = 46 GeV). For linear trast, the absorption of several pho- m ust have di mensions of only a few processes the cross section is inde- tons is a nonlinear process. When the nanometers. This results in an ex- pendent of laser energy. The open laser intensity is doubled the rate tremely high electric field inside the circles show the predictions of the electron (or positron) bunch; the in- theory. of 2-photon absorptions increases by a factor of four, the rate of 3-photon coming positron (electron) that

36 FALL 1995 e g e o B e v e t S enters the oppositely moving bunch each of the beams. In this respect E- Experiment 144 group photograph. is affected by the field and can lose a 144 has been the pilot experiment for Pictured left to right are (kneeling) significant amount of energy in the future gamma-gamma colliders both Glenn Horton-Smith, SLAC; Theofilos form of radiation and/or pair pro- in terms of the hardware involved Kotseroglou, Wolfram Ragg, and Steve Boege, University of Rochester; duction. This effect is already ob- and in measuring the fundamental (standing) Kostya Shmakov, University servable in the collisions at the Stan- processes that govern such collisions. of Tennessee; David Meyerhofer and ford Linear Collider and has been There are still many unanswered Charles Bamber, University of given the name “beamstrahlung.” It questions and much work to be done Rochester; Bill Bugg, University of will be much more important for fu- before gamma-gamma colliders can Tennessee; Uli Haug, University of ture linear colliders, and designers be designed with certainty. Rochester; Achim Weidemann, are making every effort to minimize Investigation and exploitation of University of Tenessee; Dieter Walz, David Burke, and Jim Spencer, SLAC; its impact. the electron-photon interaction has Christian Bula and Kirk McDonald, SLAC Finally, it is desired to include in had a long tradition at . High- Princeton; Adrian Melissinos, University future electron-positron colliders the er energies, new technology, and new of Rochester. Not pictured are Clive option of operating them as gamma- ideas have confirmed theoretical pre- Field and Allen Odian, SLAC; Steve gamma colliders. This has certain ad- dictions that, in some cases were Berridge, University of Tennessee; vantages in identifying the particles made decades ago. The recent ex- Eric Prebys, Princeton; and Thomas produced in the collision, especially periments present an opportunity for Koffas, University of Rochester. scalar particles such as the hypoth- probing this fundamental interaction esized Higgs boson. The high energy in a new regime. No doubt the an- electron and positron beams can be swers will continue to be important. converted to gamma-ray beams by backscattering laser photons from

BEAM LINE 37 CONTRIBUTORS

BILL CARITHERS, senior staff physicist at Lawrence Berkeley National PAUL GRANNIS completed his bach- JOHN LEARNED has been at the Uni- Laboratory (LBNL) and visiting sci- elor’s degree at Cornell University versity of Hawaii where he is Pro- entist at Fermi National Accelerator and received a doctorate from the fessor of Physics since 1980. He went Laboratory, currently serves, with University of California at Berke- there to get the DUMAND project Giorgio Bellettini, as spokesperson ley in 1965. Since 1966 he has been started and thought he would leave for CDF, the Collider Detector at on the faculty of the State Univer- in three or four years. Fifteen years Fermilab collaboration. He has held sity of New York at Stony Brook, later, a little wiser, and much more this position since January 1993. with visiting appointments at the experienced at sea, he and his col- Carithers did his undergraduate Rutherford Laboratory, CERN, Uni- leagues are still pushing to do high work at the Massachusetts Institute versity College London, and Fermi energy neutrino astronomy. of Technology and earned his PhD in National Accelerator Laboratory. He has also been involved in the physics from Yale University in 1968. From its inception in 1983 until IMB nucleon decay experiment, of Since then he has held positions at 1993, he was the spokesman for the which he was a co-founder, and had Columbia University and Rochester DØ experiment at the Fermilab an- his greatest scientific thrill leading University before making the move tiproton collider; since 1993, he has the discovery of the neutrino burst DØ west to LBNL in 1975. served as co-spokesman for with from supernova 1987A. He also had He was a Sloan Fellow from 1972 Hugh Montgomery. a foray into high energy gamma-ray to 1974. Grannis is a Fellow of the Amer- astronomy in the mid-1980s, build- ican Physical Society and vice-chair- ing and operating a Cherenkov air man of the Division of Particles and shower telescope on Haleakala. Fields of the American Physical For obvious reasons he prefers to Society and a Sloan Fellow from 1969 spend his holidays in the mountains to 1971. rather than at sea.

38 FALL 1995 JOHNH.KIN NEY is a research scientist in the Chemistry and Materials Science Department at Lawrence MICHAEL RIORDAN has been asso- Livermore National Laboratory. He ADRIAN MELISSINOShas been on the ciated with the Beam Line since he received his BA degree in mathe- faculty of the University of Rochester came to SLAC in 1988 ﬁrst as editor matics and physics from Whitman since 1958, the year he earned his now as contributing editor. During College and his PhD in materials sci- PhD from the Massachusetts Insti- 1995 he was on leave from his posi- ence from the University of Califor- tute of Technology. He has served as tion as Assistant to the Director at nia at Davis in 1983. department chairman and visiting SLAC, writing his latest book, Crystal In collaboration with researchers scientist at CERN. His recent re- Fire, a history of the transistor to be from the University of California, search efforts are in the application published by W. W. Norton in 1997. San Francisco, he has been applying of high power lasers to particle He is the author of The Hunting of physics and particle accelerators. the Quark and co-author of The synchrotron X-ray methods to the study of diseases in calciﬁed tissues. Over the years Melissinos has Shadows of Creation in case you Most recently, this work has been worked on experiments at the Cos- can’t tell from the above photograph. extended to the study of calcifica- motron, AGS, Fermilab, SLAC, and He did his PhD research at SLAC tions in coronary artery disease with Cornell. He has been involved in de- on the famous MIT-SLAC deep in- researchers from Stanford University. veloping techniques for detecting elastic electron scattering experi- He has published over sixty arti- gravitational interactions and in ments. cles in peer-reviewed journals. searches for cosmic axions. He is the author of a widely used physics textbook. He is a Fellow of the American Physical Society and a corresponding member of the National Academy of Athens.

BEAM LINE 39 DATES TO REMEMBER

Jan 7–13 Aspen Winter Conference on Particle Physics: The Third Generation of Quarks and Leptons, Aspen, CO (Aspen Center for Physics, 600 W. Gillespie St., Aspen, CO 81611).

Jan 14–20 Aspen Winter Conference on Gravitational Waves and Their Detection: Data Analysis LIGO Research Community Space Based Detectors—LISA, Aspen, CO (Aspen Center for Physics, 600 W. Gillespie St., Aspen, CO 81611).

Jan 15–26 1996 US Particle Accelerator School (USPAS 96), San Diego, CA (US Particle Accelerator School, c/o Fermilab, MS 125, PO Box 500, Batavia, IL 60510 or USPAS@FN A LV.F NAL.GOV).

Mar 18–22 General Meeting of the American Physical Society, St. Louis, MO (ASP Meetings Depart- ment, One Physics Ellipse, College Park, MD 20740-3844 or [email protected]).

Apr 1–3 Phenomenology Symposium 1996, Madison, WI (Linda Dolan, LDOLAN@ PHENXD.PHYSICS.WISC.EDU).

Apr 9–12 International Magnetics Conference (INTERMAG 96), Seattle, Washington (Courtesy Associ- ates, 655 15th Street, N W, Suite 300, Washington, DC 20005 or MAGNETISM@ MCIMAIL.COM).

Apr 22–27 CERN Accelerator School, Course on Synchrotron Radiation and Free-Electron Lasers, Grenoble, France (Mrs. S. von Wartburg, CERN Accelerator School, AC Division, 1211 Geneva 23, Switzerland or [email protected]).

May 26–Jun 9 Aspen Center for Physics Workshop on High Energy Neutrino Astrophysics, Aspen, CO (Aspen Center for Physics, 600 W. Gillespie, Aspen CO 81611).

Jun 10–14 5th European Particle Accelerator Conference (EPAC 96), Sitges, Spain (C. Petit-Jean Genaz, CERN-AC, 1211 Geneva 23, Switzerland or [email protected]).

Jun 24–Jul 12 Division of Particles and Fields/DPB Snowmass Summer Study of Future Directions in US Particle Physics, Snowmass, CO (C. M. Sazama, Fermilab, MS 122, PO Box 500, Batavia, IL 60510 or SAZAMA@FN ALV. GOV).

Jul 25–31 28th International Conference on High Energy Physics, Warsaw, Poland (A. K. Wroblewski, Institute of Experimental Physics, Warsaw University, ul. Hoza 69, PL-00-681, Warsaw, Poland or [email protected]).

40 FALL 1995