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SLAC Beam Line, September 1988 SLAC BEAM LINE Like artists, creative scientists must occasionally be able to live in a world out of joint. - Thomas Kuhn 1___ I Volume 18, Number 2 September 1988 The Mark II Detector at the SLC 2 SLAC Beam Line, September 1988 FIRST SLC RUN ENDS SLAC BEAM LINE Mail Bin 80, (415) 926-2282 The initial colliding-beam run on the Stanford Lin- Bitnet Address: BEAMLINE @ SLACVM ear Collider (SLC) ended on Monday, September 12. Work began immediately to retune the linac Editor: Michael Riordan so that it can again inject electrons Assistant Editors: Nina Adelman, Bill Kirk and positrons into the Photography: Joe Faust PEP and SPEAR storage rings. High-energy physics Graphic Arts: Walter Zawojski research at these facilities resumes this fall while ex- Production: PublicationsDepartment tensive improvements are in progress at the SLC. Major advances in accelerator physics occurred IN THIS ISSUE: during the recent SLC run. For the very first time First SLC Run Ends .......... 2 ever, bunches of electrons and positrons smaller than The Mark II Detector at the SLC . 3 5 microns in radius were collided successfully. The Search Quickens for Ultimate Particles . 10 slight deflection of one beam by the other, first wit- 16th SLAC Summer Institute .. ... 12 nessed in June (SLAC Beam Line, June 1988, p. 2), Twenty Years Ago . .. .... 15 has now been developed into a powerful steering aid Riordan Wins Science Writing Award . .. 15 that can be used to aim the beams to a relative ac- Dates to Remember .. ....... 15 curacy of 1 micron. And in early September strong, Joe Faust Retires. ...... .. 16 clear signals were observed for a phenomenon unique Cortez Heads DOE Office .. ..... 16 to the SLC called beamstrahlung- bursts of electro- Stanford University operates SLAC under magnetic radiation thrown forward by two compact contract with the U.S. Department of Energy. bunches as they cross. Although the basic principles of a linear collider With this issue the SLAC Beam Line begins a have been demonstrated, the SLC must maintain its small shift in direction. We will be attempting to beams in collision for long periods of time in order reach a slightly different audience than in past years. to serve as a useful tool for physics research. In all, The community of physicists from other institutions beams collided for about 60 hours this summer, and who use the SLAC facilities need a publication that 34 hours worth of data were logged on the big Mark addresses their interests, too. Therefore readers will II detector, but no Z°'s were observed during this pe- find increasing emphasis in these pages on scien- riod. Hardware problems and untimely component tific matters. Other topics of interest purely to failures, together with the long time presently re- SLAC staff will be reserved mainly for the Personnel quired to retune the collider, have been the principal Newsletter edited by Marian Wehking. reasons for this performance. Heading this issue is an article beginning on page An important development that occurred during 3 about the Mark II detector and how it will extract August was the successful test and subsequent use of information on the Z° particle. Following that are a subtle technique known as "BNS damping." Never a piece by Malcolm Browne excerpted from the New before attempted, it restricts the growth of trans- York Times and a survey of the recent SLAC Sum- verse bulges in the particle bunches as they travel mer Institute. On the last two pages are several short down the two-mile linac, leading to large reductions articles of general interest to the community. in the size of their troublesome tails. Other promis- We invite contributions from SLAC physicists, ing techniques have helped the SLC achieve smaller staff, and visitors. Deadline for the December issue spots (3-5 microns) at the interaction point and con- is Monday, October 31. Special thanks on this issue trol slow drifts seen there in the beam positions. are due Shirley Boozer for typesetting and Sylvia While improvements are under way at the SLC, at- MacBride for illustrations and paste-up work. Un- tention turns to the SPEAR and PEP rings. Physi- less otherwise indicated, all photos are by Joe Faust. cists working on the Mark III detector at SPEAR -the Editors plan to search for rare objects in decays of the O' particle and hope to study the production and de- COVER PHOTO: The Mark II detector being cays of the rare F meson at a combined energy of prepared for its first data-taking run at the about 4 GeV. At PEP SLC interaction point. Bunches of high-energy the TPC collaboration plans extensive studies electrons enter from the lower left and collide with of the r lepton and B mesons at an positrons inside this detector, which records the energy of 27 GeV. After the improvements have been tracks and energy deposits of particles emerging completed in January, top priority will return to the from these collisions. (Photo by Joe Faust) SLC, which will run through August 1989. -Michael Riordan r-~~~~~ 9%n~l SLAC Beam Line, September 1988 3 1 I -_ I -I THE MARK II DETECTOR AT THE SLC by Kathy O'Shaughnessy For over a decade the Mark II has been a familiar, upgrade for the Mark II, to be used for initial ex- successful particle detector at SLAC. Originally built periments at the SLC. After the proposal was ac- by the SLAC-LBL collaboration in 1977 to replace cepted, the major new parts were built and installed the Mark I detector at SPEAR, it was designed to for a check-out run at PEP in 1985. During the sum- study in detail the , and r particles discovered there. mer of 1986, the 1800-ton detector was disassembled and moved, piece by ponderous piece, to the Collider Meanwhile SLAC was building the PEP storage Experimental Hall. The reconstructed detector was ring with a higher energy, expecting the first beams tested using cosmic rays from the fall of 1986 until to be available in 1980. The collaboration proposed November 1987, when it was finally rolled into the to upgrade and move the Mark II detector to PEP beamline. after it had finished taking data at SPEAR. The idea SLC was that a seasoned, well-understood detector could The main physics goal of the Mark II collabora- be moved to PEP and be able to analyze data soon tion is to study the Z° particle, the heaviest known after beams began colliding. The Mark II finished elementary particle. The total electron-positron col- running at SPEAR, was moved to PEP during the lision energy of the SLC was designed to be approxi- summer of 1979, and started logging data in 1980. mately 92 billion electron volts (92 GeV), the energy Although no new particles were discovered with the needed to create the Z°. Largely because of its huge detector, the experimenters were able to study the mass-energy, only about 100 events containing Z°'s b quark (discovered at Fermilab in 1977) and fulfill have been observed so far at CERN and Fermilab. other important goals. As one of the particles that carries the weak nu- clear force, the Z° lives only for a very short time, decaying into the elementary particles that make up matter. There are two types of matter particles - called quarks and leptons. The Z° decays into a quark or lepton plus its corresponding antiparticle (see Figure 2). All of the Z° events observed so far have been its decays into leptons: Z° -- e+e- or Z° -, ~+,- . Mark II physicists will have the first opportunity to see the Z° decay into quarks. I e+ Q / ei. eX qC I e- 0 FIGURE 2. Diagrams illustrating production FIGURE 1. Part of the Mark II detector being of the Z° particle and its subsequent decays lowered into the Collider Experimental Hall. into either quarks (q = u, d, s, c, b, ...) or leptons A similar sequence of events happened several (I = e, ei, I/,, ,Vr, T ...) and their antiparticles. building years later. In 1983, SLAC officially began Quarks never appear in nature as single particles; Collider a new type of collider. The Stanford Linear rather they come in groups of two or three. These times higher (SLC) would reach energies over three groups of quarks, called hadrons, are the particles team joined with than PEP. The original SLAC-LBL we can actually detect. There are many different still another six other universities* and proposed hadrons, but the ones that commonly leave tracks *Caltech, UC Santa Cruz, University of Colorado, Uni- in detectors are among the lightest - pions, kaons versity of Hawaii, Johns Hopkins University and the and protons. Heavier hadrons that contain the initial University of Michigan. Indiana University joined later. quarks from the decay of the Z° (the primary quarks) I I 44 SLAC Beam Line, September 1988 Flux Muon System Cent Drift Liquid Argon End ( Calorimeter Calor Magnet Coil Time-of-Flight Counters FIGURE 3. Cut-away view of the 1\ark II detector, with the end cap removed and the central drift chamber shifted left along the beam axis. quickly decay into these hadrons and sometimes into pieces fit together to give us a complete picture of leptons as well. So we have a difficult task, trying the Z° particle and its decays. to work back to the primary decay using the end products of all of the subsequent decays. Magnet The result of a Z° decay will be anywhere from two We need to measure the momenta of the particles to fifty particles spraying out radially from the colli- emerging from a decay of the Z°.
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