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Summer 1994 Vol p A PERIODICAL OF PARTICLE PHYSICS SUMMER 1994 VOL. 24, NUMBER 2 Editors RENE DONALDSON, MICHAEL RIORDAN Executive Editor BILL KIRK Editorial Advisory Board JAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN, JOEL PRIMACK, NATALIE ROE, HERMAN WINICK Illustrations page z TERRY ANDERSON Production VANI BUSTAMANTE RAY ISLE Photographic Services TOM NAKASHIMA Distribution CRYSTAL TILGHMAN page 21 The Beam Line is published quarterly by the Stanford Linear Accelerator Center, PO Box 4349, Stanford, CA 94309. Telephone: (415) 926-2585 INTERNET: [email protected] BITNET: BEAMLINE@SLACVM FAX: (415) 926-4500 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: A transverse profile of an electron beam in the SLAC linac. Particle densities have been color coded. The bright, high-density core contains 3 x 1010 electrons. These elec- tron (and positron) beams are focused to diameters of less than a micron at their collision point, which is located at the center of the SLD detector. Printed on recycled paper As CONTENTS FEATURES 2 THE HEART OF A NEW MACHINE After years of effort, the world's first linear collider is producing exciting physics results at Stanford. Nan Phinney 12 WHATEVER HAPPENED TO THE THEORY OF EVERYTHING? A brief report on the current status of string theory. Lance Dixon 21 OPINIONS ABOUT EVERYTHING: EPAC '94 In which Werner tells about his summer vacation in London. Werner Joho -, -. I DEPARTMENTS 28 THE UNIVERSE AT LARGE Fossil Radioactivities & How We Knm .""''i 3 5 the Solar System Formed in a Hurry 10 m | Virginia Trimble 38 CONTRIBUTORS page 12 40 FROM THE EDITORS' DESK DATES TO REMEMBER page 28 The e0 New |yI .:, byNA...D :2:- i iE I I..- PHINNEY After-::IE:I:: years of effort, st linear collider is producinf results at Stanford. N THE EARLY 1980S physicists at the Stanford Linear Accelerator Center made a bold proposal to build a new type of particle accelerator, an electron-positron linear collider, to explore the properties of the Z boson. The Stanford Linear Collider, or SLC, was intended as both a prototype for a new generation of particle colliders and as an inexpensive entry into the exciting physics available with the Z. 2 SUMMER 1994 It was-a:particularly challenging task not only because it ventured into the uncharted territory of a new accel- erator technology, but also because the SLC was built us- ::ing the existing :i20-year-old SLAC linac instead of con- structing :ainew structure explicitly tailored to the requirements of a linear collider. The difficulties turned ::Oi::ut to be even greater than anticipated; it was almost two years from the completion of construction in 1987 until the first Z particle was observed by the Mark II de- tector on April 11, 1989. Later that year, the world's la::rgest electron-positron storage ring collider was com- letedatathe :CERN laboratory in Switzerlandand quick- ly grbbhed the lead: in the totalnumber iof Z:rticles. SLAG::Chas continued to push back the frontiers of A....:linear colliieri:: development and to concentrate on the ipysics advatages of two special features of the SLC- itspoilarized lectronibeam and the very small beam size :a th.i coiiision point.: In 1991 the new state-of-the-art SLDadetectorwas installed and commissioned. A po- larizedelectron source was built for the 1992 run and upgradeidi for993, provi:ding an electron beam polar- ization of over 60 percent.Steady improvements in ma- chine perform|ance have resulted in a hundredfold in- crease in lumino|sity since 1990. Now, with over 50,000 Zs recorde asttyear by the SLD collaboration, SLAC's big gamble is beginning to pay off. The SLC's com- bination of high polarization and improved luminosity provides a unique and exciting physics program. THE SLC Intended as an inexpensive first test of the linear collider concept, the SLC was designed to use the existing two- mile linac to accelerate electron and positron beams to 50 GeV. This structure was upgraded with higher pow- er klystrons, stronger focusing and steering magnets, and a sophisticated control system. Many additional pieces were required: a high-intensity electron source and positron-production system, two damping rings to re- duce the inherent size of the beams, a pair of curving beam lines each nearly one mile long to bring the two beams into collision, and a complex system of mag- netic elements to focus the beams to micron size at the collision point. A cycle of the SLC begins with two electron bunches stored in the north damping ring and two positron BEAM LINE this very complex machine turned out to be much more difficult than anticipated. Many years of effort have been required to develop the knowl- edge and experience to operate this first linear collider reliably. A major part of this effort has gone into the development of automated control and tuning procedures. By 1992 the SLC was finally ready to start a se- rious physics program with the new SLD detector. After a polarized elec- tron source was installed and suc- cessfully commissioned, more than 11,000 Zs were recorded with an av- Artist's conception of the SLC, schematically showing its principal erage beam polarization of 22 per- components. bunches stored in the south ring-all cent. of them being compressed to di- At the very end of the 1992 run, mensions of about 100 microns. Each a new operating mode with "flat" 1/120 second, one positron bunch beams (vertical size much smaller and both electron bunches are kicked than horizontal) was tested in prepa- out of the rings and accelerated down ration for the Final Focus Test Beam, the linac. The positron bunch and the a linear collider development project first electron bunch are accelerated [See Beam Line, Summer 1990, to about 47 GeV before they are sep- page 2]. Flat beams have the advan- arated by a dipole magnet at the end tage of a smaller beam area, which of the linac; they are then transported increases the rate of collisions. It is through the collider arcs and brought easy to create a small vertical beam into collision. These bunches then size in the damping rings; the chal- travel back through part of the beam lenge comes in preserving it during line until they are ejected into ex- acceleration and transport. The orig- traction lines to beam dumps. The inal SLC design had relied on opera- second electron bunch is deflected tion with round beams because of the onto a target two-thirds of the way difficulty of controlling the vertical- down the linac to produce the next to-horizontal coupling in transport bunch of positrons. Positrons are col- lines such as the arcs. In the brief test lected from this target, accelerated run, beam size ratios of 10:1 were eas- to 200 MeV and transported back to ily achieved out of the damping rings. the beginning of the linac. There they To everyone's surprise, the long years are joined by two bunches of elec- of effort developing techniques to trons from the source, accelerated to control the beams throughout the 1.2 GeV and stored in the damping SLC allowed their very small verti- rings for the next cycle. cal size to be maintained and deliv- Construction of the SLC began in ered for collisions. The luminosity, October 1983 and was completed in or event rate, doubled almost mid-1987, but the commissioning of immediately. 4 SUMMER 1994 The number of Z bosons produced by 50 the SLC from 1991 through 1993. The collider began operation with a 4 polarized electron beam in April 1992. rt 4040 | 0 30 n N 2 C 20 .o Flat beam operation was success- P1 co z fully commissioned early in the 1993 N 1 run. In this configuration, the beams 10 | can be focused to an area of 0 1.7 square microns, much smaller 0 than the original SLC design. This 1991 1992 1993 was a big step in the steady im- provement of SLC luminosity. In 1993 over 50,000 Zs were detected 80 by the SLD. POLARIZED BEAMS 60 .,-4°o At the SLC the highly polarized elec- tron beams, in which most of the 0 electrons have their spin vectors 40 aligned with (or opposite to) the di- a) rection of motion, allow physicists ct to make unique measurements. A 0o complex system of components pro- N 20 duces the polarized beam and trans- 0t ports it to the collision point. The story begins with the polarized source, which produces longitudi- 0 20000 40000 60()00 nally polarized electrons by illumi- Total Number of Z Bosons nating a photocathode with circu- The polarization of the SLC electron larly polarized photons from a high beam at the interaction point. Use of a laser power laser. In order to preserve this performance. A new Ti:Sapphire photocathode with a "strained-lattice" polarization while the beam is in the pumped by a doubled YAG laser has GaAs crystal tripled the polarization over damping ring, the spin orientation a hundred times more power and a 1992. must be rotated from the longitudi- tunable wavelength. The present nal into the vertical direction before photocathode uses a "strained lat- entering. Once the beam leaves the tice" technology to achieve much damping ring, the spin direction higher polarization. In a bulk GaAs must be correctly oriented so that cathode the laser light excites elec- it is longitudinal when it arrives at trons from two different quantum the collision point after going states.
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