SLAC (STANFORD LINEAR ACCELERATOR CENTER)

The Stanford Linear Accelerator Center (SLAC) is a National Laboratory funded chiefly by the U.S. Department of Energy. Located on the campus of Stanford University in Menlo Park, California, it began operation in 1966 as a laboratory dedicated to high-energy physics, with a two-mile-long linear electron accelerator as its tool to study matter on very tiny scales. This accelerator followed the design of earlier machines in the Stanford High Energy Physics Laboratory developed by William Hansen. SLAC's early development was led by Edward Ginzton and Wolgang K. H. Panofsky.

The high-energy physics program of physics at SLAC has garnered numerous awards, including three Nobel Prizes for experimental discoveries that are key to the understanding of particles. The program continues to be at the forefront of particle physics, in part because every ten years or so a new addition or upgrade to the facility has been made, opening up new research opportunities. These include the Stanford Positron Electron Asymmetric Ring (SPEAR), a 3.6 GeV electron-positron storage ring (1972); the Positron Electron Project (PEP) a similar but larger facility capable of storing 9 GeV electrons and positrons (1980); the SLAC Linear Collider (SLC) (1989); and most recently PEPII, an upgrade and rebuilding of the PEP ring with the addition of a second lower energy storage ring in the same tunnel to make the SLAC asymmetric B factory(1998).

The first round of SLAC experiments, conducted in the late sixties and early seventies, earned a Nobel Prize in Physics in 1990 for Richard Taylor of SLAC and Jerome Friedman and Henry Kendall of MIT. Collision of electrons from the accelerator with stationary targets (such as a tank filled with hydrogen or deuterium) probed the structure within protons and neutrons and provided evidence that these are made from yet smaller objects known as quarks.

Research done at the SPEAR ring in the midseventies garnered two Nobel Prizes. In 1976, the Nobel Prize in Physics went to Burton Richter, who led the project to build the SPEAR facility and its first physics detector. The prize honored the discovery of the particle known as J/ψ , the first particle that indicated the existence of the fourth type of quark: the charm quark. This prize was shared by Samuel Ting, leader of group at Brookhaven National Laboratory,

who announced the same discovery on the same day. The 1995 Nobel Prize in Physics was shared by Martin Perl for the discovery of the tau lepton, which is the third electronlike particle (the second being the muon), and Federick Reines of the University of California, Irvine, for detection of the neutrino. The discovery of the tau lepton suggested an entire third generation of quarks and leptons, all of which have since been found. These two discoveries were key in the development of the theory now known as the Standard Model in particle physics. Particle physicists refer to the discovery of the J/ψ as the November Revolution, so great was its impact on their worldview.

Beginning as a sideshow to the high-energy physics program at SPEAR, a new idea was explored that led to a worldwide program of synchrotron light sources being used to perform a great variety of scientific research. The Stanford Synchrotron Radiation Laboratory (SSRL) at SPEAR was a pioneer in this field. In a synchrotron electron storage ring the particles are made to circulate by bending their path with strong magnetic fields. This causes them to radiate energy. This effect must be compensated for by reaccelerating the particles at intervals around the ring. So, as far as the high-energy physicists were concerned, synchrotron radiation was an annoying but unavoidable side effect of putting electrons into a storage ring. However, this radiation at SPEAR was also found to produce an intense swath of X rays. SLAC and Stanford physicists recognized this as the world's best source of X rays for diffraction scattering and other studies of the atomic-scale structure of materials. The rich program that developed from this recognition continues, with the SPEAR ring now(2002) fed by its own cyclotron accelerator and devoted solely to SSRL use. Researchers from industry and academia worldwide come to SSRL to carry out their research, studying topics as diverse as the structure of an enzyme (knowledge that helped develop the protease inhibitor treatment of AIDS), and the distribution of impurities in a silicon wafer. Worldwide there are now a number of other synchrotron-based light sources built specifically to do this work

The PEP storage ring experiments also made significant contributions to particle physics. The patterns of particles produced in the electron-positron collisions gave evidence for the part of the Standard Model theory that describes strong interaction physics, a theory known as Quantum Chromodynamics (QCD). Particles were produced in groups or jets. The existence and angular distribution of the jets confirmed predictions from the QCD theory. Meanwhile the linear accelerator, working with a fixed target, made another key contribution. Physicists devised a way to create a polarized beam of electrons, that is, a beam in which electron spins were preferentially aligned in a predetermined direction. The dependence of the outcome of collisions on the direction of the polarization of the beam tested details of the emerging Standard Model theory of weak interactions. The results confirmed predictions of this theory.

The SLC facility was built for two reasons. The first was to demonstrate that the principle of a linear collider was a feasible approach for exploring very high-energy electron-positron collisions. In a storage ring the bunches collide many times. With a linear collider, which uses two linear accelerators head to head, one avoids the problem (for high-energy physics) of synchrotron radiation energy losses. The price is that there is only one chance at colliding each bunch of electrons with a bunch of positrons. So to make the payoff in interesting events large enough (to do the experiments in reasonable time), one must make the bunches much smaller and denser at the collision point than in a storage ring. This required new technology to control and monitor the beams, and, while it took some time to get the facility running well, SLC has shown this can be done. Because SLAC has only one two-mile accelerator, a design with two arcs (and some concomitant energy loss) was used. Designs for a higher-energy true linear collider are under development worldwide as a likely next step in the high-energy physics agenda.

The second role for SLC was to produce Z bosons and study their decays. SLC began operation a little earlier and was the first to show that the Z boson decays into only three types of neutrinos, an indication that the three known repeating sets of quarks and leptons may be the complete set. This result was later confirmed with higher precision in the LEP storage ring at the European Laboratory for Particle Physics (CERN) near Geneva, Switzerland. One area where SLC could make measurements that were not feasible at LEP was in using polarized electron beams. Measuring the dependence of the Z production on the beam polarization gave an additional probe of predictions of the Standard Model.

The next new addition at SLAC was not a new higher-energy facility but instead a rebuilding of the PEP ring into an asymmetric B factory. B mesons are mesons containing b quarks. The neutral B mesons, made from a b quark and an anti-d quark, or vice versa, provide a laboratory in which to study the predictions of the Standard Model about the differences in the laws of physics for matter and antimatter. Physicists think that these differences are key to understanding why our universe contains predominantly matter and very little antimatter. When physicists try to understand how this imbalance developed in the history of the universe using the Standard Model theory, they fail to get answers that match the observations for the ratio of matter to radiation in the universe. So it is possible that physics beyond the Standard Model comes into play here. One way to look for such effects is to carefully check the patterns of differences between the decay time distribution of B and anti-B mesons to see whether they match the Standard Model predictions. The first such difference was observed at SLAC, in the decays of B and anti-B mesons to a J/ψ and a K -short meson. Similar observations were made at about the same time at a B factory facility at the KEK laboratory in Tsukuba, Japan. An asymmetry between the B and the anti-B results was established. The magnitude of the effect is consistent with the Standard Model expectations. There are still many other rates to be measured and cross-checks to be made. The facility will continue to operate, possibly with some increases in its rate of B production. It will take all this and more, experiments elsewhere also contributing to the picture, to check whether the full pattern of Standard Model predictions is borne out, or whether some anomalies suggesting new physics are found.

While the primary purpose of the laboratory is basic research in high energy physics and the synchrotron radiation applications to both basic and applied science, there are a number of ways in which this work has developed tools that have much broader application. Electron accelerators of the type developed at Stanford are found in hospitals around the world as the source of X rays for medical treatments. The computer code EGS that models the interactions of electrons and photons with matter, developed at SLAC to allow the design of radiation shielding for the experiments, has provided ways to refine X-ray treatments to give a greater radiation dose to a tumor and a lesser dose to surrounding tissue. SLAC mounted the first U.S. web site, helping to develop this particle-physics-initiated technology that has so changed the world of information technology. Synchrotron radiation studies have provided clues to help develop new medical treatments, new ways to detect small quantities of pollutants and to develop and test pollution remediation approaches, and improvements in the production of silicon wafers, to name but a few developments. Technology developed for particle physics detectors is now being used at SLAC to build a gamma ray observatory (the Gamma Ray Large Area Space Telescope [GLAST]) to be stationed in space. These rich and varied effects, often called spin-offs, are a second payoff for the money invested in such a facility.

Bibliography

Bienenstock, A., and Winick, H. "Synchrotron Radiation Research—An Overview." Physics Today 36 (6), 48–58(1983).

Feldman, G., and Steinberger, J. "The Number of Families of Matter: How Experiments at CERN and SLAC, Using Electron-Positron Collisions, Showed that There Are Only Three Families of Fundamental Particles in the Universe." Scientific American 352 (2), 70–78 (1991).

Kendall, H. W., and Panofsky, W. "The Structure of the Proton and the Neutron." Scientific American 224 (6), 60–77(1971).

Perl, M. L., and Kirk, W. T. "Heavy Leptons." Scientific American 238 (3), 50–57 (1978).

Richter, B. "Nobel Lecture." Reviews of Modern Physics 49 , 251–266 (1977).

Stanford Linear Accelerator Center. <http://www.slac.stanford.edu>.

Witherell, M., and Quinn, H. "The Asymmetry Between Matter and Antimatter." Scientific American 279 (4), 76–81(1998).

Helen R. Quinn