Z FACTORY

The highest-energy electron-positron colliders ever built were LEP and SLC. Both projects were initiated around 1980 as Z factories to study the properties of the Z boson at a center-of-mass energy of approximately 91 GeV. The Large Electron Positron project (LEP) was a conventional circular storage ring built at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. The Stanford Linear Collider (SLC), built at the Stanford Linear Accelerator Center (SLAC) in California, was a prototype for an entirely new approach to electron-positron colliders. The switch to a new technology was motivated by the fact that the circumference of an electron storage ring has to increase as the square of the desired energy. To reach an eventual energy of 100 GeV per beam, LEP required a 27-km tunnel bored underneath the Jura Mountains in France and Switzerland. To reach ten times higher energy in a storage ring, the tunnel would have to be more than 2,000 km long, too big to be practical. The ring has to be so long because electrons radiate energy in the form of light whenever they bend out of a straight path. The sharper the bending angle, the more energy is lost. This energy must be replaced on each turn the electrons make around the ring, so a large ring is necessary to limit the energy loss.

The new technique pioneered at the SLC was to use a linear accelerator, the 3-km-long SLAC linac, to raise the electrons and positrons to the desired energy. Because the electrons are not bent in a circle, the tunnel length only grows linearly with energy, and the cost can be kept reasonable. Present designs for linear colliders to reach 1,000 GeV are about 30 km long, only slightly longer than LEP. The disadvantage of the linear collider approach is that the bunches only collide at the machine repetition rate, about 100 times a second. In a storage ring the bunches collide tens of thousands of times a second. In a collider the rate of events per second, or luminosity, depends on the bunch crossing frequency and the density of particles in the bunch. Since a linear collider has many fewer crossings per second, it must use bunches that have a much smaller size. The challenges for a linear collider are to produce bunches of electrons cooled to a very small phase space, or emittance; to preserve that emittance as the bunches are accelerated; and then to focus the bunches to a very small size at the interaction point.

LEP built on the experience from a long series of electron-positron storage rings, and many problems were already familiar and their solutions well understood. LEP began operation in 1989, was commissioned rather quickly, and reached design performance in 1993. In 1992 the number of bunches was doubled from four to eight by using a pretzel scheme to separate the orbit of the two beams at unwanted extra collision points. In 1995 for the last Z physics run, four trains of up to three bunches were used. LEP was then converted to LEP2, and the energy increased to eventually 104.4 GeV per beam. To reach these energies required the largest superconducting rf system ever built, with more than 3 GV of accelerating voltage. One of the most interesting challenges at LEP was determining the exact energy of the beams for precision measurements of the Z mass. Because the LEP ring is so large, the circumference changes by ±1 mm due to the tidal stretching of the Earth's surface. Seasonal changes in the local water table have a measurable effect on the tunnel. It was also found that the field of the bending dipoles increased steadily during each fill as a result of the thermal expansion of the concrete cores on which the magnet coils were wound and of leakage currents from the rails of nearby high-speed electric trains. To accurately predict the beam energy, the temperature and stray currents as well as numerous other quantities had to be carefully monitored.

The SLC was the first linear collider ever attempted, and it took many years to develop the understanding and techniques required to collide very small beams. New diagnostics and new procedures were needed to center the beams through the focusing magnets and accelerating structures to minimize effects that could increase the beam size. An innovative algorithm for finding the best beam trajectory, called dispersion-free steering, was used first at SLC and later at LEP to improve beam quality. A key breakthrough was moving micrometer-sized wires through the beams to measure the beam size noninvasively during routine operation. Feedback systems were required to stabilize the energy and position of the beams throughout the machine. Specialized feedback was used to bring the tiny beams into collision and even to optimize the final tuning of the beam size. The SLC eventually had more than forty wire scanners and fifty feedback systems. The beam size at the collision point was 1.4 micrometers horizontally and 0.7 micrometers vertically, much smaller than a human hair and about a factor of 100 smaller than in storage rings. With such small intense beams, the interaction of the two beams causes them to shrink even further in size. This effect is called pinch enhancement and was measured for the first time at the SLC, where it increased the luminosity (event rate) by as much as a factor of 2.

LEP had four experimental detectors: Aleph, Delphi, L3, and Opal. SLC had only one interaction point and began with the Mark-II, which was replaced by the SLD experiment in 1991. Because LEP reached high luminosity more quickly, the LEP experiments had the advantage of a much larger data sample. The SLD exploited two unique features of a linear collider: the very small interaction point and a highly polarized electron beam, where the electron spins point predominantly in the same direction. These advantages allowed SLD to make measurements that were complementary to the high-precision LEP measurements, including the world's most precise measurement of the critical electroweak mixing angle.

Building on the success of the SLC, many groups around the world are actively developing proposals for a new linear collider project to start at an energy of 500 GeV and eventually reach 1,000 GeV or higher. High-energy physics advisory groups in the United States, Europe, and Asia have all recently endorsed such a collider as the next major project needed to advance their research. Several different technologies for accelerating the beams are under study. The TESLA proposal led by the Deutsches Elektronen-Synchroton Laboratory (DESY) in Hamburg, Germany, uses superconducting cavities. The NLC proposal led by SLAC and the JLC proposal led by the Japanese High-Energy Accelerator Research Organization (KEK) in Japan use accelerating structures similar to those at the SLC, but at a higher frequency. The CLIC proposal led by CERN uses a novel two-beam design, which could form the basis for a later linear collider of even higher energy.

See also:Accelerators, Colliding Beams: Electron-Positron

Bibliography

Assmann, R., et al. "Calibration of Center-of-Mass Energies at LEP-1 for Precise Measurements of Z Properties." European Physical JournalC6 , 187–223 (1999).

Barklow, T., et al. "Experimental Evidence for Beam-Beam Disruption at the SLC." <http://accelconf.web.cern.ch/ AccelConf/p99/PAPERS/WEBR3.PDF>.

SLD Collaboration (Kenji Abe, et al.) "A High Precision Measurement of the Left-Right Z Boson Cross-section Asymmetry." Physical Review Letters84 , 5945–5949 (2000).

Sally Dawson