SLAC

acronym of the Stanford Linear Accelerator Center located in Stanford, California, U.S. An exemplar of post-World War II Big Science, SLAC is a laboratory for research in particle physics (the study of the fundamental constituents of matter). It is run by Stanford University for the U.S. Department of Energy but used by physicists from across the United States and from other countries. It houses the longest linear accelerator (linac) in the worlda machine 3.2 km (2 miles) long that accelerates electrons up to energies of 50 gigaelectron volts (GeV). The concept of a multi-GeV electron linac grew from the successful development of smaller electron linacs at Stanford University, culminating in the early 1950s in a 1.2-GeV machine. In 1961 plans for the new machine, designed to reach 20 GeV, were authorized, and the 3.2-km linac was completed in 1966. In 1968 experiments at SLAC found the first direct evidence for further structure (i.e., quarks) inside protons and neutrons. As early as 1961, design work began for an additional machine at SLACan electron-positron collider called SPEAR (Stanford Positron-Electron Asymmetric Rings). Construction did not begin until 1970, but the machine was completed within two years, producing collisions at energies of 2.5 GeV per beam. In 1974 SPEAR was upgraded to reach 4.0 GeV per beam, and physicists working with it soon discovered a new type of quark, which became known as charm, and a new, heavy relative of the electron, called the tau. SPEAR was followed by a larger, higher-energy colliding-beam machine, the PEP (Positron-Electron Project), which began operation in 1980 and took electron-positron collisions to a total energy of 36 GeV. The SLAC Linear Collider (SLC) was completed in 1987. SLC uses the original linac, upgraded to reach 50 GeV, to accelerate electrons and positrons before sending them in opposite directions around a 600-metre (1,970-foot) loop, where they collide at a total energy of 100 GeV. This is sufficient to produce the Z particle, the neutral carrier of the weak nuclear force that acts on fundamental particles. Christine Sutton