Geneva, 6 November 1995. CERN1's Large Electron-Positron Collider LEP has moved up a gear. On 31 October, particle collisions were observed for the first time at 130 GeV, the highest energy ever achieved in an electron-positron collider. After six years of studying the elementa ry particle known as the Z, LEP moved smoothly up to its new energy, bringing the possibility of discovering new particles and furthering our understanding of how the Universe works.
LEP was designed to study the weak nuclear force, which fuels the sun, and is responsible for some forms of natural radioactivity. The weak force is carried by particles called W+, W - and Z. In its first phase, LEP produced collisions with just the rig ht energy to make the Z, which 'weighs' just over 90 GeV (about 10-22 (ten to the minus 22) grammes). In its second phase, scheduled to begin in the second half of next year, LEP will run at nearly twice this energy, sufficient to produce W+ and W - partic les in pairs, completing studies of the weak force.
LEP's Z era came to an end in the last week of September. Then, as a further step of the LEP Upgrade Programme, 16 new superconducting accelerating cavities were installed, in addition to the 44 already in LEP. With these 60 superconducting cavities, the LEP accelerating system provides a total voltage of 800 Million Volts, and is already by now the most powerful superconducting particle accelerating system ever built. LEP started up again on 28 October: three days later, physicists were enthusiastically awaiting new beams of higher energy. They were not disappointed as LEP produced collisions at the first attempt.
Events are available from ALEPH, DELPHI, L3 and OPAL.
The speed with which the accelerator was upgraded and restarted is unprecedented, and a remarkable achievement by the LEP team. LEP's energy record was broken several times on the way. Beam energies reached 69.875 GeV during tests and then the two beam s were collided at 65 GeV each. The machine's performance already rivals that achieved at lower energies, and is set to improve over the coming weeks, by careful tuning; the plan is then to wind up the collision energy to 140 GeV before the annual winter shutdown.
With every increase in energy comes the possibility of discovery, and CERN physicists are eagerly waiting to see what the 130-140 GeV run will reveal. Theory predicts that new particles, called supersymmetric particles, may exist at energies higher than those reached until now. Supersymmetry says that for every particle we currently know, there exists a supersymmetric partner. This idea answers many of the questions left open by our present theory, the Standard Model. If supersymmetry is correct, one of the lightest supersymmetric particles, the chargino, could now be within LEP's reach, and CERN physicists are keeping their eyes wide open for tell-tale signs.
1. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, Turkey, Yugoslavia (status suspended after UN embargo, June 1992), the European Commission and Unesco have observer status.