The major role of British participants at LEP lies with the experiments that record the aftermath of the electron-positron annihilations. Physicists and engineers from 15 British universities, as well as from the Rutherford Appleton Laboratory (RAL), are involved in three of the experiments - ALEPH, DELPHI and OPAL funded by the PPARC.
At Imperial College, London, a team has built the inner tracking chamber for ALEPH. This detector fits around the beam pipe in which the annihilationsoccur. It reveals the first few points on the tracks of charged particles that spray out into the remainder of the apparatus and allows the physicists working with ALEPH to reconstruct these tracks more precisely. The detector has pioneered the use of a new timing technique to locate the positions of tracks along the length of the detector.
The next layer of ALEPH is a large tracking detector. This consists of a gas-filled chamber, with wires and metal pads at the ends to pick up the tiny amounts of electrical charge left in the wake of an energetic charged particle. This detector was built at CERN, but a team from Glasgow University worked on a system to calibrate it. The technique is to use a laser beam to release charge in the chamber, thereby simulating the passage of charged particles. In addition, a team at Royal Holloway and Bedford New College, London, has worked with RAL on the trigger electronics, which use signals from the tracks in deciding whether to record an annihilation.
The U.K.'s other major contribution to ALEPH was to build the end caps that close up the barrel of the electromagnetic calorimeter. These are layers of lead and particle detectors, which were constructed in the workshops at Glasgow University and at RAL. Royal Holloway and Bedford New College and Lancaster and Sheffield Universities also provided valuable assistance in building these parts.
Some of the most innovative work for the LEP detectors lies in the DELPHI experiment, for which RAL designed and built the largest superconducting solenoid in the world. RAL has also contributed to the smallest part of DELPHI - a detector made of silicon that reveals precisely the first points on the tracks of particles as they emerge from the beam pipe. One of DELPHI's strongest points lies in a comprehensive system for identifying particles based on detectors called ring imaging Cherenkov counters. To accommodate these, the tracking chamber is smaller than usual, but this does not mean that DELPHIloses out in precision in tracking charged particles. A layer of additional tracking detectors lies outside the Cherenkov counters to provide more points on the tracks. These chambers, which have been built at Liverpool University in collaboration with local industry, can locate tracks to a precision of the width of a human hair.
Oxford University has also contributed to DELPHI in providing the muon detectors that form the outer layer of the apparatus. Identifying muons and measuring their tracks precisely is an important feature of all the experiments. Muons are emitted when particles containing heavier quarks, such as charm and bottom, change into lighter particles through the agency of the weak force. The muons are a signature of the original charm and bottom particles.
In OPAL, British universities are again involved in the initial measurement of tracks emerging from the beam pipe. A team from Queen Mary Westfield College, London, Cambridge University and RAL has provided electronics to process the signals from the innermost detector, which was built in Canada. This team has also worked on the 'trigger' that uses information from the tracking detectors within OPAL's magnet. In addition, the universities developed a system to monitor the detectors in the end portions of the electromagnetic calorimeter, which were built at RAL. They consist of large, shaped blocks of polished lead-glass in which electrons, positrons and photons produce sprays of light. The light is detected and converted to electric signals by devices designed especially to work in the high magnetic field in the region of the end-caps. This collaboration has also designed and built a recent addition - a silicon inner vertex detector.
The UK has also supplied the outermost layer of OPAL - the muon detectors. The cylindrical barrel of detectors that surrounds OPAL was built at Manchester University. The physicists at Manchester developed a new way of finding the positions where tracks cross wires in the detector. This enables them to locate the tracks to an accuracy of 1 millimetre, although the wires themselves are 10metres long. The ends of the muon barrel are closed by detectors of a slightly different construction. These end-caps are the responsibility of Birmingham University and were built at RAL.
All the LEP experiments contain 'forward' detectors to track particles that emerge close to the path of the original electron and positron beams. In theOPAL experiment, these detectors are partly the responsibility of a team from Birkbeck and University Colleges, London, and Brunel University. Components to track the forward-going particles have been built in the workshops at University College and at Brunel. The team from University College intends to apply the same techniques in future to X-ray detectors for medical use.
