As plans for the LHC proton collider to be built in CERN’s 27 km LEP tunnel take shape, interest widens to bring in the experiments exploiting the big machine. The first public presentations of ‘expressions of interest’ for LHC experiments featured on 5–8 March at Evian-les-Bains on the shore of Lake Geneva, some 50 km from CERN, at the special ‘Towards the LHC Experimental Programme’ meeting.
This event followed soon after CERN Council’s unanimous December 1991 vote that the LHC machine, to be installed in the existing 27 km LEP tunnel, is ‘the right machine for the advance of the subject and for the future of CERN’. With detailed information on costs, feasibility and prospective delivery schedules to be drawn up before the end of next year, and now with plans for experiments under discussion, the preparations for LHC move into higher gear. The Evian meeting was a public forum for a full range of expressions of interest in LHC experiments, setting the stage for the submission of Letters of Intent later this year and cementing the proto-collaboration arrangements.
Participants at the meeting also heard the latest news on LHC machine studies, and the thinking on preparations for experimental areas and LHC physics potential. As well as its main objective of proton–proton collisions, LHC also opens up possibilities for ion–ion collisions, for fixed-target studies and eventually for electron–proton collisions as well. Most of these areas were covered at Evian.
LHC beams can in principle collide at eight points. Four of these coincide with the four big experiments at the LEP electron–positron collider. Of the remaining four points, one, deep under the Jura mountains, will have to be used for an LHC ‘beam-cleaning’ system to ensure high performance by reducing troublesome beam halo. Another will be reserved for the beam dump where the LHC protons will be absorbed once the circulating beams are no longer required. This leaves room for two big new LHC-collider detectors, plus the potential of the existing LEP experimental areas, using either adapted LEP experiments or new apparatus mounted in push–pull to alternate with LEP running.
At Evian, four major detectors for studying proton–proton collisions were being tabled, three of which are new, and one a development from an existing LEP experiment. The ASCOT (Apparatus with SuperCOnducting Toroids) general purpose detector is proposed by a team from CERN, the UK (Edinburgh and Rutherford Appleton Laboratory), Germany (Wuppertal and Munich MPI and University), France (Saclay) and Russia (Moscow, Dubna and Protvino). It is based on a 24 m long superconducting toroid instrumented with drift tubes for precision muon detection.
Inside the magnet, the emphasis is on electrons, with a lead/liquid argon electromagnetic calorimeter, and tracking through interleaved layers of scintillators and transition radiation detectors, with semiconductor pads close to the beam pipe. A 1.5 T superconducting solenoid in front of the electromagnetic calorimeter distinguishes electrons and positrons. Hadron calorimetry uses iron and liquid argon.
The EAGLE (Experiment for Accurate Gamma, Lepton and Energy measurements) collaboration proposes a comprehensive detector to cover a wide range of physics, and already involves physicists from 14 CERN member states, plus Canada, Russia, Australia, Brazil and lsrael. EAGLE foresees a powerful inner-electron detector inside a 2 T central superconducting solenoid. The design features high-quality electromagnetic sampling calorimetry combined with fine-grained electron and photon preshower detection, a high-precision vertex detector for lower collision rates, hadron calorimetry and a conventional toroid muon spectrometer.
The Compact Muon Solenoid (CMS) LHC detector is designed to be compatible with the highest LHC collision rates, and is built around a 15 m long superconducting solenoid providing a 4 T field. The strong field gives relatively compact muon measurement. R&D work for the muon detectors is looking at resistive-plate chambers and parallel plate chambers for timing information and honeycomb-strip chambers and wall-less drift chambers for spatial information. The central tracker will use small cells, based on silicon (or gallium arsenide) strip detectors and microstrip gas chambers, to ensure good pattern recognition under the stringent LHC conditions. Also inside the coil is a high-resolution electromagnetic calorimeter and a hadron calorimeter. CMS involves a team from 12 CERN member states, plus Byelorussia, Bulgaria, Estonia, Georgia, Hungary, Russia and the US.
The L3 experiment at LEP was originally designed for use at both LEP and LHC, with a large experimental hall and magnet. Upgrade for LHC would involve improving the muon resolution, adding a fine-grain hadron calorimeter, increasing the magnetic field, and being able to lift the detector 120 cm from the LEP position to the LHC beams above. For the work, 39 institutes from the L3 LEP line-up have been joined by 20 more, mainly from China and the former Soviet bloc.
Supplementing the main proton–proton LHC programme are a range of other experiments, including fixed-target studies. Expressions of interest received so far include ideas for two neutrino experiments and three studies concentrating on CP violation in B-particle decays, one using a gas-jet target, one using extracted beams and one a colliding-beam setup.
Although not the spearhead of LHC physics, ion–ion collisions will still play a major role, continuing a CERN tradition in this field. For ion collisions, three teams are interested – one using CMS, another using the (suitably modified) Delphi experiment at LEP and a third using a new dedicated detector.
More than 600 members of the potential LHC user community met at Evian. Introducing the event, Organizing Committee chairman Gunter Flügge of Aachen traced the previous history of major international get-togethers and other milestones which have delineated LHC progress: from the 1984 Lausanne workshop where the LHC idea was launched, through the valuable 1987 recommendations of the CERN Long Range Planning Committee under the chairmanship of Carlo Rubbia, to the 1989 Barcelona meeting on Instrumentation Technology and the 1990 Aachen workshop to study the physics objectives. Wrapping up at Evian, CERN director-general Carlo Rubbia proposed an ongoing schedule for the selection of LHC experiments, with Letters of Intent to be submitted after the summer for selection at the end of the year. The selected experiments would then proceed with a full design report. Whatever the outcome of this selection, Evian will always be remembered as the stage where these ideas made their public debut.
• Compiled from April 1992 pp1–3 and May 1992 pp1–3.
