En mars 1984, un atelier tenu à Lausanne est l'occasion pour la communauté de la physique des particules de réfléchir à l'étape faisant suite à la construction et à l'exploitation du LEP. C'est là qu'on parle pour la première fois de construire un collisionneur de hadrons dans le tunnel du LEP.

The installation of a hadron collider in the LEP tunnel, using superconducting magnets, has always been foreseen by ECFA and CERN as the natural long term extension of the CERN facilities beyond LEP. Indeed such considerations were kept in mind when the radius and size of the LEP tunnel were decided. The recent successes of the CERN proton–antiproton collider now give confidence that a hadron collider would be an ideal machine to explore physics in the few TeV range at the particle constituent (quarks and gluons) level. The present enthusiasm for the Superconducting Super Collider (SSC) in the US reflects the impressive potential of such machines.

Although the installation of such a hadron collider in the LEP tunnel might appear still a long way off (LEP is scheduled for initial operation in 1988), it was still an opportune moment for ECFA, in collaboration with CERN, to organize a 'Workshop on the Feasibility of a Hadron Collider in the LEP Tunnel' from 21–27 March. The first four days of detailed work were held in Lausanne, at the kind invitation of the University, and were followed by two days of summary talks and discussion at CERN.

The workshop was initiated particularly by the then ECFA Chairman, John Mulvey, in keeping with ECFA's role in stimulating and coordinating plans for future particle-physics facilities in Europe. The workshop was timed to enable CERN to communicate present ideas on long-term prospects to an ICFA (International Committee for Future Accelerators) seminar held in Tokyo on 15–19 May and entitled 'Perspectives in High-Energy Physics'.

In his opening address at the workshop summary session, CERN director-general Herwig Schopper emphasized that CERN's top priorities remain the completion of LEP Phase I (to achieve electron–positron collisions up to 50 GeV per beam), followed by Phase II (taking the beam energies to around 100 GeV). Thus the Large Hadron Collider (LHC) means looking as far ahead as the middle of the next decade.

Nevertheless, LHC would have to use the infrastructure permitted by LEP. Present ECFA Chairman Jean Sacton emphasized what LEP and CERN would offer. Besides the LEP tunnel itself, the PS and SPS provide excellent proton (and antiproton) injectors. In particular, with the experience of the Intersecting Storage Rings (ISR) and the proton–antiproton Collider under its belt, CERN can claim unique experience and expertise with bunched-beam hadron colliders. The European particle-physics community is also well aware of the competition from the SSC in the US breathing down its neck.

Giorgio Brianti summed up the outcome of the LHC machine studies so far. After confirming that the LEP tunnel would indeed be suitable for such a machine, the next conclusion was that construction moreover need not interfere significantly with LEP operation, given the foreseen LEP operating schedule. Four excavated colliding beam regions are still vacant, although this may not still be the case by the time of LEP Phase II.

To be competitive, the LHC has to push for the highest-possible energies given its fixed tunnel circumference. Thus the competitivity lives or dies with the development of high field superconducting magnets. The long gestation period of LHC fits in with the research and development required for 10 T magnets (probably niobium-tin), which would permit 10 TeV colliding beams. The keen interest in having such magnets extends into the thermonuclear fusion field, and development collaborations in the US, Japan and Europe look feasible.

There are two main options – either to build a single ring and have proton–antiproton colliding beams, as in the CERN SPS Super Proton Synchrotron and scheduled for Fermilab's Tevatron, or to build two rings and have colliding proton beams. Two considerations turned the thinking firmly towards the second option. The first is the advantage of the higher luminosity (up to 1033/cm2 per s) of proton–proton collisions. The second is the complications in separating the multi-bunch proton and antiproton beams outside the collision regions, which would require cumbersome separators. These considerations outweigh the intrinsic economy of having protons and antiprotons circulating in the same ring. At the workshop, designs were presented of two-in-one magnets in single cryostats with the two proton-beam channels less than 20 cm apart.

At such high energies, there are aspects of machine operation which need special attention. For example – the enormous stored energy in the beams means that the beam-abort system would have to cope with 60 MJ, the vacuum chamber design has to take account of synchrotron radiation heating, the refrigeration system has to distribute liquid helium over tens of kilometres and be able to cope with several superconducting magnet quenches at a time. The growing experience at the Fermilab Tevatron, where the world's first superconducting synchrotron has come so impressively into operation, would provide important input into design decisions.

Preceding the workshop, studies of machine design, magnets and cryogenics had been (and continue to be) underway at CERN, with periodic meetings to review progress. This work was summarized at Lausanne, including a panel discussion on superconducting magnet design and technology.

On the experimental side, eight working groups had been set up: Jets (convener P Jenni), Electron and photon detection (P Bloch), Muon detection (W Bartel), Tracking chambers (A Wagner), Vertex detection (G Bellini), Triggering (J Garvey), Data acquisition (D Linglin) and Forward physics (G Matthiae). There was also a great deal of input from theorists, and the Lausanne theory talks were also attended by many experimentalists.

The reports of these working groups provided much valuable input, and several general conclusions emerged. The highest energy would be a valuable asset but there is no actual threshold known now. The key point is to have at least 10 TeV collision energy in order to have typically at least one TeV at the hadron constituent level. There is also a trade-off between energy and luminosity, a gain in luminosity for a loss in energy and vice versa. According to present wisdom, differences between proton–proton and proton–antiproton reactions would be in most cases too small to be detectable. Information from proton collisions should hence be adequate.

Production rates for hitherto unknown objects are 'expected' to decrease quickly with the mass of these objects, so that here high luminosity would be an advantage. Multi-bunched beams were envisaged with 3564 bunches per ring, giving 25 ns between bunches and an average of one interaction per bunch crossing. Much thought is going into particle detector performance and there is confidence that the high luminosities could be handled.

Another attractive possibility with both proton and electron rings in the same LEP tunnel is the provision of high-energy electron–proton collisions 'for free'.

No attempt was made at the workshop to arrive at even a tentative cost estimate for LHC in the LEP tunnel. The project has only been under consideration for a few months and a great deal of further study is needed. However, as Carlo Rubbia emphasized in his concluding remarks, the feasibility of the LHC has been demonstrated, a good physics case has been outlined and CERN is able to promise a great deal when future perspectives in high-energy physics are discussed.

• June 1984 pp185–187 (abridged).