Interview: Lessons from LEP

The Large Electron Positron collider (LEP) changed particle physics, and CERN, forever. Former Director-General Herwig Schopper describes what it took to make LEP happen.

When was the first LEP proposal made, and by whom?

Looking forward

Discussions on how to organise a “world accelerator” took place at a pre-ICFA committee in New Orleans in the early 1970s. The talks went on for a long time, but nothing much came out of them. In 1978 John Adams and Leon Van Hove – the two CERN Director-Generals (DGs) at the time – agreed to build an electron–positron collider at CERN. There was worldwide support, but then there came competition from the US, worried that they might lose the edge in high-energy physics. Formal discussions about a Superconducting Supercollider (SSC) had already begun. While it was open to international contribution, Ronald Reagan’s “join it or not” approach to the SSC, and other reasons, put other countries off the project.

Was there scientific consensus for a collider four times bigger than anything before it?

Yes. The W and Z bosons hadn’t yet been discovered, but there were already strong indications that they were there. Measuring the electroweak bosons in detail was the guiding force for LEP. There was also the hunt for the Higgs and the top quark, yet there was no guidance on the masses of these particles. LEP was proposed in two phases, first to sit at the Z pole and then the WW threshold. We made the straight sections as long as possible so we could increase the energy during the LEP2 phase.

What about political consensus?

The first proposal for LEP was initially refused by the CERN Council because it had a 30 km circumference and cost 1.4 billion Swiss Francs. When I was appointed DG in February 1979, they asked me to sit down with both current DGs and make a common proposal, which we did. This was the proposal with the idea to make it 22 km in circumference. At that time CERN had a “basic” programme (which all Member States had to pay for) and a “special” programme whereby additional funds were sought. The latter was how the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS) were built. But the cost of LEP made some Member States hesitate because they were worried that it would eat too much into the resources of CERN and national projects.

How was the situation resolved?

After long discussions, Council said: yes, you build it, but do so within a constant budget. It seemed like an impossible task because the CERN budget had peaked before I took over and it was already in decline. I was advised by some senior colleagues to resign because it was not possible to build LEP on a constant budget. So we found another idea: make advance payments and create debts. Council said we can’t make debts with a bank, so we raided the CERN pension fund instead. They agreed happily since I had to guarantee them 6% interest, and as soon as LEP was built we started to pay it back. With the LHC, CERN had to do the same (the only difference was that Council said we could go to a bank). CERN is still operating within essentially the same constant budget today (apart from compensation for inflation), with the number of users having more than doubled – a remarkable achievement! To get LEP approved, I also had to say to Council that CERN would fund the machine and others would fund the experiments. Before LEP, it was usual for CERN to pay for experiments. We also had to stop several activities like the ISR and the BEBC bubble chamber. So LEP changed CERN completely.

How do LEP’s findings compare with what was expected?

It was wonderful to see the W and Z discovered at the SPS while LEP was being built. Of course, we were disappointed that the Higgs and the top were not discovered. But, look, these things just weren’t known then. When I was at DESY, we spent 5 million Deutsche Marks to increase the radio-frequency power of the PETRA collider because theorists had guaranteed that the top quark would be lighter than 25 GeV! At LEP2 it was completely unknown what it would find.

What is LEP’s physics legacy?

These days, there is a climate where everything that is not a peak is not a discovery. People often say “not much came out from LEP”. That is completely wrong. What people forget is that LEP changed high-energy physics from a 10% to a 1% science. Apart from establishing the existence of three neutrino flavours, the LEP experiments enabled predictions of the top-quark mass that were confirmed at Fermilab’s Tevatron. This is because LEP was measuring the radiative corrections – the essential element that shows the Standard Model is a renormalisable theory, as shown theoretically by ’t Hooft and Veltman. It also showed that the strong coupling constant, αs, runs with energy and allowed the coupling constants of all the gauge forces to be extrapolated to the Planck mass – where they do not meet. To my mind, this is the most concrete experimental evidence that the Standard Model doesn’t work, that there is something beyond it.

How did the idea come about to put a proton collider in the LEP tunnel?

When LEP was conceived, the Higgs was far in the future and nobody was really talking about it. When the LEP tunnel was discussed, it was only the competition with SSC. The question was: who would win the race to go to higher energy? It was clear in the long run that the proton machine would win, so we had the famous workshop in Lausanne in 1983 where we discussed the possibility of putting a proton collider in the LEP tunnel. It was foreseen then to put it on top of LEP and to have them running at the same time. With the LHC, we couldn’t compete in energy with the SSC so we went for higher luminosities. But when we looked into this, we realised we had to make the tunnel bigger. The original proposal, as approved by Council in October 1981, had a tunnel size of 22 km and making it bigger was a big problem because of the geology – basically we couldn’t go too far under the Jura mountains. Nevertheless, I decided to go to 27 km against the advice of most colleagues and some advisory committees, a decision that delayed LEP by about a year because of the water in the tunnel. But it is almost forgotten that the LEP tunnel size was only chosen in view of the LHC.

Are there parallels with CERN today concerning what comes next after the LHC?

Yes and no. One of the big differences compared to the LEP days is that, back then, the population around CERN did not know what we were doing – the policy of management was not to explain what we are doing because it is “too complicated”! I was very surprised to learn this when I arrived as DG, so we had many hundreds of meetings with the local community. There was a misunderstanding about the word “nuclear” in CERN’s name – they thought we were involved in generating nuclear power. That fortunately has completely changed and CERN is accepted in the area.

What is different concerns the physics. We are in a situation more similar to the 1970s before the famous J/ψ discovery when we had no indications from theory where to go. People were talking about all sorts of completely new ideas back then. Whatever one builds now is penetrating into unknown territory. One cannot be sure we will find something because there are no predictions of any thresholds.

What wisdom can today’s decision-makers take from the LEP experience?

In the end I think that the next machine has to be a world facility. The strange thing is that CERN formally is still a European lab. There are associates and countries who contribute in kind, which allows them to participate, but the boring things like staff salaries and electricity have to be paid for by the Member States. One therefore has to find out whether the next collider can be built under a constant budget or whether one has to change the constitutional model of CERN. In the end I think the next collider has to be a proton machine. Maybe the LEP approach of beginning with an electron–positron collider in a new tunnel would work. I wouldn’t exclude it. I don’t believe that an electron–positron linear collider would satisfy requests for a world machine as its energy will be lower than for a proton collider, and because it has just one interaction point. Whatever the next project is, it should be based on new technology such as higher field superconducting magnets, and not be just a bigger version of the LHC. Costs have gone up and I think the next collider will not fly without new technologies.

You were born before the Schrödinger equation and retired when LEP switched on in 1989. What have been the highs and lows of your remarkable career?

I was lucky in my career to be able to go through the whole of physics. My PhD was in optics and solid-state physics, then I later moved to nuclear and particle physics. So I’ve had this fantastic privilege. I still believe in the unity of physics in spite of all the specialisation that exists today. I am glad to have seen all of the highlights. Witnessing the discovery of parity violation while I was working in nuclear physics was one.

How do you see the future of curiosity-driven research, and of CERN?

The future of high-energy physics is to combine with astrophysics, because the real big questions now are things like dark matter and dark energy. This has already been done in a sense. Originally the idea in particle physics was to investigate the micro-cosmos; now we find out that measuring the micro-cosmos means investigating matter under conditions that existed nanoseconds after the Big Bang. Of course, many questions remain in particle physics itself, like neutrinos, matter–antimatter inequality and the real unification of the forces.

With LEP and the LHC, the number of outside users who build and operate the experiments increased drastically, so the physics competence now rests to a large extent with them. CERN’s competence is mainly new technology, both for experiments and accelerators. At LEP, cheap “concrete” instead of iron magnets were used to save on investment, and coupled RF cavities to use power more efficiently were invented, and later complemented by superconducting cavities. New detector technologies following the CERN tradition of Charpak turned the LEP experiments into precision ones. This line was followed by the LHC, with the first large-scale use of high-field superconducting magnets and superfluid-helium cooling technology. Whatever happens in elementary particle physics, technology will remain one of CERN’s key competences. Above and beyond elementary particle physics, CERN has become such a symbol and big success for Europe, and a model for worldwide international cooperation, that it is worth a large political effort to guarantee its long-term future.

About the author

Interview by Matthew Chalmers editor.