by Virginia Greco. Published: 06 December 2016

The third speaker coming on stage at the 30 Years of Heavy Ions event was Herwig Schopper, the DG of CERN at the time in which hard decisions had to be taken about the LEP tunnel’s size and the heavy-ion programme at CERN.

Herwig Schopper, Director General of CERN from 1981 to 1988, has been one of the main players responsible for the establishment of heavy-ion physics at CERN. At the 30 Years of Heavy Ions celebration, he provided a very interesting account of the events that signed the beginnings of this field from the point of view “not of an expert in heavy ions but of a very interested observer and sometimes a helper”, as Schopper himself stated.

He reminded the audience that one of the earliest pioneers of the field was Rolf Hagedorn, who speculated on the heating up of strongly interacting matter in high-energy collisions. He introduced the idea that the energy content of strongly interacting matter can be increased without raising the temperature only if new degrees of freedom become available. This temperature limit took his name (Hagedorn-temperature).

On the experimental side, Schopper explained, “the study of heavy-ion reactions grew out of nuclear physics in the 1970’s and 1980’s and eventually became an interdisciplinary field of its own”.

Discussions about injecting heavy ions into CERN’s machines started in the early 1970’s and, according to Schopper, one of the first who suggested it was Giuseppe Cocconi (who had been director of the PS and worked with LBL, among the others). “I remember meeting him at CERN after long time we hadn’t seen each other,” Schopper said chuckling, “as soon as he saw me, he said: ‘Ah, you are here. You know, the whole world has become crazy, except the two of us, and I am not sure about you’”.

In 1982 a triangle was formed by GSI, LBL, and CERN around the idea of getting heavy ions to CERN. GSI and LBL decided to submit a proposal for studying relativistic nucleus-nucleus reactions at CERN, with Rudolf Bock and Reinhard Stock being the main players. A Memorandum of Understanding was also signed in which GSI promised to take to CERN an Electron Cyclotron Resonance-ion source and LBL a Radio Frequency Quadrupole Linac.

In those years Schopper was DG and his main tasks were two. First of all he had the mandate to reunite the ‘two CERN’s’ (“at that time there were CERN I and CERN II, each with its own DG; almost nobody nowadays knows this story,” he highlighted). Secondly, he had to propose LEP, get it approved and build it. But no extra money would have been put into CERN, so the new accelerator needed to be built with a constant budget, which was actually lower than before.

In order to respect these conditions, the DG had to take the difficult decision to stop many research programmes in favour of LEP (among the others, the ISR, which at the time was the only proton-proton collider in the world). “Because of this I lost many friends,” Schopper admitted.

Only one new project was started (apart from the LEP): the heavy-ion in the SPS. This was a decision that Schopper took against the recommendations of all advisory committees, who were worried for the financial restrictions. Since the budget was short, in order to be able to run their programme, each experiment had to be financed externally, by the groups proposing it, and had to exploit as much as possible existing equipment. “Despite these problems, the reaction of the interested physicists was enthusiastic,” Schopper commented, “this is when the age of heavy-ion physics actually started at CERN.”

Another very important decision that he had to take against the opinion of the advisory committee concerned the size of the new accelerator tunnel: “This is a story practically forgotten.” The first proposal was to build a 30 km tunnel, but because of geological problems due to proximity to the Jura Mountain, the tunnel size had to be reduced. The advisory committee suggested making a 22 km tunnel, but Schopper insisted to have a 27 km circumference to allow higher energies for a proton machine to be possibly installed later in the LEP tunnel. ALICE as well is now benefitting of this.

After this digression about the LEP tunnel, Schopper retied the discussion to the history of heavy-ion experiments at CERN. Five experiments started to work in 1986. The first measurements were performed accelerating oxygen ions and, at a second state, sulphur ions. Then, in 1993 Linac1 was replaced by Linac3 and an ion-source for heavier nuclei was developed, so that lead could be accelerated. These experiments led the field until 2000, when the Relativistic Heavy-Ion Collider (RHIC) came into operation at the Brookhaven National Laboratory.

Over almost two decades of operation, the experiments at the SPS achieved various results, the most important of which was the observation of the QGP. The two clearest indications for the production of a deconfined medium were the production enhancement of strange and multi-strange baryons and the production suppression of the J/Ψ meson, both with respect to the rates extrapolated from proton-proton collisions.

When the press conference on the first indications of QGP formation was held at CERN,” Schopper commented, “there were still some doubts about it and somebody claimed that there was not evidence yet, but later it was confirmed.”

When in 1990 the RHIC project was approved in the US, the European physics community had to decide what position to take. It was decided to participate in RHIC with very small efforts, while starting in parallel a new project for a large heavy-ion detector at CERN. This opened up the way to ALICE.

Sam Ting proposed to upgrade the L3 experiment for the LHC, but it was not approved. So he invited the heavy-ion group to use the magnet and part of the infrastructure of L3. In March 1993 the Letter of Intent for ‘A Large Ion Collider Experiment’ was submitted and ALICE could take off with full steam.

 

At the end of the seminar, we asked a few questions to Schopper.

Professor Schopper, during your appointment as DG of CERN you took decisions against the recommendations of the advisory committee, in particular you insisted on having a 27 km tunnel for the LEP and on starting a heavy-ion programme at CERN. What drove you to take these big risks?

For what concerns the size of the tunnel, it was the competition with the US Superconducting Super Collider (SSC). At that time the SSC had been proposed, for which a tunnel of 60 km was foreseen. In the same period during a workshop in Lausanne the hypothesis to build – after LEP - a proton machine in the same tunnel was discussed. The idea was to have an electron ring and a proton ring in the same tunnel, so that we could have both proton-proton and electron-proton collisions. This turned out to be not possible because there wasn’t enough space in the tunnel.

Nevertheless, in order to be able to compete with the SSC, the proton-accelerator-to-be (LHC) needed to have the biggest possible size. 27 km was still much less than the 60 km of the SSC, but we thought that reaching high luminosity we could be competitive, while with a ring of only 22 km we would have been put offside. So that’s why I pushed so much to have a higher circumference, but of course I couldn’t be sure that it would go well.

And concerning the heavy-ion programme?

Well, I believed in it. The decision to start such programme was actually taken at a lunch in the glass box at CERN and the main arguments to convince me came from Tsung-Dao Lee.

What happened when RHIC came into play? Was it considered as a competitor by CERN’s physicists?

Yes and no. When RHIC started its programme, the European groups were faced with the possibility to give up this kind of physics here in Europe, or at least reduce their participation in it, and give priority to RHIC. But a decision was taken to go in the opposite direction, in other words, they decided to put a relatively small effort in RHIC and focus on a new project at CERN. Indeed some groups were even prepared to give up activities in the field at their national laboratories and concentrate on CERN. In that sense, at the time of the SPS, RHIC was a competitor.

In fact there was also a discussion on weather the QGP was discovered at CERN or at RHIC. I think that this new state of matter was first observed at SPS, but RHIC found the first evidence of its peculiar properties.

Now, with LHC, the situation is different. The two machines are complementary because at LHC we can scan a range of high energies that cannot be reached by RHIC.

So what is RHIC useful for nowadays?

It is useful because indeed sometimes it is easier to study some phenomena at low energies. I think that it is very valuable to have both machines, they allow us to investigate complicated phenomena both at low and at high energy.

Heavy-ions experiments, among which ALICE, have already achieved important results, but you said that there are good chances of more to come. What do you think of? What is coming next in heavy-ion physics?

I cannot say because I am not a theorist, I am an experimentalist; I can only speculate. There were predictions from QCD that we would discover the QGP, but the rest was not predicted, for instance the fact that the QGP would act as a liquid with an extremely low viscosity. This came unexpected.

The QGP is very complicated, quark-gluon matter has many degrees of freedom like ordinary matter, and QCD is not yet a fully developed theory, so I wouldn’t be surprised if we found some other new phenomena.

Did anybody who was against your decisions to have a 27 km tunnel and to start the heavy-ion programme at SPS ever come back to you to admit that you were right in the end?

Some of them, yes. Anyway, whether they came or not, for sure there are lots of people who were against it that are very happy now.