Heavy-ion physics and the SPS: a long way together

Virginia Greco

We talked with Hans J. Specht about his view of the beginnings of heavy-ion physics and the performance of the experiments at the SPS.

Following Reinhard Stock, Hans J. Specht took the floor at the 30 Years of Heavy Ions celebration at CERN to tell the story of the birth of heavy-ion physics from a different point of view. He also made a summary of the great results achieved by the experiments at the SPS over these 30 years of activity.

At the time of the events he recounted, Specht was already Professor in Heidelberg. He worked in low-energy nuclear and atomic physics up to the end of 1982, when he decided to switch to high-energy physics joining R807/808 for the last year of ISR running in 1983.

Professor Specht, you saw the birth of heavy-ion physics as you have been involved in the field since its very beginning. Could you tell us about the key moments of its early history and your role in it?

The roots of the field of high-energy heavy-ion physics are dual, resting on both nuclear and particle physics. To draw the history of the former, we have to go back to 1974, when in the Bevalac at LBL the first ions were accelerated up to relativistic energies of 2 GeV/nucleon. Professor R. Bock, member of the Directorate of GSI and one of the pioneers of low-energy heavy ion physics world-wide, and H. Grunder at LBL, a Swiss accelerator physicist and responsible for the Bevalac project, decided to form a collaboration between GSI and LBL to participate in the experimental program. Bock brought in the younger GSI scientists R.Stock and H.Gutbrod (both PhDs of him), who later became the team leaders for the ‘Streamer Chamber' and ‘Plastic Ball' experiments, respectively.

One year later, in 1975, the idea of the possible existence of a new state of strongly interacting matter, reflecting quark deconfinement, started to spread within the community. This increased the interest in the topic, but initially more among particle than nuclear physicists, due to the need for much higher energies than available at the Bevalac.

In 1979 GSI presented the SIS-100 project, a proposal for a new synchrotron that would accelerate heavy ions up to 10 GeV/nucleon. In the same period German universities and other institutions submitted also new proposals. The Government of Germany therefore set up an ad-hoc committee in 1979 that would evaluate the projects and decide which ones to fund. I happened to be one of the members of this board, called the ‘Lindenberger-Ausschuss' committee, from the name of its chairman. The SIS-100 proposal was quite controversial - on one side because GSI did not have the expertise and tradition of LBL and the energy even seemed to be too low for the novel ideas of deconfinement, on the other side because the great majority of GSI physicists still saw an attractive potential in low energies thus was not interested in high energies. This rather bitter conflict went on for 5 years and was only solved in 1984 with the SIS-18 and the associated heavy ion cooler ring ESR.

I myself got very interested in experiments at sufficiently high energies to address quark deconfinement and therefore saw no alternative to opening this new domain at CERN. All six experimental and theoretical colleagues of the Committee were finally unanimous on the following recommendation: ‘It is proposed to reinvestigate whether the field of ultra-relativistic heavy ions could not been opened at an accelerator at CERN in a collaboration of CERN with GSI.'

As a follow-up to the Committee Report in May 1980, Bock and Stock organized in October 1980 the “Workshop on Future Relativistic Heavy Ion Experiments” and invited me to give the summary talk, knowing that I was very much behind that recommendation.  This was a clever move, and so was the fact that they invited not only people involved in the Bevalac experiments, but also particle physicists including W. Willis. The workshop thus represented a real milestone since it was the first time in history that nuclear and particle physicists gathered together to discuss the future of heavy-ion experiments. For me, the title ‘Quark Matter Conference I' is therefore more justified here than for all other contenders.

Two important events happened right after. First, a Letter of Intent for two experiments (direct follow-ups of the two at the Bevalac) was submitted to CERN by the GSI/LBL Collaboration. They were meant for the proton synchrotron (PS), thus they would address the SIS-100 regime and not the much higher energies required for deconfinement.  In addition, W. Willis, who was the intellectual leader among the interested experimental physicists at CERN at that time, invited me to meet with him the CERN DG L. van Hove to discuss the different options for the laboratory. As a result of that day, which was very memorable for me, I got convinced that the future of ultra-relativistic heavy ion physics at CERN would be the use of the SPS instead of the ISR.

So we arrive to 1982, which was a key year…

Yes, it was a fundamental year because two events occurred in the span of a few months.

GSI and LBL, together with Heidelberg, Marburg and Warsaw, submitted to CERN the final proposal on the ‘Study of Relativistic Nucleus-Nucleus Reaction Induced by 16O Beams of 9-13 GeV/nucleon at the CERN PS'.

At the same time, a group of leading physicists at CERN (T. Ericson, M. Jacob, W. Willis) together with H. Satz  organized a conference on ‘Quark Matter Formation and Heavy Ion Collisions' in Bielefeld, Germany. Here nuclear and particle physicists (with the latter being more than half of the total participants) discussed within the frame of about six working groups a set of specialized experiments to be performed at the CERN SPS, drawing as much as possible from existing set-ups.

It is important to note that the almost simultaneous occurrence of the two events excludes the frequent interpretation that the PS proposal triggered the SPS programme at much higher energies. It was the accompanying offer of a novel (ECR) ion source and an RFQ, more than any other thing, which opened the way to a comprehensive programme on a very broad intellectual basis.

Why were you so enthusiastic about these new experiments?

Well, I had been working in low-energy atomic and nuclear physics since the beginning of my career and had reported successes in these fields to the point that I made it quickly up to the position of full Professor in Heidelberg in 1973. The successes continued, but the last field I opened in the late 1970s, working on 3- or 4-body decays in heavy ion collisions at the new UNILAC at GSI Darmstadt, raised increasing doubts in myself. The technique was based on newly developed square-meter position-sensitive parallel-plate detectors for heavy ions, which was revolutionary for nuclear physics at a time when tiny silicon detectors were the standard around. This in itself was great, the physics output was matched to the efforts and even unique, but what one really learnt was only accessible to narrow specialists and not to a broader community as I was used to before. The visibly diminished attraction for students to work in this particular field was also consistent with that.

It was therefore the right moment for me to make a drastic jump to something else. Time proved that I was right: the following 10-15  years of successful research on the Quark Gluon Plasma, coupled to very challenging hardware developments, brought interest and visibility to my work, up to a level that it could attract some of the best students of my whole professional life.

Herwig Schopper, DG of CERN at the time, was enthusiastic too and pushed for it against his committees advice. Why do you think he did so?

There are physicists who are very visionary, and H. Schopper is one of that kind.

Did he know that you were in favour of his choice?

Yes, we knew each other in person since before. He was aware of my previous role in German nuclear physics and also knew that not only he would have my support, but that I would actively get involved in the programme.

I have always had a good relationship with Schopper. When I came in 1983 to CERN for a full year, I actually knew only a tiny fraction of people in the particle physics community outside Germany, so he did a lot to help my integration, including dinner invitations to his home. At the first one I met for the first time the Nobel Prize Laureate Samuel Ting.

In 1982 Schopper took the decision to start a heavy-ion programme at the SPS. What happened then?

A total of 6 experiments were formally approved for the first round of ion physics, with oxygen beams. CERN had an extremely tight budget situation at that time due to the construction of LEP, and Schoppers support of the heavy-ion program was subject to the strict condition that we would recuperate and recycle as much instrumentation as we could. My own first experiment was HELIOS/NA34-2 where I was elected as the spokesperson from the beginning: an unforgettable experience for somebody used until then solely to small groups.

Why so many experiments in parallel?

Mainly for physics reasons, since in order to have detectors optimized for different types of observables you need to separate the tasks, in particular in fixed-target experiments. NA34-2 was an experiment designed for the study of e+e- pairs, μ+μ- pairs, real photons, hadron spectroscopy and hadron calorimetry in almost 4pi, all at the same time. We paid a price for the necessary compromises and finally decided to divide NA34 up into three experiments.

This happened a few years later. When it was decided to inject lead ions, a second generation of experiments came, right?

Yes, but it was a fluent transition. The three experiments following NA34-2, for example, were NA34-3 for μ+μ- pairs, CERES/NA45 for e+e- pairs and NA44 as a small-angle focusing hadron spectrometer, which all run already with sulphur beams. The latter two continued into the Pb era, accompanied by several new experiments. The solid production runs for Pb took place in 1995 and 1996. CERES was really my personal baby, with the original conceptual design literally done by hand calculations. It was considered by most of the colleagues as a mission impossible, but finally turned out to be a great success.

The SPS control room in 1979 (CERN Archive).


In your talk you presented the most important results achieved by the experiments at the CERN SPS. What is your personal opinion on the role played by the SPS?

The SPS has been the pioneering machine for the field of ultra-relativistic heavy-ion collisions worldwide and has performed spectacular measurements. In 2000 the first observation of the Quark Gluon Plasma, based on the Pb results of the second-generation experiments at the SPS, was announced. With some caution, though, in fact the words used were ‘a new state of matter'. Only in 2009 the NA60 μ+μ- pair experiment topped this with a direct measurement of the temperature of the early stage of the fireball clearly above the critical temperature for deconfinement.

But between the two there was RHIC…

Yes, indeed. RHIC started taking data the same year; actually, the CERN Press Conference to announce the observation at the SPS was held just before RHIC started its run. In the first few months RHIC produced already very good results. I happened to be the summary speaker of the first Quark Matter Conference after the start-up of RHIC (2001 in Stony Brook). There was a peaceful coexistence of the two worlds, RHIC quickly came up with first results on new observables like jet production and elliptic flow. It was also responsible for discovering the characteristics of ‘the most perfect liquid' of all liquids ever, which is a solid statement on the properties of the plasma rather than just the detection. Direct accurate temperature measurements have, however, remained the domain of NA60 at the SPS. There is no chance to ever beat the superiority by a factor of 1000 at any collider, be it RHIC or the LHC, simply due to the principle luminosity difference to fixed-target machines. 

Is there a future for heavy-ion physics at the SPS?

The SPS is still the best machine in the world for performing precision studies of the two different phase transitions - deconfinement and chiral symmetry restoration - within the QCD phase diagram. There is an experiment that is somehow the follow-up of Stock's NA49 experiment, called NA61, which is already running and is performing both energy and atomic number scans. A future follow-up of the NA60 experiment is also under discussion, with an increase of another factor of 100 in data quality. This would allow clarifing the remaining open issues, including the chiral transition (rho-a_1 mixing), the onset and the order of the transitions.

Alice Matters