by Virginia Greco. Published: 06 December 2016

Reinhard Stock gives us his view of the birth of heavy-ion physics and how it landed at CERN.

 

After the welcome address, the first who came on stage at the 30 Years of Heavy Ions celebration at CERN was Reinhard Stock, who told the story of the birth of heavy-ion physics and of its “entry in the high-energy sanctuary”, i.e. CERN.

At that time – the late 1970’s and early 1980’s - Stock was working on the Streamer Chamber experiment, set at the Laurence Berkley National Laboratory (LBNL) in US, in which high energy heavy-ion reactions were studied and collision events were analyzed by eye on scan-tables (“It took two or three hours to analyze just an event!” Stock commented). In 1980 the Bevatron accelerator at LBNL had been upgraded to work with heavy-ion injection from the the SuperHILAC linear accelerator, and the complex of the two had been called Bevalac. Under the direction of Hermann Grunder, it was the first universal facility to study relativistic collisions of light or heavy nuclei. Its research programme was oriented to studying the Nuclear Matter Equation of State, which - according to Stock - was “the holy grail of Bevalac’s physics” and was of key importance to understand the structure of neutron stars and supernova dynamics.

At the beginning of the 1980’s the idea rose to to search for the plasma state of QCD with heavy-ion collisions at the higher CERN energies, so a group of researchers in nuclear and heavy-ion physics (mainly of GSI and LBL, but also of Heidelberg, Marburg and Warsaw) in 1982 wrote a proposal and submitted it to the CERN Proton-Synchrotron Committee, principally under initiative of Bock, Stock and Gutbrod from GSI, and Pugh and Poskanzer from LBL.

 

When and how was heavy-ion physics born?

The transition from the goal of the nuclear (or hadronic) matter Equation of State to the investigation of deconfined QCD matter with high-energy heavy-ion collisions happened when physicists, both in the environment of the Bevalac experiments and among particle physics groups at CERN and BNL, began to address the question of whether a deconfined (colour conducting plasma) QCD state might be created in very high energy collisions of nuclei. In this situation our 1982 proposal to CERN, which included construction of heavy-ion injection instrumentation at the Proton Synchrotron (PS), set a lot of things rolling.

Why did you and your colleagues decide to write this proposal? Why going to CERN?

The idea to investigate a deconfined state of QCD in very high energy collisions was somehow irresistible to us. The Quark-Gluon Plasma (QGP) state had been postulated theoretically to arise from the newly discovered so-called asymptotic freedom limit of a system of quarks and gluons. This QGP is a much more elementary state than that of the matter we are made of and plays an important role in the early evolution of the Universe because such a state has apparently dominated the attosecond to microsecond era of the Big Bang expansion. Of course, in the early 1980’s the properties of the QGP were not yet known. It has turned out to be fundamental quantum liquid with unexpected properties, very far away indeed from QCD asymptotic freedom. This is the key result of the research we are talking about here.

Although the idea was already there since the late 1970’s, iit took time to set up the EOS Bevalac experiments, at a more modest energy, and gain experience with the new physics, as well as instrumentation of substantial cost (including providing for funding) novel to the Nuclear Physics community. Most importantly, it soon turned out that even the maximal Bevalac energy did not suffice to create the QCD deconfinement energy. So it was natural to turn to CERN where about tenfold higher center of mass energy was available. 

The proposal was for an experiment at the PS, but it actually ended up at the SPS…

Yes and that was a terribly fortunate development of the events. My colleague from MPI Munich, Volker Eckardt, had ignited my interest in QCD deconfinement by suggesting that there was a heritage of potentially useful instrumentation and infrastructure, from former SPS experiments NA5 and NA24, that could be adaptable to QGP research. But these experiments were located in the North extraction area of the SPS. There was a modern Streamer Chamber, as well as a host of calorimetry, plus beam line and other structures, fully intact. Actually it was this prospect that had prompted the 1982 proposal, but we were outsiders to CERN and did not dare to propose an SPS experiment, with much more weightful implication to CERN’s  programme, so the experiment called for two PS experiments, one based on the LBL Streamer Chamber experiences, the other on an extended scheme of the GSI – LBL Plastic Ball experiment.

But the historically most shaking element of our proposal was that GSI and LBL offered to procure a pre-installed and tested new injector complex for heavy ions, consisting of a Gellert-Grenoble ECR ion source bought by GSI, and an LBL-built RFQ preaccelerator. This latter facility proved irresistible to CERN as it invited the idea that other groups at CERN, coming from particle physics background, could also articulate their intentions to participate in QCD plasma research. The combined impact of our interest in inheriting the NA5/24 setup in the North SPS extraction hall and the push of the (North area based) former experiments now preparing for QCD matter physics finally convinced R.Klapisch (then CERN research director) to initiate the launch of a full-fledged SPS programme. In retrospect this was an overwhelmingly constructive  development because the CERN SPS nuclear collision research facility provided for the first, crucial inroad into QCD matter physics.

Oddly enough, though, your proposal was never formally approved, was it?

It is true, indeed.  But from the above you see that this, our initial proposal for PS experiments, was swiftly “washed away” by a complex set of ideas and decision-making at CERN, which also entailed a formal CERN-GSI-LBL agreement to build the new heavy ion injector system. As a personal note – as my talk was about entering the CERN sanctuary from the outside Nuclear Physics community – I included the recollection that the initial young physicists proposing this research to CERN, notably H. Gutbrod and myself, were obviously mostly regarded as “catalysts” in this development, somehow “idiots utiles” for the major good, as was the object of CERN internal deliberations. Indeed we were never received in committee hearings about our planned experiments. However, putting this aside, of course the transformations of our initial proposal were then well taken care of in the ensuing discussion of the resulting SPS experiment programme, and our experiments ended up as NA35 and WA80. As I said, NA35 was the result of MPI Munich joining and providing all the invaluable instrumentation in the North hall, without which I do not know how we would have managed to set up NA35 (if based on Nuclear Physics funding only).

 

Following these events, a first generation of experiments was installed at CERN’s SPS and measurements were performed with beams of fully stripped oxygen first and sulphur later. Stock was the leader of the NA35 experiment.

The second act of this story began with another proposal, dated 1986. In this case the idea was to study lead collisions in the SPS and the proposal went through all the formalities up to reaching approval. It was decided that the upgrade of the infrastructure would be paid with “in kind” contribution by CERN and other eight European agencies.

Thus, a second generation of experiments was established at the SPS. The follow-up of NA35 was NA49, in which streamer chambers were replaced by Time Projection Chambers (TPC), based on a more recent technology. In addition, an automatic system to analyze the data was developed, since the eye scanning method could not cope with the increased number of collisions.

 

How do you feel, now that we are celebrating 30 years of heavy ions, being one of the beginners of this kind of physics?

I think that intellectually it was a very interesting voyage. At SPS we have learnt a lot both about QCD and about detector technology. Great things came also from RHIC, which has been the world leader in this field in particular in the period from 2000 to 2005.

Then LHC followed in the footsteps of RHIC. At first, remarkably, the QCD matter research was a shared initiative of the two - hitherto essentially separately marching - communities of nuclear and particle physicists. A landmark accomplishment of the  open horizon of CERN research! LHC brought a transition from qualitative to quantitative results, much closer to the fundamental goals of determining the transport coefficients of quark matter. And, thus, radiating a sharp stimulus to our theoretical colleagues.

What can we expect from the next lead-ion runs at LHC?

First of all, more statistics to have higher confidence levels for already performed measurements. Then, hopefully, something new that could trigger a different theoretical view. It could be something like a generalization of matter under the government of QCD, or going back earlier in time in the reconstruction of the development of matter after the Big Bang, or finding a more fundamental theory of which QCD is some sort of low energy limit. These are our hopes, beyond accumulating more statistics. There is actually a need for a new theoretical paradigm.

 

SaveSave