From CERN ALICE: “QGP: 17 years after the public announcement…”

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31 January 2017
Virginia Greco

Interview with Luciano Maiani, DG of CERN from 1999 to 2003, who gave the announcement talk of the discovery of QGP at the SPS.

CERN  Super Proton Synchrotron
CERN Super Proton Synchrotron

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About 25 years after its first theoretical prediction, the new state of matter called quark-gluon plasma (QGP) was observed at CERN’s SPS. The public announcement was made on the 10th of February 2000 by Luciano Maiani, Director General of CERN back then. At the event organized by ALICE to celebrate the 30-year anniversary of the first heavy-ion collisions at the SPS, Maiani gave his account of this piece of history of physics.

We had an interview with him after the seminar.

After one year of mandate as DG of CERN you had the honour and the responsibility to announce that evidence of the existence of QGP had been found at the SPS. How did you live these happenings?

At that time I was not an expert in heavy ion physics, because I hadn’t worked in the field. Nevertheless, I was aware of the phase transition issue and of the two existing visions about what happens to nuclear matter at very high temperature. On one side there was the theory that matter would break down into a gas of quarks and gluons (and temperature could be freely increased), on the other side the model of Hagedorn about the existence of an upper limit of the temperature reachable, which could be estimated from the hadron spectrum to be 170-180 MeV.

With the development of QCD it was possible to combine these two models. In particular, in 1975 Nicola Cabibbo and Giorgio Parisi suggested that the Hagedorn limit temperature is just the critical temperature of a phase transition from a gas of hadrons, made of confined quarks, to a gas of deconfined quarks and gluons (the QGP). These works had convinced the experts in the field.

When the moment came to decide whether to make a public announcement about what the SPS had found, I discussed with many of the people involved, such as Claude Detraz, who was Director for Fixed Target and Future Programmes during my mandate, Reinhard Stock and Hans Specht. After examining the data and collecting opinions, I concluded that we had convincing signals that what we were observing was indeed the quark-gluon plasma.

But the public announcement was cautious, wasn’t it? Was there still some doubt?

I think that the announcement was quite clear. I have the text of it with me, it reads: “The data provide evidence for colour deconfinement in the early collision stage and for a collective explosion of the collision fireball in its late stages. The new state of matter exhibits many of the characteristic features of the theoretically predicted Quark-Gluon Plasma.” The key word is “evidence”, not discovery, and the evidence was there, indeed.

In the talk I gave at that time I also described the concept of quark deconfinement using an analogy with the snow on the Jura Mountain, which I particularly like. We can consider a quark as a skier: when the temperature is not very low, on the mountain there are only patches of snow in which the skier can move. When the temperature decreases and the snow increases, the skier can move along bigger and bigger spaces, up to a point where he or she can freely sweep long distances. The same can be said for a quark confined in a hadron (the patch), which becomes free when temperature increases.

Of course at that moment the idea still popular was that we were dealing with a phase transition to a gaseous state in which quarks and gluons would be asymptotically free. Later RHIC showed that the situation is more complicated and that this new state is much more like a liquid with very low viscosity rather than like a gas.

The announcement came just a few months before the start of the programme of RHIC. Were there some polemics about this “timing”?

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The Solenoidal Tracker at the Relativistic Heavy Ion Collider (RHIC)

We were almost at the conclusion of a long and accurate experimental programme at the SPS, so making a summing up was needed. In addition, as I said, we thought there were the elements for a public announcement. And this has been proved right by later experiments.

Somebody thought that it would make RHIC, which was going to enter in operation, appear useless. But that was not the case, since much more was left to study. Indeed in the same announcement talk I said: “the higher energies of RHIC and LHC are needed to complete the picture and provide a full characterization of the Quark-Gluon Plasma”.

In your opinion, what is the future of this branch of research?

Well, there are still many open problems, things that need to be studied further.

It is very important to explore the properties of this new state of matter and the connected phenomena, to get a more precise physical picture of the new state.

Personally, I think that there is also another possible line of research in this field: to study the production of those exotic hadronic resonances that are not included in the scheme of baryons and mesons (i.e. three quarks or quark-antiquark structures). These resonances have been observed in CMS and LHCb in pp collisions, and it would be interesting to study how they are produced in heavy-ion collisions. It could give us indications about what these objects are, tell us if they are molecules made of colourless hadrons or new states which are configurations of quarks and antiquarks (different from mesons) that include subcomponents connected by colour bounds.

ALICE could provide an important contribution to this research. It is not easy to observe such exotic states in heavy-ion collisions but I think it is worth trying.

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Inflationary Universe. NASA/WMAP
Inflationary Universe. NASA/WMAP

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