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A process – not an event

Jürgen Schukraft joined CERN’s heavy ion programme when it started data taking in 1986. He worked on the NA34 (HELIOS) experiment and the NA45 (CERES) experiment before joining ALICE, CERN’s new heavy ion experiment at the Large Hadron Collider, for which he has been spokesperson since it started in the early 1990s.

Jürgen Schukraft“The beginning of the heavy ion programme at CERN was both exciting and confusing. We were making a big step in energy with the SPS, compared to other programmes at the time, and thought it could be enough to make a quark-gluon plasma. So everyone was in ‘gold rush’ mood, but nobody really knew what we would find, and when. During the first days, the Austrian TV came to CERN and wanted to film live the ‘first detection of the quark–gluon plasma’. They were rather disappointed when it became clear that this would take more time than they had budgeted for...

Nevertheless, already in the first year of the heavy ion programme, two striking signatures predicted for a quark gluon plasma were found: particles containing the strange quark were enhanced and particles containing the charm quark were suppressed. The interpretations of the first results reported from heavy ion experiments in 1987 alternated repeatedly between ‘trivial’ and ‘exciting’. The problem: one set of data and two sets of explanations. Better theory was needed — and more data. In the end, it took 15 years to sort out and digest the results.

I was a little bit disappointed and disillusioned after the first couple of years. We had tried long and in vain with light ions, but did not see any ‘smoking gun’. Ever better data, but no unique interpretation. At a conference just a few months before the first data taking in 1986, Léon van Hove, former Director-General of CERN, had said: “To analyse and interpret [the data] will be a time-consuming and tedious activity, all the more so that most results will have little glamour.” He was right.

However, starting in 1994 we could use really heavy ions (lead), and this made a difference. In addition, a new generation of detectors opened up new possibilities, and evidence that we were looking at something ‘different’ was accumulating. In early 2000, the discovery of ‘a new state of matter’ was announced by CERN. This discovery has been a process and not a single event. It was the conclusion of all the data gained in the previous years. And still we could not be sure if this ‘new state’ is already the real QGP, or only some precursor.

Today the quark–gluon plasma is still being discovered, but in pieces. Looking at many signals, one is getting a little more insight with every new set of data. At the moment, this process is rapidly proceeding in Brookhaven, but eventually the focus will come back to CERN with ALICE and the ultra-high energy of the LHC. By then, I hope the QGP has been finally ‘discovered’ and we can go on with the business of studying its properties in detail.”