CERN Accelerating science

Jürgen Schukraft receives the Niels Bohr Institute Medal of Honour

by Virginia Greco

Former ALICE spokesperson Jürgen Schukraft has been recently awarded the Niels Bohr Institute’s Honorary Medal, an honour for his outstanding career in heavy-ion physics and his role as leader of the ALICE experiment. Established in 2010 as a tribute to excellent physicists who have also fostered international collaborations, this Medal has been already awarded from the Danish institution to – among the others – CERN Director-General Fabiola Gianotti, and Nobel Laureates Andre Geim and Gerard ’t Hooft.

Now a senior staff researcher at CERN, Schukraft started his career at GSI, in Darmstadt, where he studied Uranium-Uranium processes and in 1983 completed his PhD under the supervision of Hans Specht. Ultra-Relativistic Heavy-Ion physics did not exist yet as an independent field of research, but those were the years when the idea of accelerating and studying heavy-ions at CERN was being discussed. In the meanwhile, following his supervisor’s suggestion, Schukraft joined CERN as a fellow in 1984 to work in the R808 experiment at the Intersecting Storage Ring (ISR), the first proton-proton collider in the world, which operated at a maximum energy in the centre-of-mass of 62 GeV.

“Before moving to CERN I was doubtful, but once I came here I enjoyed it so much that I decided to stay,” recounts Schukraft. “What I liked most, apart from the mountains and the wine,” he adds laughing, “was the international and stimulating environment. I was coming from a small lab. Here, even if the experiments were small compared to what they are now – I think the biggest collaboration was less than 100 people – you could discuss and share opinions and doubts with many colleagues, thus you were to progress in your research much faster.” He also found the scientific atmosphere quite exciting, “because it was the time when jets were observed in proton-proton collisions: they had already been discovered in e+/e- collisions, but it was not evident that they could be seen in pp as well”.

When, in 1986, the heavy-ion programme was launched at CERN’s Super-Proton-Synchrotron (SPS), Schukraft moved to work first for NA34/HELIOS, a fixed target experiment using proton-nucleus collisions, and later for the NA45/CERES experiment, utilizing beams of heavier nuclei. “We started colliding ions but, in fact, at the beginning we did not use heavy ions, but oxygen and sulphur ions,” clarifies Schukraft, “because, in order to accelerate heavy ions, a special injector is needed, which had to be built and thus would require investing money. So, we first started an exploratory programme using light ions, with the idea that, if the results were interesting, the scientific community would support a heavy-ion programme at CERN. The first period was actually a bit strange: we didn’t really know how to interpret data, so we would continuously oscillate between excitement and disillusion…”

Even though not yet clear, these first results convinced the CERN management to go to the next step and start a heavy-ion programme. The decision was taken in 1990 and new experiments were built –NA45/CERES was one of them – which started taking data in 1994.

In the meanwhile, CERN had also decided to build a new more-powerful accelerator: the Large Hadron Collider (LHC). Thus, discussions started about building an LHC experiment dedicated to heavy-ion physics. Schukraft started working with a small group on a possible design for such experiment. “Both Paolo Giubellino and Federico Antinori [former and current spokesperson] were members of that group”, Schukraft highlights. “We thought of a possible detector and made many calculations and then we presented our proposal at a meeting in Evian in 1992”. Then a letter of intent followed in 1993, a Technical Proposal (1996) and later many Technical Design Reports (TDR).

Already leader of what was the first nucleus of the ALICE collaboration, Schukraft was officially elected as Spokesperson in 1992 and kept this role for the following 18 years. As he explains, along this period the experiment passed through three different stages. The first one, which lasted 4 or 5 years, was the most exciting, because it was when decisions were taken about the physics to do, the kind of technologies to use for the various sub-detectors, as well as how to get funding. The second five-year-long phase was about making sure that the design could be transformed in an actual detector: “We had to lay down the structure of the project, do lots of R&D to check what could be technically done, line up the money, decide who would pay for what, and so on,”, Schukraft comments. In 2000 the third phase started: the building of the detector. “This was a technical phase, which lasted 10 years. During this, the heaviest responsibility was on the shoulders of the technical coordinator, who had to follow the construction and keep activities on track. My tasks, instead, were to secure funding and make the collaboration work smoothly.”

Along the 20 years of Schukraft’s mandate, the Collaboration grew significantly, passing from a handful of people to a thousand members. More collaborators joined it afterwards, until reaching the present number of about 1800 people.

In 2010 the ALICE experiment was steady and operational, so the moment came for Schukraft to step back from his position of spokesperson. This allowed him to dedicate again to research and have a less stressed life. “When you are the spokesperson of such a big experiment, you spend most of your time managing people and money, as well as selling results.”

At the moment, his interest is mainly focused on understanding the recent observations in small systems (proton-nuclei and proton-proton collisions): “The most intriguing question for me is why we see in small systems so many of the things we observe in heavy-ion collisions;” he explains, “this is a big question and I think that, when we answer it, we will really better understand heavy-ion physics”.

According to Schukraft, in order to tackle this issue new ideas are needed on how to interpret the results and about what different analysis to carry out on both existing and new data. “The results are many and often clear cut,” he underlines, “but we are missing the interpretation.”

Significant new data will be collected in the future runs, in particular in LHC Run 3, with the upgraded detector. The present limitations of the ALICE experiment are in the data rate that it can handle and in the event acceptance of the detector, which is basically related to the fraction of solid angle around the collision point that the detector covers. The upgraded system will be able to cope with the increased data rate, but the acceptance will not change a lot. “It is a pity that we didn’t build a bigger detector, it covers only an angle of +/- 45 degrees,” comments Schukraft. “This choice was conditioned by costs, of course, it was a compromise we had to do, but if I could go back in time I would try to have a bigger detector. Unfortunately, it is not something that can be fairly easily upgraded, we would have to build a brand-new detector.”

Nevertheless, with increased statistics and better resolution, the ALICE experiment will be able to study in more detail heavy-ion collisions. “With the new detector, we would also like to investigate the low-mass lepton pairs, for which the upgraded ITS will be essential. This topic falls in the field of thermal radiation phenomena,” Schukraft adds. “A big jump forward will be also possible in the charm physics.”

This prestigious career award arrives a few months before Schukraft’s retirement. As it can be expected, he will not leave research and the ALICE experiment, only his official status will be different. “If you have passion for what you do, you want to keep doing it even if you have to retire” he comments. “Besides, being at CERN you are not only involved in your own research, but some way you are part of the activities of the whole community: it is definitely a great place to spend your time”.

The article originally appeared in ALICE Matters.