Interview with Urs Wiedemann (Part A')

Panos Charitos

A.M. How did you start your career in physics and for how long have you been here at CERN?

U.W. I did my PhD in Cambridge in the mid ‘90s on a mathematical subject and after my PhD I joined the research group of Professor Ulrich Heinz in Regensburg, Germany . This was the first time that I started familiarizing myself with the phenomenology of heavy-ion collisions which was studied at the CERN SPS at that time. After my habilitation on Hanbury-Brown Twiss identical two-pion correlations in the 1998 I moved as a postdoc to Columbia University, to work with Miklos Gyulassy. There, I started working on heavy-ion collisions at collider energies. Following this position I came to CERN as a fellow, then as junior staff and then with a short interlude in the U.S. as senior staff. So since 1999 I am working in the theory unit at CERN.

A.M. What are your main research interests?

U.W My research focuses on the theory of strong interactions and its applications to heavy ion collisions. This entails all phenomenological aspects of heavy ion physics and aims to connect them in the best possible way to the fundamental interaction, which is described by the theory of QCD.


Dr. Urs Wiedemann - CERN Division of Theory

A.M. How does LHC currently help us towards this direction?

U.W. First of all it is a tremendous jump in energy from the facility of RHIC. That means that we can embed much harder processes, which means processes with very much higher resolution scale in dense matter. But it also means that the matter in which we embed these processes is much denser because of the high energy. So there are quantitative gains that come from the fact that we have increased initial density, and therefore increased system size and, increased lifetimes that probes spend in the system. At the same time there are also large gains in the way we are able to separate the hard probes from the medium. In my opinion the first years of running LHC have shown that both are of tremendous advantage. Think of the increased multiplicity – which may be a mild factor – but is tremendously helpful in identifying collective phenomena and precisely characterising them. For example the higher precision regarding flow phenomena and centrality selection is also due to higher event multiplicity. Concerning the hard side, we have seen for instance Dijet asymmetries from CMS and ATLAS, showing that even the most energetic hard processes are strongly modified and highly sensitive. So, at LHC, both the soft and the hard sector have demonstrated already their potential for a detailed characterization of ultra-dense matter.

One of the main challenges for theory in my view is to understand now in which observables and up to which accuracy theory can keep pace with the precision of LHC experiments. Let me recall that in all research on the fundamental forces, there are some measurements that theory describes very accurately (and where one can learn a lot from measuring with increased precision), and there are many other interesting measurements for which the theoretical framework for an accurate description is weaker (and where the argument for a more precise measurement is therefore arguably weaker). In this way, theory motivates us to look closer not in a random fashion but in a very pointed one. I believe that this applies also to heavy ion physics. By going from a qualitative to a more and more quantitative description of a necessarily selected set of LHC data, theory will help to identify those measurements that advance our understanding of ultra-dense matter in the most direct way. This is needed for data interpretation and for focussing future experimental efforts. As many of my colleagues, I am convinced that both the wealth of flow phenomena, and the wealth of jet quenching measurements will be part of this selected set of LHC data for which a theoretical framework of established accuracy should be achievable in the coming years. This guides my personal research.


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