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.
<200> "I know that this complementarity is also competition, competition between experimentalists and competition between different experimental approaches towards studying ultra-dense matter. And I think that this competition is very good for the science done at LHC"200>
A.M. So maybe this is a good point to ask you about the importance of the ALICE experiment for theorists working on heavy-ion physics?
U.W. ALICE is clearly the central experiment for theorists of heavy-ion physics. Of course it is not the only experiment in the field as ATLAS and CMS have significant complementarity. ALICE offers the most precise way of getting a handle on bulk properties of matter such as hadron chemistry and low pt processes, but it also has a significant window into hard processes. Vice versa, ATLAS and CMS have better reach for many hard processes but they also have a window into soft processes. I know that this complementarity is also competition, competition between experimentalists and competition between different experimental approaches towards studying ultra-dense matter. And I think that this competition is very good for the science done at LHC. Let me mention as an aside that in the mid-term, ALICE has at least one interesting advantage over its competitors: it is probably the only LHC experiment with a potential to upgrade its hardware in direct and optimal response to discoveries made in heavy ion collisions. At least in this sense, ALICE is the dedicated heavy ion experiment at the LHC, and it is important to discuss how this dedication could be used optimally.
ALICE experiment - event from lead collisions
A.M. What does it mean to make heavy ion physics and how different is it from the rest of particle physics?
U.W. If you do heavy-ion physics at collider energy you certainly need to know particle physics. But what you are looking for is something that goes beyond particle physics in the strictest reductionist sense. The question you pose is related to the way in which properties of matter and collective phenomena emerge from fundamental laws. That means you go beyond studying the scattering or production of the most elementary constituents, you are interested to study how material properties emerge from the fundamental laws of nature which these elementary constituents abide. So in order to do this you need to know elementary particle physics to define the baseline on top of which you identify what we name collective phenomena. But then you have to connect this theory of elementary particle physics to theoretical concepts and frameworks that are equally well established but lie outside the framework of elementary particle physics. This includes connecting particle physics to a wide range of modern thermodynamics and relativistic hydrodynamics. You have to build bridges between the different areas of physics. Heavy ions is one of the bridges that connects high energy physics with other branches of physics. Astroparticle physics or cosmology provide other bridges in a similar sense. However, heavy ions physics is a bridge that is – at least at present - much closer connected to the activities of the CERN laboratory.
Perhaps you have also noticed that I phrased my answer by avoiding explicitly mentioning the notion of quark gluon plasma. This is not because I think that the notion of QGP is meaningless but because I see experimental evidences that our way of looking at data from LHC goes far beyond testing Quark Gluon Plasma. It would not be very helpful to reply to your question about the differences to particle physics with the statement that I am studying QGP. I feel that in giving such a short-cut answer to our friends in high-energy physics, we often unduly reduce the scientific potentials that our field has. It’s much better to point to the intellectual and experimental richness of our field.
<200> "You have to build bridges between the different areas of physics. Heavy ions is one of the bridges that connects high energy physics with other branches of physics. Astroparticle physics or cosmology provide other bridges in a similar sense"200>
A.M It is often mentioned that at LHC we reproduce the conditions of the early Universe just a few moments after the Big Bang. I am wondering whether there is anything we could learn from the study of QGP about the unification of the fundamental forces and particularly about gravity?
U.W. The energy scale at which degrees of freedom in QCD change from hadronic to partonic (i.e. phase transition) is much lower and it is not related to the unification of fundamental forces. In fact the first unification of forces occurs at the electroweak scale and that is a factor of 500 higher than the energy where the QCD transition occurs. However, we know that the unification of forces occurs in non-abelian field theories and we also know that non-abelian field theories can show phase transitions. What makes the study of QCD relevant with this respect is that it is the only non-abelian theory where the transition range lies within our experimental reach. The QCD transition is not the one that has left the most pronounced signals in the early universe. As you might know, the corresponding cosmological discussions are mainly about the electroweak phase transition, or about transitions at even higher temperatures.. However QCD is the only theory in which lab-based experiments can instruct us about the way in which the collective dynamics of non-abelian field theories manifest themselves. And the analogous question, posed for the electroweak theory or grand unified theories, is clearly relevant for our understanding of the Early Universe.
(end of part A...)