Since January 2017, Marco van Leeuwen - researcher at Nikhef (The Netherlands) - is the new Physics Coordinator of the ALICE experiment. We asked him about his career, research interests, and vision of the future of heavy ion physics.
What has been you career path and when did you join the ALICE collaboration?
I entered the field of heavy ion physics when I started my Ph.D. at the University of Nikhef in Amsterdam. For my thesis, I worked in the NA49 experiment at the CERN’s SPS, where I measured kaon production in Pb-Pb collisions at different energies. I also looked for charmed D mesons, but in the end we did not have a large enough data sample to see a signal. After graduating in 2003, I went to Berkeley as a postdoc to work in STAR, one of the RHIC experiments. There I switched analysis topic, passing from the study of soft probes (strange production) to hard probes (jets and correlated particle production). I was also involved in the proposal of the Electro-Magnetic Calorimeter of ALICE. When I went back to The Netherlands, in 2007, I started working full time for the ALICE experiment. At the start, I participated in the calibration and alignment of the Silicon Strip Detector, which was built by the Utrecht group.
You are Physics Coordinator of ALICE since January of this year (2017). What other roles did you cover before?
In the STAR experiment I was co-convener for the ‘High pT’ physics working group. In ALICE, I was co-coordinator of the particle identification task-force, which later became a part of the physics performance working group. Later on, I was one of the conveners of the physics working group for jets.
What are the most interesting and the most challenging aspects of being Physics Coordinator?
I think that the most interesting aspect is that you get to participate in the entire physics programme of the experiment, instead of focusing on one group, as it normally happens; although it is challenging at the same time, because you cannot really follow everything in detail.
What I find also challenging is the management aspect: when a problem is found, you often have to look for people who could work on that issue and bring them together to solve it.
Do you still find time to work on your own analysis?
A little bit but not much. I still co-supervise some Ph.D. students from Utrecht and I contribute to the performance study of the Forward Calorimeter (FoCAL) upgrade. I like to be involved in data analysis and modeling of the physics that we do, but the common tasks have to take priority.
What is your specific field of interest?
I have a quite broad spectrum of interest since, as I mentioned, I started working on soft probes and strange production, while in the last years I focused in particular on jets and parton energy loss studies.
Something that I find really attractive in heavy ion physics is that, in principle, one can use well known QCD modeling to understand the quark–gluon plasma: there is a nice bridge between parts of QCD which are quantitative and well understood and the type of research that we do in this field. In practice, making this bridge is actually very challenging, but I find very interesting and fun being able to move back and forth between data analysis and phenomenological calculations. I hope that we will keep moving forward with getting a more quantitative grip on our understanding of the physics of heavy ion collisions.
What is the status of heavy ion physics and what is next to come?
I think that in some topics we have reached a point where we have a fairly complete picture, in the sense that the modeling has become very detailed and we have performed many measurements. Thus, now we can start to figure out what are the elements required to understand the physics; in other words, the mechanisms to be considered to explain what we observe.
This is the case, for example, of flow, for which we have measured for example event plane correlation and flow coefficients, which are sensitive to relatively subtle aspects of the initial geometry and the properties of the plasma.
On the other side, there are topics for which we are still trying to understand how to approach the problem, what to take into account and what the best way is to make the connection between the calculation and the measurements. This is more the case of hard probes, where we have a basic understanding, but also many open questions, some of which are quite fundamental, such as on the role of angular ordering and the mechanisms behind momentum transport to large angles.
What do you expect from the analysis of the data taken in 2016 and 2017?
In 2016 we had p-Pb data, which are very important to study the connection between what we observe respectively in pp and in Pb-Pb collisions. We will have a much larger data sample, about a factor of 7 more events, which means that things that used to be hardly visible will become clear. One of the things that hopefully we will be able to look at is the flow-like effect (long range correlations in pseudo-rapidity) in heavy flavour in p-Pb collisions.
We took mainly data at 5 TeV, which is the same energy of the data sample collected during the previous p-Pb run, as well as of other Pb-Pb and pp data sets. We had also some runs at 8 TeV, which are important to understand the parton distribution in the nucleus.
At the end of 2017 we should have another pp run at 5TeV, which will help us measure the baseline of pp events and have a solid reference for the study of particle production modifications in p-Pb and Pb-Pb collisions at the same energy.
The next lead run will take place in 2018, when we will be able to record a large amount of central events, about 10 times the number of events that we collected in the previous run.
This will allow us to study in detail many phenomena; for example, we will be able to look for event plane correlations and flow effects in ultra-central collisions with much larger precision, study substructures of jets, and measure heavy flavour suppression at low pT, which is important to understand the total charm production cross section.