by Panos Charitos. Published: 15 April 2013

This years' ALICE Thesis Awards were announced during the recent Collaboration Board during an official ceremony. The thesis committee announced the two winners: i) Antonin Maire for the best physics thesis on the “Multi-strange baryon production at the LHC in proton-proton collisions with the ALICE experiment” and Magnus Mager for the best technical thesis on "Studies on the upgrade of the ALICE central tracker".

The members of the thesis committee who made the selection are: Bruno Ghidini, Christina Markert, Nicole Bastid, Peter Braun-Munzinger, Sudhir Raniwala and Paul Kuijer.

Since 2008 two awards are given once a year for the most outstanding PhD thesis by members of the ALICE Collaboration in the field of physics and instrumentation, respectively. The awards celebrate the excellence of young researchers at ALICE and the importance of their work in the physics world. This year the ALICE Thesis Award received a kind donation which will be used to further augment the prestige of the award.

ALICE MATTERS met this years' winners and asked them about their future research plans and the highlights of their PhD thesis.

Interviewing Antonin Maire

How did you decide to pursue your PhD in the field of Quantum Chromodynamics?

I have been in touch with the group ALICE/STAR in the University of Strasbourg (IPHC Institute) since the 3rd year of my undergraduate studies. At that time, I had a two-month training period, working closely with the group. My work was based on one of the early hydrodynamic models and more specifically, on its comparison with some heavy-ion data from the STAR experiment based at RHIC.

I started working in the field of heavy-ion physics in 2005 and I went on to complete my MSc and my PhD thesis in the same field. Since that research training, I have established a very good relationship with the group. For each decisive step of my degree course, I could get some opinions and advice from the team; I had the chance to regularly meet and discuss physics and research. The group had already acquired expertise in strangeness and, in 2008, as the LHC was about to start collecting data, we considered reasonable to build on the previous experience and study multi-strange baryons Ξ– (dss) and Ω– (sss) at the CERN machine.

What was the main research question of your PhD thesis?

The research on quantum chromodynamics can go via the study of different collisions, the “benchmark” systems a priori free of Quark Gluon Plasma (QGP), i.e. proton-proton (p-p) or proton-nucleus (p-A), and the “QGP-ready” systems, with heavy-ions (A-A). In any of these systems, it is essential to be able to identify the type of a particle. In this respect, strange quarks define valuable probes and the multi-strange baryons Ξ- and Ω- go along this line.

The importance of these particles can be better understood if we think of the big picture. Consider all the quarks that can possibly be produced in a collision, namely: the up, down, strange, charm and beauty quarks. (For our concerns, we can omit the top quark...). What is particularly exciting about strange quarks is that they are in the middle of this mental picture. Up and down quarks are light, while charm and beauty quarks are quite heavy; strange quarks are, in a sense, balancing between these two extremes. Of course strangeness is more on the lighter side but it is still significantly heavier than up and down quarks.

Therefore, if you try to bridge the gap between light flavours and gluons on the one hand and charm and beauty on the other hand, it is necessary to inspect the role of strangeness. Although strangeness is not the most abundant population produced in a collision, it appears to play a special role. Strangeness is one middle piece in the big picture, that may help to understand strong interactions in p-p and characterise the QGP in A-A.

What are the results, or if you prefer, the highlights of your thesis?

The idea was to measure Ξ and Ω production as a function of transverse momentum (pT) in p-p collisions. It was meant to be the first ALICE measurement on this kind of particles, so we had to prepare the ground; certainly this goal was one of the key challenges of my thesis. A Ξ pT spectrum was first measured at 0.9 TeV ; it was a combined measurement of particles and anti-particles, done with a limited sample of data. As Ω are particles even rarer than Ξ, it was not possible to get an Ω result at this energy. At 7 TeV, the situation changed to something more comfortable : with the large quantity of available data, we were able to extract particles and anti-particles separately, allowing the production measurements for each of the four species : Ξ–, Ξ+, Ω– and Ω+. Here, I would like to thrust another actor into the limelight, somebody that was also a PhD student at that time : David Chinellato. The team work with him has really been a great chance!

Having cross section measurements at different energies has been a prime incentive : we wanted to know how the production of strangeness evolved with the collision energy. Similar measurements at lower energies were available in p-p, e.g. by the STAR experiment, at 0.2 TeV. So our work aimed at extending the “excitation function” into the TeV region, providing a much wider energy coverage.

Getting to know this energy evolution is important for at least two reasons.

The first one is related to A-A studies. In the 80's, strangeness had been proposed as the basis of one QGP signature ; the idea is that strangeness production should be enhanced within the plasma in A-A, enhanced relative to the p-p system. A consequence is that if you don’t understand the p-p benchmark, you can spoil your comprehension in heavy ions. Understanding what is happening in p-p provides the baseline of further analysis in A-A at LHC energies.

The second reason is related to p-p physics itself. We often derive some physics from Monte Carlo simulations, being themselves the results of phenomenological models. For models like EPOS, HERWIG, PHOJET, PYTHIA, SHERPA, ... it might be easy to describe pions, i.e. particle type that correspond to about 83% of the whole particle production in p-p. Compared to that, Ξ and Ω are just negligible quantities. However, if you want to come to more and more robust models, providing fine descriptions and a comprehensive picture, strangeness can no longer be ignored. And there, multi-strange baryons can become crucial for testing one's theoretical model. At the moment, Ξ and Ω in p-p collisions are described poorly.

Comparisons between our experimental spectra and spectra as produced by different benchmark phenomenological models show an unequivocal underestimate by the Monte Carlo generators in their current versions (up to a factor ~4 for Ξ, ~15 for Ω). This shows there is room for physics discussion and for complementing the p-p picture.

What are your ongoing plans?

I have started a postdoc in Heidelberg where I am working on J/ψ analysis. This is a step to heavier states but I always try to keep in mind the broader picture that I described before. In fact, I now share my time between two working groups. On the one hand, I still follow light flavours via one of our PhD students who studies strangeness content in jets. On the other hand, on the J/ψ topic, I participate to the analysis of Pb-Pb data and follow another PhD student on the J/ψ production in p-Pb.

Discussing with Magnus Mager

What has been your previous background before starting your PhD?

Before starting my PhD, I was writing my diploma thesis within the ALICE TPC group of GSI, where I dealt with the space point distortions due to the slight inhomogeneity of the magnetic field (ExB effect) as well as with an adaptation of the thermal model to interpolate from central heavy-ion to pp collisions. Apart from that I am since always enjoying math, algorithms, electronics and computer programming.

Magnus Mager receiving the ALICE thesis award by Paul Kuijer.

How did you decide to pursue your thesis on the ITS upgrade? 

The ALICE upgrade (my thesis also dealt with the TPC) is fascinating, because it gives the opportunity to design a partly new detector from scratch. I personally have been enjoying very much the physics driven design process, which given a physics observable (in my case the Lambda-c) searches for the best possible detector configuration to measure it.

How were you nominated for the prize? What has been the process?

This year, one had to nominate oneself. I was, however, encouraged to do so by different people (to whom I express my gratitude since otherwise I would probably not have nominated myself).

What are your feelings now and your future plans?

I am continuing with the ITS upgrade project. I shifted focus towards the silicon sensor chips and I am currently coordinating the sensor characterisation activities for the ITS upgrade. I enjoy very much working on the hardware side of the detector and I am looking forward to the integration of the new ITS and the installation in 2018.