by Panos Charitos. Published: 01 March 2014

Marta finished her MSc thesis at Utrecht University where she worked with the ALICE group on the study of charm quarks. During her master she realized that she was well fitted to the research environment and the field of heavy-ion physics seemed really exciting.

Following the successful completion of her MSc she moved on working on her PhD thesis with the same group under the supervision of Raimond Snellings and Marco van Leeuwen. However, she decided to move to a slightly different field, namely jet quenching, which, as Marta explains: “is another hard probe for the medium recreated in ultra-relativistic heavy-ion collisions. Heavy quarks also cause jet quenching but the focus now is on light flavours as they are produced in larger numbers during heavy-ion collisions”.

When Marta started her PhD she focused on phenomenological models, describing the interaction of high-energetic partons with the medium formed in ultrarelativistic heavy-ion collisions. She pursued a fairly systematic study taking into account previous results from RHIC that were showing strong suppression for high pT particles.

Marta says: “There were several models on the market but there was an inconsistency regarding the density of the medium that they were predicting. The density of the medium that is produced during heavy-ion collisions is rather complicated. In the beginning it is hot in the centre but as the medium expands it cools down. The spatial and time evolution of the medium adds a lot of complexity to the study of the interaction between partons and the medium”.

In order to study this effect, Marta assumed a medium with constant density and fixed length. High pT partons travel through the medium and the energy loss of partons is calculated with different models. “For each model we did the calculations: first we assumed that the high energetic parton radiates only one gluon and calculated the probabilities of the different energies of this gluon. In reality, partons radiate multiple gluons and hence we did a second step taking this into account while finally we calculated a suppression factor in order to compare suppression factors for different models” and she adds: “Different models result in different energy loss while all models try to describe the same physics and use similar principles; one wouldn’t expect these large discrepancies. This study taught us that many details in the modelling matter. This was a valuable lesson as we learned how we could improve our calculations".

For Marta this has been one of the most intensive periods in her life. She familiarized herself with many new notions that also gave her a deeper understanding of the physics of heavy-ion collisions. It also gave her the chance to interact with many theorists working on the different models that she compared her analysis.

Moreover, Marta’s PhD had a strong experimental focus. Using data from the 2010 heavy-ion run of the LHC she started working on jet reconstruction in ALICE.

She studied jets as a function of the centrality of the collision and it was found that less jets are observed in central collisions compared to peripheral. In the presence of a medium a jet is quenched which can result in the broadening of the jet profile and out-of-cone jet radiation. When the gluons are radiated inside the jet cone, the constituents of the jet are expected to be softer and have a broader density profile compared to jets in vacuum. In other words when we see suppression the simplest interpretation is that gluons are not emitted close to the jet axis but at larger angles that are not recovered in the jet.

For jet measurements in heavy-ion collisions one has to deal with the background induced by the large number of soft particles. She recalls: “large part of my thesis was to think how the background could be removed. We decided to adopt a two-step process: first we calculated an average background for each event that was then subtracted from the jet. After that we used a statistical method called unfolding to correct for all the remaining fluctuations”.

It should be noted that one cannot adopt a very broad jet cone as jets are highly collimated with most of the energy lying near the centre. As we move further out from the centre of the jet-axis we have less energy coming from the leading parton and we “see” more and more background particles.

Last May, Marta started working at Wayne State University in the US. She studies cold nuclear matter effects by using measurements of di-jet production in proton-lead collisions.

“What we would like to know is whether the phenomena observed in lead-lead collisions are due to hot nuclear matter effects or not. In order to answer this question we compare our measurements with data from proton-lead collisions".

Jets in p-Pb collisions look quite similar to those in p-p collisions and we don’t see a significant modification when we compare these data with p-Pb results. Marta notes: “This is a sign that we are looking in the right direction as we might assume that the differences observed in Pb-Pb collisions are due to hot nuclear matter effects”.

However, there is more room for research and “we still don’t know exactly the path-length dependence of the energy loss”. This is one of the topics on which Marta is currently working and we are looking forward to learning more about one of the most puzzling phenomena in heavy-ion physics.

You can read Marta Verweij's PhD thesis here