by Virginia Greco. Published: 12 October 2018

Mohamad Tarhini, from the Institut de Physique Nucléaire (IPN) in Orsay (France), was assigned one of the 2018 ALICE Thesis Awards for his work on Z-boson and J/ψ production measurements in p-Pb and Pb-Pb collisions. We talked with him about his studies and research interests.

After completing his Bachelor’s and part of his Master’s studies at the Lebanese University in Lebanon, his home country, Mohamad obtained a scholarship to continue his Master in Particle Physics at the Université Paris-Sud in Orsay (France). There, he joined the ALICE collaboration and started to work with the Institut de Physique Nucléaire (IPN) on J/psi and double J/psi production in p-Pb collisions at the LHC.

As soon as he concluded the Master, he got the opportunity to continue his research enrolling for a PhD in Orsay, which he started at the end of 2014. For his thesis, Mohamad developed in parallel two analyses: the study of the Z-boson production and of the J/psi-meson production in different collision systems with the ALICE detector.

The original focus of my thesis was meant to be an extension of the measurements of the J/psi production, on which I was already working, to Pb-Pb collisions at 5.02TeV, using data that would be taken during Run 2,” explains Mohamad. “But, while waiting for these data to be available, I started another study on the Z boson. My colleagues and I performed the first measurement in ALICE of the Z boson production in p-Pb collisions.” When the 2015 run of Pb-Pb collisions at 5 TeV was completed, he continued both analyses on these new data.

The objectives and motivations of the two studies are quite different. While the main interaction at work in the Quark Gluon Plasma is the strong one, the Z boson does not interact strongly because it has no colour charge. As a consequence, in principle, the Z boson production should not be affected by the QGP formation in Pb-Pb collisions in ALICE. Any difference observed in this production, compared with what happens in pp collisions, provides indications about cold nuclear matter effects that must play a role in the dynamics of these systems. 

There are various models parametrizing the nuclear modification of the parton distribution function (nPDF), which is the main phenomenon that can affect the Z boson production,” Mohamad explains, “and indeed we have observed a suppression of this production in Pb-Pb collisions with a significance of 2.3 sigma. This result not only confirms that there are other effects involved in the collisions in which the QGP is formed but are also very useful to help theorists constrain the nPDF models.” This study, of course, will be extended to the new data that ALICE will take this year and, later, in Run 3.

The J/psi meson, on the contrary, can interact strongly, thus its production can be influenced by the QGP. A suppression or an enhancement of the J/psi production in Pb-Pb collisions, with respect to pp, reveals the presence of the QGP and these measurements allow studying the characteristics of this state of matter.

Here the picture is less clear than it is for the Z-bosons. The J/psi production in heavy-ion collisions is not well understood theoretically. This production is affected by different mechanisms, some of them can cause a suppression of the J/psi production in Pb-Pb collisions, while others its enhancement. As a consequence, we need to perform precise measurements of different observables to be able to constrain the theory.”

Since fall 2017, Mohamad is working as a postdoc at the SUBATECH laboratory, in Nantes, on the J/psi polarization in Pb-Pb collisions. At the moment, he is also involved in hardware activities for the upgrade of the Muon-Forward-Tracker (MFT) detector of ALICE.

I am passionate about research and I would really like to keep working in heavy-ion physics,” Mohamad concludes.