CORDIS - EU research results

Quark-Gluon Plasma through dilepton studies with the CMS experiment at the Large Hadron Collider

Final Report Summary - QUARKGLUONPLASMACMS (Quark-Gluon Plasma through dilepton studies with the CMS experiment at the Large Hadron Collider)

This project, led by Raphael Granier de Cassagnac, aims at studying the state of matter created in the heavy-ion (lead-lead) collisions at the Large Hadron Collider. This state, often referred to as the Quark-Gluon Plasma, must have prevailed in the Universe during the first microseconds after the big-bang.
The team supported by this grant works in the CMS experiment and spearheaded the use of leptons to characterise the plasma, via leading all the first analyses of particles decaying in muons, and developing electron reconstruction in the challenging heavy-ion environment. It also organised the storage and processing of the lepton-related data.
In heavy-ion collisions, the variety of produced particles can be used to probe different properties of the matter. We focus on quarkonia (bound states of a charm or bottom quark with its antiquark) and electroweak bosons.
The latter were never observed before in heavy-ion collisions and are of particular interest: Since they are not affected by the medium, they can serve as standard candles. In November 2010, we observed the first Z bosons in heavy-ion collisions, at the start of the first lead-lead campaign of LHC, in both their dielectron and dimuon decay channels. Since then, about 1400 Z were collected, confirming that the usually-assumed scaling holds: for such hard processes, a lead-lead collisions can be considered as a simple superposition of nucleon-nucleon collisions. The W bosons, more copiously produced but more difficult to detect also confirmed the hypothesis, proving themselves to be unmodified in the plasma, with a hint of nuclear effects observed in proton-lead collisions.
In contrast, quarkonia are supposed to be strongly affected by the medium, the leading expected effect being their melting if the temperature is high enough. Looking at their decay in two muons, we indeed observed the disappearance of five of them, two of which made of charm quarks (the psi family), three of bottom quarks (the upsilon family). A by-product of the psi-analysis was the suppression of the psi originating from B mesons decaying far outside the medium, a first unambiguous observation of the b quark energy loss in the plasma. Putting this aside, the ordered suppression of the prompt quarkonia, especially of the upsilon family, indeed suggest a temperature-related effect. The five quarkonia could ultimately be used as a 5-grade thermometer of the medium, once the rich phenomenology of competing effects is taken into account.