Bharadwaj Harikrishnan soutient sa thèse

"Mesures de la sous-structure des jets marqués par des photons dans les collisions pp et PbPb avec le détecteur CMS".

Measurements of the girth (g) and groomed radius of jets (Rg) recoiling against isolated photons in lead-lead (PbPb) and proton-proton (pp) collisions at the LHC at 5.02 TeV using the data collected at the CMS experiment are presented in this thesis. These measurements provide insight into the properties of the Quark Gluon Plasma (QGP), a hot and dense medium created in PbPb collisions. Modifications of jets in the QGP medium are studied relative to a reference provided by pp collision events. The observables g and Rg provide a quantitative measure of how narrow or broad the structure of the jet is. Measurements of these in inclusive jet events have shown a narrower jet substructure in heavy ion collisions compared with pp collisions for jets reconstructed with the same jet transverse momentum (pTjet) in previous measurements. A measurement of these observables in photon-tagged jet events provides a complementary interpretation about the jet energy loss in the QGP medium by considering the photon transverse momentum (pTgamma) as a proxy for the pT of the parton that initiates the jet shower.

Events are required to have a photon with transverse momentum pTgamma > 100 GeV and at least one jet back-to-back in azimuth with respect to the photon and with transverse momentum pTjet such that pTjet/pTgamma > 0.4. The measured Rg and g distributions are unfolded to the particle level, which facilitates the comparison between the PbPb and pp results and with theoretical predictions. It is found that jets with pTjet/pTgamma > 0.8, i.e., those that closely balance the photon pT, are narrower in PbPb than in pp collisions. Relaxing the selection to include jets with pTjet/pTgamma > 0.4 reduces the narrowing of the angular structure of jets in PbPb relative to the pp reference. This shows that selection bias effects associated with jet energy loss play an important role in the interpretation of jet substructure measurements.