Studies of Jet Modification in Heavy Ion Collisions with the CMS Experiment

Abstract

In the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC), lead ions are collided at ultra-relativistic velocities to produce Quark-Gluon Plasma (QGP), a state of matter where quarks and gluons are deconfined and move collectively. Jets are produced in high-momentum transfer parton scatterings prior to and independently of QGP formation, and serve as natural probes of its properties. As the high-energy partons pass through the QGP, they lose energy through medium-induced gluon radiation and elastic scattering, resulting in jets that are modified with respect to the vacuum baseline. In this thesis, jet modification is quantified by measuring the jet production cross section as a function of jet radius in inclusive jets and the jet axis decorrelation in jets recoiling from isolated photons in Lead-Lead (PbPb) and Proton-Proton (pp) collisions. Both measurements indicate that effects of medium-induced jet broadening may be balanced by survivor bias in PbPb collisions, potentially due to differences in the magnitude of quenching of wide versus narrow jets. The results underline the importance of constraining the initial jet kinematics with bosons, which are unmodified by the QGP.