Jet-Track Correlation Studies of the Quark Gluon Plasma
The properties of the quark gluon plasma (QGP) are explored using correlations between jets and charged-hadron tracks in lead-lead (PbPb) and proton-proton (pp) collision events at 2.76 and 5.02 TeV recorded with the CMS detector. Jets with high transverse momentum (pT) may serve as in situ probes of the QGP because they originate from partons that are produced in initial hard-scatterings in heavy ion collisions, and subsequently pass through and interact with the medium. Comparing measurements in PbPb collisions to pp reference data can be used to study jet energy loss in the medium, medium-induced modifications to jet structure, and possible hydrodynamic medium response to the high-pT propagating parton. In this analysis, jet-track correlations are used to extract a number of different observables that can serve to constrain theoretical models of interactions between jets and the QGP. These observables include the number and angular distribution of charged hadron tracks with respect to the jet axis as a function of track-pT (which characterizes jet fragmentation), as well as the jet transverse momentum profile (jet shape). Jet properties in PbPb data are studied differentially in event centrality (i.e. the degree of overlap between the two colliding nuclei) and track-pT for an inclusive sample of highly-pT jets and for back-to-back dijets.