Detailed Characterization of Jets in Heavy Ion Collisions Using Jet Shapes and Jet Fragmentation Functions

The CMS detector has excellent capabilities for studying high-$p_{\mathrm{T}}$ jets formed in heavy ion collisions. Previous CMS analyses have characterized the energy loss of hard-scattered partons traversing the medium produced in such collisions at a center of mass energy of 2.76 TeV using the momentum imbalance of di-jet and photon-jet events. In this paper, the fragmentation properties of inclusive jets with $p_{\mathrm{T,jet}}>100$~GeV/$c$ in PbPb collisions are characterized by measuring differential and integated jet shapes, as well as charged particle fragmentation functions. A data sample of PbPb collisions collected in 2011 at a center of mass energy of $\sqrt{s_{_{\mathrm{NN}}}}=$2.76~TeV corresponding to an integrated luminosity of $L_{int} = 140~\mu\mbox{b}^{-1}$ is used. The results for PbPb collisions as a function of collision centrality are compared to reference distributions based on pp data collected at the same collision energy. For both PbPb and pp collisions, jets are reconstructed with the anti-$k_T$ clustering algorithm with a resolution parameter of 0.3 and using ``Particle Flow'' objects that combine tracking and caloritmetry information. The jet shapes and fragmentation functions are measured for reconstructed charged particles with $\pt > 1$~GeV/$c$ within the jet cone. For the most central collisions indications of a broadening of the differential jet shape in PbPb collisions are observed, as well as a significant rise of the PbPb/pp fragmentation function ratio for the softest fragmentation products with $p_{\mathrm{T}} < 3$~GeV/$c$.