Comparison of jet fragmentation for isolated-photon+jet pairs in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$

Measurements of fragmentation functions for jets paired with an isolated photon in pp and, for the first time, PbPb collisions are presented. The analysis uses data from the CMS detector at the CERN LHC, with both systems at a nucleon-nucleon center-of-mass energy of $5.02\,\mathrm{TeV}$. Fragmentation functions are constructed using charged particles with transverse momentum $p_{\mathrm{T}}^{\mathrm{trk}} > 1\,\mathrm{GeV}/c$ inside jets with transverse momentum $p_{\mathrm{T}}^{\mathrm{jet}} > 30\,\mathrm{GeV}/c$ for events containing an isolated photon with $p_{\mathrm{T}}^{\gamma} > 60\,\mathrm{GeV}/c$. For central PbPb collisions, modifications of the jet fragmentation function with respect to that found for pp collisions are observed, while no significant differences are found in the 50\% most peripheral collisions. The modifications seen in central events indicate an enhancement for particles at low $p_{\mathrm{T}}$ and a depletion at high $p_{\mathrm{T}}$, with a transition around $3\,\mathrm{GeV}/c$. These measurements provide information about the longitudinal modifications of a parton shower whose initial kinematics are tightly constrained by the properties of the associated photon.