Measurement of groomed jet mass in PbPb and pp collisions at $\sqrt{s_{NN}} = 5.02~\mathrm{TeV}$

A measurement of the groomed jet mass ($M_g$) normalized by the jet transverse momentum ($p_\text{T}^\text{jet}$) for anti-$k_\text{T}$ jets with the radius parameter $0.4$ in PbPb and pp collisions at a center of mass energy of $5.02\,\mathrm{TeV}$ per nucleon pair is presented. The jet grooming algorithm is a recursive procedure which sequentially removes soft constituents of a jet until a pair of hard subjets is found. The results of this grooming can be used to study modifications to partons and their evolution while traversing the hot and dense medium created in heavy ion collisions, via small angle splitting of quarks and gluons insie the jet cores. The CMS detector at the LHC is used to perform this analysis for $p_\text{T}^\text{jet}$ between 140 and $300\,\mathrm{GeV}$ and pseudo-rapidity less than 1.3 in both PbPb and pp collisions, and for a range of PbPb collision centralities. The measurements in pp collisions are compared to predictions from the PYTHIA and HERWIG++ event generators and agreement at the $20\%$ level is found. When compared to pp data, an increase of jets with large jet mass is observed for the PbPb results. However, the core of the jet is observed to be unmodified for all event centrality classes.