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

TL;DR

This paper measures groomed jet mass in PbPb and pp collisions at 5.02 TeV to study medium effects on jet structure, finding no significant modification in PbPb compared to pp, challenging some theoretical models.

Contribution

First measurement of groomed jet mass in heavy ion collisions at the LHC, providing insights into medium-induced modifications of jet substructure.

Findings

Agreement of pp results with PYTHIA and HERWIG++ predictions

Hint of increased large-mass jets in central PbPb collisions

No observed modification of groomed jet core mass in PbPb collisions

Abstract

A measurement of the groomed jet mass in PbPb and pp collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV with the CMS detector at the LHC is presented. Jet grooming is a recursive procedure which sequentially removes soft constituents of a jet until a pair of hard subjets is found. The resulting groomed jets can be used to study modifications to the parton shower evolution in the presence of the hot and dense medium created in heavy ion collisions. Predictions of groomed jet properties from the PYTHIA and HERWIG++ event generators agree with the measurements in pp collisions. When comparing the results from the most central PbPb collisions to pp data, a hint of an increase of jets with large jet mass is observed, which could originate from additional medium-induced radiation at a large angle from the jet axis. However, no modification of the groomed mass of the core of the jet is observed for all PbPb centrality classes. The PbPb results are also compared to predictions from the JEWEL and Q-PYTHIA event generators, which predict a large modification of the groomed mass not observed in the data.