# First measurement of jet mass in Pb-Pb and p-Pb collisions at the LHC

## TL;DR

This study reports the first measurement of jet mass in Pb-Pb and p-Pb collisions at the LHC, providing insights into jet quenching effects in quark-gluon plasma using reconstructed jets and comparing results with models.

## Contribution

It introduces the first measurement of jet mass in heavy-ion collisions at the LHC and compares it with p-Pb references and theoretical models, highlighting discrepancies with jet quenching models.

## Key findings

- Jet mass in Pb-Pb collisions aligns with p-Pb references within uncertainties.
- Jet mass measurements are consistent with PYTHIA expectations.
- Current jet quenching models do not reproduce the observed jet mass in Pb-Pb collisions.

## Abstract

This letter presents the first measurement of jet mass in Pb-Pb and p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV and 5.02 TeV, respectively. Both the jet energy and the jet mass are expected to be sensitive to jet quenching in the hot Quantum Chromodynamics (QCD) matter created in nuclear collisions at collider energies. Jets are reconstructed from charged particles using the anti-$k_{\rm T}$ jet algorithm and resolution parameter $R = 0.4$. The jets are measured in the pseudorapidity range $|\eta_{\rm{jet}}|<0.5$ and in three intervals of transverse momentum between 60 GeV/$c$ and 120 GeV/$c$. The measurement of the jet mass in central Pb-Pb collisions is compared to the jet mass as measured in p-Pb reference collisions, to vacuum event generators, and to models including jet quenching. It is observed that the jet mass in central Pb-Pb collisions is consistent within uncertainties with p-Pb reference measurements. Furthermore, the measured jet mass in Pb-Pb collisions is not reproduced by the quenching models considered in this letter and is found to be consistent with PYTHIA expectations within systematic uncertainties.