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

Abstract

This letter presents the first measurement of jet mass in Pb–Pband p– Pbcollisions at √sNN=2.76TeV and √sNN=5.02TeV, 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-kTjet algorithm and resolution parameter R =0.4. The jets are measured in the pseudorapidity range |ηjet| <0.5and in three intervals of transverse momentum between 60 GeV/cand 120 GeV/c. The measurement of the jet mass in central Pb–Pbcollisions is compared to the jet mass as measured in p–Pbreference collisions, to vacuum event generators, and to models including jet quenching. It is observed that the jet mass in central Pb–Pbcollisions is consistent within uncertainties with p–Pbreference measurements. Furthermore, the measured jet mass in Pb–Pbcollisions is not reproduced by the quenching models considered in this letter and is found to be consistent with PYTHIA expectations within systematic uncertainties.