  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.
