  Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy
of 2.76 TeV was studied with the CMS detector at the LHC, using a data sample
corresponding to an integrated luminosity of 6.7 inverse microbarns. Jets are
reconstructed using the energy deposited in the CMS calorimeters and studied as
a function of collision centrality. With increasing collision centrality, a
striking imbalance in dijet transverse momentum is observed, consistent with
jet quenching. The observed effect extends from the lower cut-off used in this
study (jet transverse momentum = 120 GeV/c) up to the statistical limit of the
available data sample (jet transverse momentum approximately 210 GeV/c).
Correlations of charged particle tracks with jets indicate that the momentum
imbalance is accompanied by a softening of the fragmentation pattern of the
second most energetic, away-side jet. The dijet momentum balance is recovered
when integrating low transverse momentum particles distributed over a wide
angular range relative to the direction of the away-side jet.
