Studies of jet quenching in PbPb collisions at sqrt(s) = 2.76TeV

JStudying the interaction of fast partons with the exotic matter produced in high-energy heavy ion collisions can help to elucidate the properties of the produced medium. The high resolution calorimeters and high precision silicon tracker in the Compact Muon Solenoid detector at the Large Hadron Collider (LHC) make it ideal for investigating these hard scattering processes. The first heavy ion run at the LHC provided PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV. Parton energy loss should result in a significant dijet transverse momentum imbalance, when comparing to a pp reference distribution. A large imbalance is, in fact, observed in the PbPb data, with the magnitude of the effect increasing with centrality. The details of the energy loss process were studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, low momentum particles emitted at large angles with respect to the jet axis constitute a large fraction of the momentum balance for asymmetric dijets in PbPb collisions. Fragmentation functions for leading and subleading jets were extracted and were observed to be unmodified compared to those in pp. These data provide a wealth of information on the propagation of hard partons through hot QCD matter.