Observation and studies of jet quenching in PbPb collisions at √sNN = 2.76 TeV

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the Compact Muon Solenoid (CMS) detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7 μb^(−1). 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 cutoff used in this study (jet pT = 120 GeV/c) up to the statistical limit of the available data sample (jet pT ≈ 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

Search for resonances in the dilepton mass distribution in pp collisions at √s = 7 TeV

A search for narrow resonances at high mass in the dimuon and dielectron channels has been performed by the CMS experiment at the CERN LHC, using pp collision data recorded at √s=7 TeV. The event samples correspond to integrated luminosities of 40 pb^(−1) in the dimuon channel and 35 pb^(−1) in the dielectron channel. Heavy dilepton resonances are predicted in theoretical models with extra gauge bosons (Z′) or as Kaluza-Klein graviton excitations (G_(KK)) in the Randall-Sundrum model. Upper limits on the inclusive cross section of Z′(G_(KK)) → ℓ^+ ℓ^− relative to Z → ℓ^+ ℓ^− are presented. These limits exclude at 95% confidence level a Z′ with standard-model-like couplings below 1140GeV, the superstring-inspired Z'_ψ below 887 GeV, and, for values of the coupling parameter kM_(Pl) of 0.05 (0.1), Kaluza-Klein gravitons below 855 (1079) GeV