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 p T = 120 GeV/c) up to the statistical limit of the available data sample (jet p T ≈ 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.

The first measurement of jet shapes, defined as the fractional transverse momentum radial distribution, for inclusive jets produced in heavy-ion collisions is presented. Data samples of PbPb and pp collisions, corresponding to integrated luminosities of 150 µb −1 and 5.3 pb −1 respectively, were collected at a nucleon-nucleon centre-ofmass energy of √ s NN = 2.76 TeV with the CMS detector at the LHC. The jets are reconstructed with the anti-k T algorithm with a distance parameter R = 0.3, and the jet shapes are measured for charged particles with transverse momentum p T > 1 GeV/c. The jet shapes measured in PbPb collisions in different collision centralities are compared to reference distributions based on the pp data. A centrality-dependent modification of the jet shapes is observed in the more central PbPb collisions, indicating a redistribution of the energy inside the jet cone. This measurement provides information about the parton shower mechanism in the hot and dense medium produced in heavy-ion collisions.

Jet fragmentation in pp and PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair was studied using data collected with the CMS detector at the LHC. Fragmentation functions are constructed using charged-particle tracks with transverse momenta p T > 4 GeV/c for dijet events with a leading jet of p T > 100 GeV/c. The fragmentation functions in PbPb events are compared to those in pp data as a function of collision centrality, as well as dijet-p T imbalance. Special emphasis is placed on the most central PbPb events including dijets with unbalanced momentum, indicative of energy loss of the hard scattered parent partons. The fragmentation patterns for both the leading and subleading jets in PbPb collisions agree with those seen in pp data at 2.76 TeV. The results provide evidence that, despite the large parton energy loss observed in PbPb collisions, the partition of the remaining momentum within the jet cone into high-p T particles is not strongly modified in comparison to that observed for jets in vacuum.

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The jet fragmentation function of inclusive jets with transverse momentum p T above 100 GeV/c in PbPb collisions has been measured using reconstructed charged particles with p T above 1 GeV/c in a cone of radius 0.3 around the jet axis. A data sample of PbPb collisions collected in 2011 at a nucleon-nucleon center-of-mass energy of √ s NN = 2.76 TeV corresponding to an integrated luminosity of 150 µb −1 is used. The results for PbPb collisions as a function of collision centrality and jet transverse momentum are compared to reference distributions based on pp data collected at the same center-of-mass energy in 2013, with an integrated luminosity of 5.3 pb −1 . A centrality-dependent modification of the fragmentation function is found. For the most central collisions, a significant enhancement is observed in the PbPb/pp fragmentation function ratio for charged particles with p T less than 3 GeV/c. This enhancement is observed for all jet p T bins studied. 4 Monte Carlo simulations The CMS detectorThe centerpiece of the CMS detector is a superconducting solenoid, 12.5 m long with an internal diameter of 6 m, that provides a uniform magnetic field of 3.8 T. In the CMS coordinate system, the z axis points in the counterclockwise beam direction, the x axis points towards the centre of the LHC ring, and the y axis points up, perpendicular to the plane of the LHC ring. The azimuthal angle φ is measured with respect to the x axis, and the polar angle θ is measured with respect to the z axis. Charged particles or charged particles reconstructed in the inner tracking system are characterized by their transverse momentum, p T = | p| sin θ, and pseudorapidity, η = − ln [tan(θ/2)]. The inner tracking system is composed of a pixel detector with three barrel layers at radii between 4.4 and 10.2 cm and a silicon strip tracker with 10 barrel layers extending outwards to a radius of 110 cm. Two endcap modules extend the acceptance of the tracking system up to |η| = 2.5. The momentum resolution for reconstructed tracks in the barrel region is about 1% at p T = 100 GeV/c and up to 2% in the endcap at the same p T .The calorimeters inside the magnetic coil consist of a lead-tungstate crystal electromagnetic calorimeter (ECAL) and a brass/scintillator hadron calorimeter (HCAL) with coverage up to |η| = 3. Steel/quartz-fibre Cherenkov hadron forward (HF) calorimeters extend the coverage to |η| = 5.2. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke of the magnet. The calorimeter cells are grouped in projective towers of granularity ∆η × ∆φ = 0.087 × 0.087 for the central rapidities (|η| ≤ 2) considered in this paper. The energy scale in data agrees with that in the simulation to better than 1% in the barrel region (|η| < 1.5) and better than 3% in the endcap region (1.3 < |η| < 3.0) [20]. Hadron calorimeter cells in the |η| < 3 region are calibrated primarily with test-beam data and radioactive sources [21,22]. A detailed description of the CMS detector can be found in Ref. [23]. Monte Carlo simulat...