Measurement of jet fragmentation in PbPb andppcollisions atsNN=2.76TeV

Abstract

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...

Small-radius jets to all orders in QCD

“…However in most practical uses of jets at hadron colliders, the jet radius is taken somewhat smaller: this makes it more straightforward to resolve multiple jets in events such as top-antitop production, which can decay to six quarks and in cascade decays of supersymmetric particles; it also significantly reduces the contamination of the jet by the underlying event and multiple simultaneous pp collisions (pileup). The most common choices for R are in the range 0.4-0.5 [3,4], and in some extreme environments, such as heavy-ion collisions, even smaller values are used, down to R = 0.2 [5][6][7][8][9]. Also, one sometimes studies the ratio of the inclusive jet cross-sections obtained with two different R values [5,[10][11][12][13].…”

“…Given a parton with transverse momentum p t , we define f hardest (z) to be the probability that the hardest resulting microjet carries a momentum zp t . 9 Now instead of a momentum sum rule, we have a probability sum rule,…”

“…The corresponding full analytical expressions are given in appendix A. 9 We should really write f hardest (z, t), but in most cases drop the t argument for compactness.…”

Event-by-event fluctuations in the medium-induced jet evolution

“…A remarkable phenomenon discovered and, by now, abundantly studied in Pb+Pb collisions at the LHC is the di-jet asymmetry -a strong energy imbalance between two jets which propagate nearly back-to-back in the plane transverse to the collision axis [1][2][3][4][5][6][7][8][9]. Imbalanced di-jets are also observed in proton-proton collisions, where they are mostly associated with 3-jet events, but the respective events in (central) Pb+Pb collisions look quite different.…”

Origins of the di-jet asymmetry in heavy-ion collisions

“…The ability to systematically reconstruct jets above the large and fluctuating background present in ultra-relativistic heavy-ion collisions [1] has opened up a versatile path [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] to study the properties of Quark Gluon Plasma (QGP). Jets are sensitive, through the wide range of scales involved in their development, to a variety of properties of the expanding QGP they traverse.…”