Abstract
The quark-gluon plasma is studied via medium-induced changes to correlations between jets and charged particles in PbPb collisions compared to pp reference data. This analysis uses data sets from PbPb and pp collisions with integrated luminosities of 166 inverse microbarns and 5.3 inverse picobarns, respectively, collected at sqrt(s[NN]) = 2.76 TeV. The angular distributions of charged particles are studied as a function of relative pseudorapidity (Delta eta) and relative azimuthal angle (Delta phi) with respect to reconstructed jet directions. Charged particles are correlated with all jets with transverse momentum (pt) above 120 GeV, and with the leading and subleading jets (the highest and second-highest in pt, respectively) in a selection of back-to-back dijet events. Modifications in PbPb data relative to pp reference data are characterized as a function of PbPb collision centrality and charged particle pt. A centrality-dependent excess of low-pt particles is present for all jets studied, and is most pronounced in the most central events. This excess of low-pt particles follows a Gaussian-like distribution around the jet axis, and extends to large relative angles of Delta eta approximately 1 and Delta phi approximately 1.
Highlights
Fit Background Fit (Fourier Only)Systematic Uncertainty ∆φS(∆η,∆φ) - B(∆η,∆φ) yield: a bias toward the selection of jets with harder fragmentation, and a bias toward the selection of jets that coincide with upward fluctuations in the background
In order to account for the influence of the underlying PbPb events, the pythia events are embedded into fully simulated PbPb events that are generated by hydjet [40], which is tuned to reproduce the total particle multiplicities, charged-hadron spectra, and elliptic flow at all centralities
The first correction addresses a jet fragmentation function (JFF) bias in which the jet energy is over-estimated for jets with hard fragmentation and under-estimated for those with soft fragmentation, resulting in a preferred selection of jets with harder fragmentation
Summary
The central feature of the CMS apparatus is a superconducting solenoid with an internal diameter of 6 m, providing an axial uniform magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Extensive hadronic forward (HF) steel and quartz fiber calorimetry complements the barrel and endcap detectors, providing coverage to |η| < 5. In this analysis, the collision centrality is determined using the total sum of transverse energy (ET) from calorimeter towers in the HF region (covering 2.9 < |η| < 5.2). The energy deposits in ECAL and HCAL cells are summed to define the calorimeter tower energies, subsequently used to provide the energies and directions of hadronic jets. The CMS silicon tracker measures charged particles within the region |η| < 2.5. A detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [33]
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