Physics Letters B 720 (2013) 52–62 Contents lists available at SciVerse ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Centrality dependence of charged particle production at large transverse momentum in Pb–Pb collisions at √ s NN = 2.76 TeV .ALICE Collaboration article info abstract Article history: Received 11 September 2012 Received in revised form 21 January 2013 Accepted 24 January 2013 Available online 31 January 2013 Editor: V. Metag The inclusive transverse momentum ( p T ) distributions of primary charged particles are measured in the pseudo-rapidity range |η| < 0.8 as a function of event centrality in Pb–Pb collisions at √ s NN = 2.76 TeV with ALICE at the LHC. The data are presented in the p T range 0.15 < p T < 50 GeV/c for nine centrality intervals from 70–80% to 0–5%. The results in Pb–Pb are presented in terms of the nuclear modification factor R AA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high- p T particles strongly depends on event centrality. The yield is most suppressed in central collisions (0–5%) with R AA ≈ 0.13 at p T = 6–7 GeV/c. Above p T = 7 GeV/c, there is a significant rise in the nuclear modification factor, which reaches R AA ≈ 0.4 for p T > 30 GeV/c. In peripheral collisions (70–80%), only moderate suppression ( R AA = 0.6–0.7) and a weak p T dependence is observed. The measured nuclear modification factors are compared to other measurements and model calculations.  2013 CERN. Published by Elsevier B.V. 1. Introduction High-energy collisions of heavy-ions enable the study of hot and dense strongly interacting matter [1–5]. At sufficiently high temperature, it is expected that partons (quarks and gluons) are the dominant degrees of freedom. During the very early stage of the collision, some of the incoming partons experience scatterings with large momentum transfers. These partons lose energy when they traverse the hot and dense medium that is formed. One of the major goals of the heavy-ion physics programme at the LHC is to understand the underlying mechanisms for parton energy loss and use this as a tool to probe the properties of the medium. Parton energy loss in heavy-ion collisions was first observed at RHIC as the suppression of high- p T particle production in Au–Au collisions compared to expectations from an independent super- position of nucleon–nucleon collisions [6–9]. At RHIC, the particle production in central (0–5%) Au–Au collisions at √ s NN = 200 GeV is suppressed by a factor of 5 at p T = 5–6 GeV/c [8,9], and is consistent with being independent of p T over the measured range 5 < p T < 20 GeV/c [10]. The increase of the charged particle density (dN ch /dη) at mid- rapidity from RHIC energies to actual LHC energies by a factor of around 2.2 [11] implies a similar increase in energy density. How- ever, the observed suppression of high- p T particle production also depends on the ratio of quarks to gluons due to their different color factors, and on the steepness of the p T spectra of the scat- tered partons. At the LHC the initial parton p T spectra are less steep than at RHIC and the ratio of gluons to quarks at a given p T is higher [12]. The measurement of high- p T hadron production at the LHC helps to disentangle the effects which cause the suppres- sion and provides a critical test of existing energy loss calculations [13]. In particular, the large p T reach provides a means to study the dependence of the energy loss on the initial parton energy. We present a measurement of the p T distributions of charged particles in 0.15 < p T < 50 GeV/c with pseudo-rapidity |η| < 0.8, where η =− ln[tan(θ/2)], with θ the polar angle between the charged particle direction and the beam axis. Results are presented for different centrality intervals in Pb–Pb collisions at √ s NN = 2.76 TeV. They are compared with measurements in pp collisions, by calculating the nuclear modification factor R AA ( p T ) = d 2 N AA ch /dη d p T 〈T AA 〉 d 2 σ pp ch /dη d p T (1) where N AA ch and σ pp ch represent the charged particle yield in nucleus–nucleus (AA) collisions and the cross section in pp col- lisions, respectively. The nuclear overlap function T AA is calculated from the Glauber model [14] and averaged over each centrality interval, 〈T AA 〉=〈N coll 〉/σ NN inel , where 〈N coll 〉 is the average num- ber of binary nucleon–nucleon collisions and σ NN inel is the inelastic nucleon–nucleon cross section. Early results from ALICE [15] showed that the production of charged particles in central (0–5%) Pb–Pb collisions at √ s NN = 2.76 TeV is suppressed by more than a factor of 6 at p T = 6–7 GeV/c compared to an independent superposition of nucleon– nucleon collisions, and that the suppression is stronger than that observed at RHIC. The present data extend the study of high- p T 0370-2693/  2013 CERN. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physletb.2013.01.051 Open access under CC BY-NC-ND license. Open access under CC BY-NC-ND license.