Centrality Dependence of Charged Particle Production at Large Transverse Momentum in Pb--Pb Collisions at $\sqrt{s_{\rm{NN}}} = 2.76$ TeV

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    a  r   X   i  v  :   1   2   0   8 .   2   7   1   1  v   2   [   h  e  p  -  e  x   ]   1   8   M  a  r   2   0   1   3 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-PH-EP-2012-233March 20, 2013 Centrality Dependence of Charged Particle Production at LargeTransverse Momentum in Pb–Pb Collisions at  √  s NN  =  2 . 76  TeV The ALICE Collaboration ∗ Abstract The inclusive transverse momentum (  p T ) distributions of primary charged particles are measured inthe 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  fornine centrality intervals from 70–80% to 0–5%. The results in Pb–Pb are presented in terms of thenuclearmodificationfactor  R AA  usinga ppreferencespectrummeasuredat thesame collisionenergy.We observe that the suppression of high-  p T  particles strongly depends on event centrality. The yieldis 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) anda weak   p T  dependenceis observed. The measurednuclear modificationfactors are comparedto othermeasurements and model calculations. ∗ See Appendix A for the list of collaboration members  Particle Production at Large Transverse Momentum 1 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 degreesof freedom. During the very early stage of the collision, some of the incoming partons experiencescatterings with large momentum transfers. These partons lose energy when they traverse the hot anddense medium that is formed. One of the major goals of the heavy-ion physics programme at the LHCis to understand the underlying mechanisms for parton energy loss and use this as a tool to probe theproperties 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 superposition 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 beingindependent of   p T  over the measured range 5  <  p T  <  20 GeV/  c  [10].The increase of the charged particle density (d  N  ch / d η ) at mid-rapidity from RHIC energies to actualLHC energies by a factor of around 2.2 [11] implies a similar increase in energy density. However, theobserved suppression of high-  p T  particle production also depends on the ratio of quarks to gluons dueto their different color factors, and on the steepness of the  p T  spectra of the scattered partons. At theLHC 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 theeffects which cause the suppression 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 initialparton energy.We present a measurement of the  p T  distributions of charged particles in 0 . 15  <  p T  <  50 GeV/  c  withpseudo-rapidity  | η | < 0 . 8, where η  = − ln [ tan ( θ  / 2 )] , with θ   the polar angle between the charged particledirection and the beam axis. Results are presented for different centrality intervals in Pb–Pb collisionsat  √  s NN  =  2 . 76 TeV. They are compared with measurements in pp collisions, by calculating the nuclearmodification factor  R AA (  p T ) =  d 2  N  AAch  / d η d  p T  T  AA  d 2 σ  pp ch  / d η d  p T (1)where  N  AAch  and  σ  ppch  represent the charged particle yield in nucleus-nucleus (AA) collisions and the crosssection in pp collisions, respectively. The nuclear overlap function  T  AA  is calculated from the Glaubermodel [14] and averaged over each centrality interval,   T  AA   =    N  coll  / σ  NNinel , where    N  coll   is the averagenumber of binary nucleon-nucleon collisions and  σ  NNinel  is the inelastic nucleon-nucleon cross section.Early results from ALICE [15] showed that the production of charged particles in central (0–5%) Pb–Pbcollisions at  √  s NN  =  2 . 76 TeV is suppressed by more than a factor of 6 at  p T  =  6–7 GeV/  c  comparedto an independent superposition of nucleon-nucleon collisions, and that the suppression is stronger thanthat observed at RHIC. The present data extend the study of high-  p T  particle suppression in Pb–Pb outto  p T  =  50 GeV/  c  with a systematic study of the centrality dependence.Moreover, the systematic uncertainties related to the pp reference were significantly reduced with respectto the previous measurement by using the  p T  distribution measured in pp collisions at  √  s  =  2 . 76 TeV[16].  2 The ALICE Collaboration Table1:  Averagevaluesofthenumberofparticipatingnucleons   N  part  andthenuclearoverlapfunction  T  AA  [14]for the centrality intervals used in the analysis. Centrality    N  part   T  AA   (mb − 1 )0–5% 383  ±  3 26.4  ±  1.15–10% 330  ±  5 20.6  ±  0.910–20% 261  ±  4 14.4  ±  0.620–30% 186  ±  4 8.7  ±  0.430–40% 129  ±  3 5.0  ±  0.240–50% 85  ±  3 2.68  ±  0.1450–60% 53  ±  2 1.32  ±  0.0960–70% 30.0  ±  1.3 0.59  ±  0.0470–80% 15.8  ±  0.6 0.24  ±  0.03 2 Experiment and Data Analysis The ALICE detector is described in [17]. The Inner Tracking System (ITS) and the Time ProjectionChamber (TPC) are used for vertex finding and tracking. The minimum-bias interaction trigger wasderived from signals from the forward scintillators (VZERO), and the two innermost layers of the ITS(Silicon Pixel Detector - SPD). The collision centrality is determined using the VZERO. In addition,the information from two neutron Zero Degree Calorimeters (ZDCs) positioned at  ± 114 m from theinteraction point was used to remove contributions from beam-gas and electromagnetic interactions. Thetrigger and centrality selection are described in more detail in [11].The following analysis is based on 1 . 6 · 10 7 minimum-bias Pb–Pb events recorded by ALICE in 2010.For this study, the events are divided into nine centrality intervals from the 70–80% to the 0–5% mostcentral Pb–Pb collisions, expressed in percentage of the total hadronic cross section. The event centralitycan be related to the number of participating nucleons  N  part  and the nuclear overlap function  T  AA  byusing simulations based on the Glauber model [14]. The average values of   N  part  and  T  AA  for each cen-trality interval,    N  part   and   T  AA  , along with their corresponding systematic uncertainties, are listed inTable 1. The errors include the experimental uncertainties on the inelastic nucleon-nucleon cross section σ  NNinel  =  64 ± 5 mb at  √  s NN  =  2 . 76 TeV [18] and on the parameters of the nuclear density profile used inthe Glauber simulations (more details in [11]).The primary vertex position was determined from the tracks reconstructed in the ITS and the TPC byusing an analytic  χ  2 minimization method, applied after approximating each of the tracks by a straightline in the vicinity of their common srcin. The event is accepted if the coordinate of the reconstructedvertex measured along the beam direction (  z -axis) iswithin  ± 10 cm around the nominal interaction point.The event vertex reconstruction is fully efficient for the event centralities covered.Primary charged particles are defined as all prompt particles produced in the collision, including decayproducts, except those from weak decays of strange hadrons. A set of standard cuts based on the numberof space points and the quality of the momentum fit in the TPC and ITS is applied to the reconstructedtracks. Track candidates in the TPC are required to have hits in at least 120 (out of a maximum of 159)pad-rows and  χ  2 per point of the momentum fit smaller than 4. Such tracks are projected to the ITSand used for further analysis if at least 2 matching hits (out of a maximum of 6) in the ITS, including atleast one in the SPD, are found. In addition, the  χ  2 per point of the momentum fit in the ITS must besmaller than 36. In order to improve the purity of primary track reconstruction at high  p T  we developeda procedure where we compare tracking information from the combined ITS and TPC track reconstruc-tion algorithm to that derived only from the TPC and constrained by the interaction vertex point. Wecalculated the  χ  2TPC − ITS  between these tracks using the following formula  Particle Production at Large Transverse Momentum 3 Table 2:  Contribution to the systematic uncertainties on the  p T  spectra (0.15–50 GeV/  c ) for the most central andperipheral Pb–Pb collisions. Also listed are the systematic uncertainties on the pp reference (0.15–50GeV/  c ) [16]. Centrality class 0–5% 70–80%Centrality selection 0.4% 6.7%Event selection 3.2% 3.4%Track selection 4.1–7.3% 3.6–6.0%Tracking efficiency 5% 5%  p T  resolution correction  < 1.8%  < 3%Material budget 0.9–1.2% 0.5–1.7%Particle composition 0.6–10% 0.5–7.7%MC generator 2.5% 1.5%Secondary particle rejection  < 1%  < 1%Total for  p T  spectra 8.2–13.5% 10.3–13.4%Total for pp reference 6.3–18.8%pp reference normalization 1.9%  χ  2TPC − ITS  =( v TPC − v TPC − ITS ) T · ( C TPC  + C TPC − ITS ) − 1 · ( v TPC − v TPC − ITS )  (2)where  v TPC ,  v TPC − ITS  and  C TPC ,  C TPC − ITS  represent the measured track parameter vectors v  = (  x ,  y ,  z , θ  , φ  , 1 /  p T )  and their covariance matrices, respectively. If the  χ  2TPC − ITS  is larger than 36 thetrack candidate is rejected. At  p T  =  0 . 15–50 GeV/  c , this procedure removes about 2–7% (1–3%) of thereconstructed tracks in the most central (peripheral) collisions. This procedure in fact removes high-  p T fake tracks, which srcinate from spurious matches of low  p T  particles in the TPC to hits in the ITS, andwould result in an incorrect momentum assignment.Finally, tracks are rejected from the sample if their distance of closest approach to the reconstructedvertex in the longitudinal direction  d  z  is larger than 2 cm or  d  xy  >  0 . 018cm  + 0 . 035cm  ·  p − 1T  in thetransverse direction with  p T  in GeV/  c , which corresponds to 7 standard deviations of the resolution in d  xy  (see [19] for details). The upper limit on the  d  z  ( d  z  <  2 cm) was set to minimize the contributionof tracks coming from pileup and beam-gas background events. These cuts reject less than 0.5% of thereconstructed tracks independently of   p T  and collision centrality.The efficiency and purity of the primary charged particle selection are estimated using a Monte Carlosimulation with HIJING [20] events and a GEANT3 [21] model of the detector response. We used a HIJING tune which reproduces approximately the measured charged particle density in central colli-sions [11]. In the most central events, the overall primary charged particle reconstruction efficiency(tracking efficiency and acceptance) in  | η |  <  0 . 8 is 36% at  p T  =  0 . 15 GeV/  c  and increases to 65% for  p T  >  0 . 6 GeV/  c . In the most peripheral events the efficiency is larger than that for the central eventsby about 1–3%. The contribution from secondary particles was estimated using the  d  xy  distributions of data and HIJING and is consistent with the measured strangeness to charged particle ratio from the re-construction of K 0s ,  Λ  and  Λ  invariant mass peaks in Pb–Pb [22]. The total contribution from secondarytracks at  p T  =  0 . 15 GeV/  c  is 13 (7)% for central (peripheral) events and decreases to about 0.6% above  p T  =  4 GeV/  c  for both central and peripheral events. From a systematic variation of the  χ  2TPC − ITS  cutand comparison of track properties in MC to data we conclude that the number of properly reconstructedtracks rejected as high-  p T  fake tracks is around 1–2% (0.5–1%) in the most central (peripheral) colli-sions. We also conclude that the contribution from the high-  p T  fake tracks to the  p T  spectra is negligibleindependently of the collision centrality and  p T .The transverse momentum of charged particles is reconstructed from the track curvature measured in themagnetic field  B  =  0 . 5 T using the ITS and TPC detectors. The  p T  resolution is estimated from the track 
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