Long-range and short-range dihadron angular correlations in central PbPb collisions at sqrt {{{s_{text{NN}}}}} = 2.76 TeV

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Long-range and short-range dihadron angular correlations in central PbPb collisions at sqrt {{{s_{text{NN}}}}} = 2.76 TeV
  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP/2011-0562011/05/12 CMS-HIN-11-001 Long-range and short-range dihadron angular correlationsin central PbPb collisions at √  s NN   = 2.76TeV The CMS Collaboration ∗ Abstract First measurements of dihadron correlations for charged particles are presented forcentral PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76TeV over a broad range in relative pseudorapidity ( ∆ η ) and the full range of relative azimuthalangle ( ∆ φ ). The data were collected with the CMS detector, at the LHC. A broadeningof the away-side ( ∆ φ  ≈  π  ) azimuthal correlation is observed at all ∆ η , as comparedto the measurements in pp collisions. Furthermore, long-range dihadron correlationsin ∆ η  are observed for particles with similar  φ  values. This phenomenon, also knownas the “ridge”, persists up to at least | ∆ η | =  4. For particles with transverse momenta(  p T ) of 2–4GeV/ c , the ridge is found to be most prominent when these particles arecorrelated with particles of   p T  = 2–6GeV/ c , and to be much reduced when paired withparticles of   p T  = 10–12GeV/ c . Submitted to the Journal of High Energy Physics ∗ See Appendix A for the list of collaboration members   a  r   X   i  v  :  s  u   b  m   i   t   /   0   2   4   6   5   1   2   [  n  u  c   l  -  e  x   ]   1   2   M  a  y   2   0   1   1  1 1 Introduction Measurementsofdihadronazimuthalcorrelations[1–7]haveprovidedapowerfultooltostudy thepropertiesofthestronglyinteractingmediumcreatedinultrarelativisticnuclearcollisions[8–11]. An early indication of strong jet-medium interactions at RHIC was the absence of high-transverse-momentum (high-  p T ) back-to-back particle pairs in dihadron correlation measure-ments[1]andthecorrespondingenhancementoflow-  p T  hadronsrecoilingfromahigh-  p T  lead-ing, or “trigger”, particle [3]. The recent observations of the suppression of high-  p T  chargedhadrons [12] and of asymmetric energies of reconstructed jets [13, 14] in PbPb collisions at the Large Hadron Collider (LHC) provide further evidence of jet quenching, suggesting a largeenergy loss for partons traversing the produced medium.At RHIC, extending dihadron azimuthal correlation measurements to larger relative pseudo-rapidities resulted in the discovery of a ridge-shaped correlation in central AuAu collisions between particles with small relative azimuthal angles ( | ∆ φ | ≈  0), out to very large relativepseudorapidity ( | ∆ η | ) [2, 6]. Although the “ridge” has been qualitatively described by severaldifferent models [15–26], its srcin is still not well understood. Some models attribute the ridge to jet-medium interactions, while others attribute it to the medium itself. The ridge has beenobserved for particles with transverse momenta from several hundred MeV/ c  to a few GeV/ c .However, the character of the ridge for even higher-  p T  particles, as well as its dependence oncollision energy, is still poorly understood from the RHIC results [2]. Recently, a striking ridgestructure has also been observed in very high multiplicity proton-proton (pp) collisions at acenter-of-mass energy of 7TeV at the LHC by the Compact Muon Solenoid (CMS) Collabora-tion [27], posing new challenges to the understanding of these long-range correlations.This paper presents the first measurement of dihadron correlations for charged particles pro-duced in the most central (0–5% centrality) PbPb collisions at a nucleon-nucleon center-of-massenergy ( √  s NN  ) of 2.76TeV over a large phase space. The results are presented in terms of the as-sociated hadron yields as a function of pseudorapidity and azimuthal angle relative to triggerparticles in different transverse momentum intervals. Traditionally, trigger particles have beenutilized to represent the direction of the leading hadron in a jet, and were required to have ahigher momentum than all the other associated particles in the jet [2, 6]. However, as shownin Ref. [27], important information can also be obtained by studying the correlation of hadronpairs from the same  p T  interval, which is particularly useful when addressing the properties of the medium itself. The current analysis employs both approaches. This measurement providesa unique examination of the ridge in the most central PbPb collisions at the highest energiesreached so far in the laboratory over a wide range in transverse momentum (2–12GeV/ c ) andup to large relative pseudorapidity ( | ∆ η |≈ 4), imposing further quantitative constraints on thepossible srcin of the ridge.Details of event readout and analysis for extracting the correlation functions are described inSection 2, the physics results found using the correlations are described in Section 3, and a summary is given in Section 4. 2 Data and Analysis The analysis reported in this paper is based on PbPb collisions at  √  s NN   = 2.76TeV collectedduring the LHC heavy-ion run in November and December 2010 with the CMS detector. Thecentral feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter.Within the field volume are the inner tracker, the crystal electromagnetic calorimeter, and the brass/scintillator hadron calorimeter. Muons are measured in gas-ionization detectors embed-  2  2 Data and Analysis ded in the steel return yoke. In addition to the barrel and endcap detectors, CMS has extensiveforward calorimetry. The nearly 4 π   solid-angle acceptance of the CMS detector is ideally suitedfor studies of both short- and long-range particle correlations. A detailed description of theCMS detector can be found in Ref. [28]. CMS uses a right-handed coordinate system, with thesrcin at the nominal interaction point, the  x  axis pointing to the center of the LHC, the  y  axispointing up (perpendicular to the LHC plane), and the  z  axis along the counterclockwise beamdirection. The detector subsystem primarily used for the present analysis is the inner trackerthat reconstructs the trajectories of charged particles with  p T  >  100MeV/ c , covering the pseu-dorapidity region | η |  <  2.5, where  η  =  − ln [ tan ( θ /2 )]  and  θ  is the polar angle relative to the beam direction. The inner tracker consists of 1440 silicon pixel and 15148 silicon strip detectormodules immersed in the 3.8 T axial magnetic field of the superconducting solenoid.The event readout of the CMS detector for PbPb collisions is triggered by coincident signals inforward detectors located on both sides of the nominal collision point. In particular, minimum bias PbPb data are recorded based on coincident signals in the beam scintillator counters (BSC,3.23  <  | η |  <  4.65) or in the steel/quartz-fiber Cherenkov forward hadron calorimeters (HF,2.9  <  | η |  <  5.2) from both ends of the detector. In order to suppress events due to noise,cosmic rays, double-firing triggers, and beam backgrounds, the minimum bias trigger used inthis analysis is required to be in coincidence with bunches colliding in the interaction region.The trigger has an acceptance of   ( 97 ± 3 ) % for hadronic inelastic PbPb collisions [14].Events are selected offline by requiring in addition at least three hits in the HF calorimetersat both ends of CMS, with at least 3GeV of energy in each cluster, and the presence of a re-constructed primary vertex containing at least two tracks. These criteria further reduce back-ground from single-beam interactions (e.g., beam gas and beam halo), cosmic muons, andlarge-impact-parameter, ultra-peripheral collisions that lead to the electromagnetic breakupof one or both of the Pb nuclei [29]. The reconstructed primary vertex is required to be locatedwithin 15 cm of the nominal collision point along the beam axis and within a radius of 0.02 cmrelative to the average vertex position in the transverse plane.This analysis is based on a data sample of PbPb collisions corresponding to an integrated lumi-nosity of approximately 3.12 µ  b − 1 [30, 31], which contains 24.1 million minimum bias collisions after all event selections are applied.The energy released in the collisions is related to the centrality of heavy-ion interactions, i.e.,the geometrical overlap of the incoming nuclei. In CMS, centrality is classified according topercentiles of the distribution of the energy deposited in the HF calorimeters. The centralityclass used in this analysis corresponds to the 0–5% most central PbPb collisions, a total of 1.2 million events. More details on the centrality determination can be found in Refs. [14, 32]. A reconstructed track is considered as a primary-track candidate if the significance of the sepa-ration along the beam axis between the track and the primary vertex,  d  z / σ  ( d  z ) , and the signifi-canceoftheimpactparameterrelativetotheprimaryvertextransversetothebeam,  d xy / σ  ( d xy ) ,are each less than 3. In order to remove tracks with potentially poorly reconstructed momen-tum values, the relative uncertainty of the momentum measurement,  σ  (  p T ) /  p T , is required to be less than 5.0%. Requiring at least 12 hits on each track helps to reject misidentified tracks.Systematic uncertainties related to the track selections have been evaluated as discussed below.Trigger particles are defined as all charged particles srcinating from the primary vertex, with | η |  < 2.4 and in a specified  p trigT  range. The number of trigger particles in the event is denoted by  N  trig , which can be more than one per event. Hadron pairs are formed by associating withevery trigger particle the remaining charged particles with  | η |  <  2.4 and in a specified  p assocT  3 range. The per-trigger-particle associated yield distribution is then defined by:1 N  trig d 2 N  pair d ∆ η d ∆ φ  =  B ( 0,0 ) ×  S ( ∆ η , ∆ φ ) B ( ∆ η , ∆ φ ) , (1)where ∆ η  and ∆ φ  are the differences in  η  and  φ  of the pair, respectively. The signal distribution, S ( ∆ η , ∆ φ ) , is the measured per-trigger-particle distribution of same-event pairs, i.e., S ( ∆ η , ∆ φ ) =  1 N  trig d 2 N  same d ∆ η d ∆ φ . (2)The mixed-event background distribution, B ( ∆ η , ∆ φ ) =  1 N  trig d 2 N  mix d ∆ η d ∆ φ , (3)isconstructedbypairingthetriggerparticlesineacheventwiththeassociatedparticlesfrom10different random events, excluding the srcinal event. The symbol  N  mix denotes the number of pairs taken from the mixed event. The background distribution is used to account for randomcombinatorial background and pair-acceptance effects. The normalization factor  B ( 0,0 )  is thevalue of   B ( ∆ η , ∆ φ )  at ∆ η  =  0 and ∆ φ  =  0 (with a bin width of 0.3 in ∆ η  and  π  /16 in ∆ φ ), repre-senting the mixed-event associated yield for both particles of the pair going in approximatelythe same direction, thus having full pair acceptance. Therefore, the ratio  B ( 0,0 ) / B ( ∆ η , ∆ φ ) is the pair-acceptance correction factor used to derive the corrected per-trigger-particle associ-ated yield distribution. Equation (1) is calculated in 2 cm wide bins of the vertex position (  z vtx )along the beam direction and averaged over the range |  z vtx |  <  15 cm. To maximize the statis-tical precision, the absolute values of   ∆ η  and  ∆ φ  are used to fill one quadrant of the ( ∆ η , ∆ φ )histograms, with the other three quadrants filled (only for illustration purposes) by reflection.Therefore, the resulting distributions are symmetric about  ( ∆ η , ∆ φ ) = ( 0,0 )  by construction.Each reconstructed track is weighted by the inverse of the efficiency factor,  ε trk ( η ,  p T ) , as afunction of the track’s pseudorapidity and transverse momentum. The efficiency weightingfactor accounts for the detector acceptance  A ( η ,  p T ) , the reconstruction efficiency  E ( η ,  p T ) , andthe fraction of misidentified tracks,  F ( η ,  p T ) , ε trk ( η ,  p T ) =  AE 1 − F . (4)Studies with simulated Monte Carlo (MC) events show that the combined geometrical accep-tance and reconstruction efficiency for the primary-track reconstruction reaches about 60% forthe 0–5% most central PbPb collisions at  p T  >  2GeV/ c  over the full CMS tracker acceptance( | η |  <  2.4) and 65% for  | η |  <  1.0. The fraction of misidentified tracks is about 1–2% for | η |  <  1.0, but increases to 10% at  | η | ≈  2.4. The weighting changes the overall scale butnot the shape of the associated yield distribution, which depends on the ratio of the signal to background distributions.A closure test of the track-weighting procedure is performed on HYDJET [33] (version 1.6)MC events. The efficiency-weighted associated yield distribution from reconstructed tracks isfound to agree with the generator-level correlation function to within 3.3%. In addition, sys-tematic checks of the tracking efficiency, in which simulated MC tracks are embedded into
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