Centrality dependence of the pseudorapidity density distribution for charged particles in Pb–Pb collisions at sNN=2.76 TeV

Abstract

Centrality dependence of the pseudorapidity density distribution for charged particles in Pb-Pb collisions at √ s NN = 2.76 TeV ✩ .ALICE Collaboration a r t i c l e i n f o a b s t r a c tWe present the first wide-range measurement of the charged-particle pseudorapidity density distribution, for different centralities (the 0-5%, 5-10%, 10-20%, and 20-30% most central events) in Pb-Pb collisions at √ s NN = 2.76 TeV at the LHC. The measurement is performed using the full coverage of the ALICE detectors, −5.0 < η < 5.5, and employing a special analysis technique based on collisions arising from LHC 'satellite' bunches. We present the pseudorapidity density as a function of the number of participating nucleons as well as an extrapolation to the total number of produced charged particles (N ch = 17 165 ± 772 for the 0-5% most central collisions). From the measured dN ch /dη distribution we derive the rapidity density distribution, dN ch /dy, under simple assumptions. The rapidity density distribution is found to be significantly wider than the predictions of the Landau model. We assess the validity of longitudinal scaling by comparing to lower energy results from RHIC. Finally the mechanisms of the underlying particle production are discussed based on a comparison with various theoretical models.ICE detector. The employed method relies on using so-called 'satellite' bunch collisions and is based on measurements from three ✩ © CERN for the benefit of the ALICE Collaboration. different ALICE sub-detectors. This method is applicable for the 30% most central events where the trigger efficiency for these 'satellite' collisions remains high. These measurements extend considerably the former results obtained at the LHC [8-10] and can be compared to the wealth of results on the charged-particle pseudorapidity density from lower energy Au-Au collisions at RHIC [6,11, 12] as well as model calculations. Experimental setupA detailed description of the ALICE detector can be found in [13]. In the following, we will briefly describe the detectors used in this analysis, namely the Silicon Pixel Detector (SPD), the Forward Multiplicity Detector (FMD), the VZERO, and the Zero Degree Calorimeter (ZDC) (see Fig. 1).The SPD is the innermost element of the ALICE inner tracking system [13]. It consists of two cylindrical layers of hybrid silicon pixel assemblies positioned at radial distances of 3.9 and 7.6 cm from the beam line, with a total of 9.8 × 10 6 pixels of size 50 × 425 μm 2 , read out by 1200 electronic chips. The SPD coverage for particles originating from the nominal interaction point at the center of the detector is |η| < 2 and |η| < 1.4 for the inner and outer layers, respectively.The VZERO detector [14] consists of two arrays of 32 scintillator tiles (4 rings of increasing radii each with 8 azimuthal sectors) placed at distances of 3.3 m (VZERO-A) and −0.9 m (VZERO-C) from the nominal interaction point along the beam axis, covering the full azimuth within 2.8 < η < 5.1 and −3.7 < η < −1.7, 0370-2693/

EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider

“…As can be seen in Figs. 17 and 18, it is possible to achieve a good description of both the pseudorapidity distribution of Pb-Pb collisions at 2.76 TeV with various centralities from the ATLAS and ALICE experiments [37,38] and the nuclear suppression factor of the most central events as measured by the ALICE experiment [39] for p t < 5 GeV/c. The large suppression observed in the simulations with EPOS LHC at larger p t is due to a lack of hard scatterings during the initial stage of the nucleus-nucleus interaction.…”

“…In fact it has been shown now that such an initial stage suppression of hard processes is not observed in to 10%, 10% to 20%, 30% to 40%, and 50% to 60%. EPOS LHC simulations (solid line) are compared to the ATLAS (band) and ALICE (point) measurements [37,38].…”

Multiplicity distributions and fluctuations in proton-proton and heavy-ion collisions

“…The charged particle pseudorapidity density distribution has been measured in Pb-Pb collisions at √s=2.76 TeV also in ALICE [7]. The measurement was performed in a wide pseudorapidity interval of −5.0 < η < 5.5 allowing the total number of charged particles produced at the LHC.…”

Studying Heavy Ion Collisions Using Methods From Cosmic Microwave Background (CMB) Analysis

“…In fact, we may estimate from the systematics presented in ref. [12] infer that at the top LHC energy for Pb+Pb collisions, which is expected to be reached in Y2015 or Y2016, the number of charged particles per single 0-5% central event wil be of the order of. 23.000 particles.…”

“…From this time on, produced hadrons stream to the detectors only affected by possible decays in flight and reinteractions with the surrounding material. In central collisions of lead ions at the LHC the energy density after about 1 f m/c exceeds 16 GeV/ f m 3 and about 17.500 charged particles are produced in a single central collision [12]. The particles that reach the detectors carry the imprint of the early stages of the collision and we may expect, dependent on the reinteractions and degree of equilibration attained during the rapid expansion expansion that fluctuations in the final channel may reflect some of the initial conditions.…”