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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/

We report on two-particle charge-dependent correlations in pp, p–Pb, and Pb–Pb collisions as a function of the pseudorapidity and azimuthal angle difference, and respectively. These correlations are studied using the balance function that probes the charge creation time and the development of collectivity in the produced system. The dependence of the balance function on the event multiplicity as well as on the trigger and associated particle transverse momentum () in pp, p–Pb, and Pb–Pb collisions at 7, 5.02, and 2.76 TeV, respectively, are presented. In the low transverse momentum region, for  GeV/c, the balance function becomes narrower in both and directions in all three systems for events with higher multiplicity. The experimental findings favor models that either incorporate some collective behavior (e.g. AMPT) or different mechanisms that lead to effects that resemble collective behavior (e.g. PYTHIA8 with color reconnection). For higher values of transverse momenta the balance function becomes even narrower but exhibits no multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low is a feature of bulk particle production.

2016-12-24T18:18:19