We report measurements of the primary charged-particle pseudorapidity density and transverse momentum distributions in p-Pb collisions at √ s NN = 5.02 TeV and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined by using different event-activity estimators, i.e., charged-particle multiplicities measured in three different pseudorapidity regions as well as the energy measured at beam rapidity (zero degree). The procedures to determine the centrality, quantified by the number of participants (N part ) or the number of nucleon-nucleon binary collisions (N coll ) are described. We show that, in contrast to Pb-Pb collisions, in p-Pb collisions large multiplicity fluctuations together with the small range of participants available generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy-centrality classes, the N part dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapidity of the number of participating nucleons is observed. Furthermore, at high-p T the p-Pb spectra are found to be consistent with the pp spectra scaled by N coll for all centrality classes. Our results represent valuable input for the study of the event-activity dependence of hard probes in p-Pb collisions and, hence, help to establish baselines for the interpretation of the Pb-Pb data.

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We report the measured transverse momentum (p T) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy √ s NN = 5.02 TeV in the kinematic range of 0.15 < p T < 50 GeV/c and |η| < 0.8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at √ s NN = 2.76 TeV, as well as in p-Pb collisions at √ s NN = 5.02 TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the p T spectra are suppressed by more than a factor of 7 around 6-7 GeV/c with a significant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modification factor R pPb , constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/c. While the spectra in both pp and Pb-Pb collisions are substantially harder at √ s NN = 5.02 TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.

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/