Centrality dependence of particle production inp−Pbcollisions atsNN=5.02TeV

Abstract

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.

Zboson production inp+Pbcollisions atsNN=5.02Abulaiti1, Akerstedt2, Åsman3 et al. 2015Phys. Rev. CSelf Cite7861010View full textAdd to dashboardCite

“…This causes a bias in the centrality-dependent yield, as the Z boson yield is enhanced in the more central events but depleted in the more peripheral ones. This effect, referred to as a centrality bias, has been noted for yields of hard processes in d + Au collisions at √ s NN = 200 GeV by the PHENIX Collaboration [28] and in p + Pb collisions at √ s NN = 5.02 TeV by the ALICE Collaboration [29]. A correction to the centrality-dependent yields of hard processes in d + Au collisions at √ s NN = 200 GeV has been studied by the PHENIX Collaboration [28] (the correction is also calculated for p + Pb at √ s NN = 5.02 TeV).…”

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Longitudinal correlations from fluctuating strings in Pb-Pb, p -Pb, and p−p collisions

“…As can be seen from the experimental data [39][40][41][42], pseudorapidity spectra mainly differ from rapidity spectra by a pronounced dip around η = 0, which trivially follows from the kinematic relation between rapidity and pseudorapidity. In order to pass from pseudorapidity to rapidity for the spectra which are largely dominated by the pions, we use the simplifying assumption of a factorization of the rapidity and p T dependence of the spectra.…”