Centrality dependence of particle production in p-Pb collisions at $\sqrt{s_{\rm NN} }$= 5.02 TeV

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5.02 TeV
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We report measurements of the primary
charged particle pseudorapidity density and transverse momentum
distributions in p-Pb collisions at sqrt(s_nn) = 5.02 TeV, and
investigate their correlation with experimental observables sensitive
to the centrality of the collision.  Centrality classes are defined
using cuts on different event activity estimators, i.e. charged
particle multiplicities measured in three disjunct 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 (Npart), or the number of nucleon-nucleon binary
collisions (Ncoll), 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.  As an alternative event-centrality estimator, we
propose to use the zero-degree energy, which we expect does not
induce any dynamical bias. Based on zero-degree energy
centrality classes, the \Npart\ 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-rapitity of the number of participating nucleons is
observed. Furthermore, at high-pt the p-Pb spectra are found to be
consistent with the pp spectra scaled by Ncoll for all centrality
classes.  Our results are expected to put constraints on the
description of particle production in high-energy heavy-ion collisions.