Centrality determination of Pb-Pb collisions atsNN=2.76TeV with ALICE

Abstract

This publication describes the methods used to measure the centrality of inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding nucleon pair with ALICE. The centrality is a key parameter in the study of the properties of QCD matter at extreme temperature and energy density, because it is directly related to the initial overlap region of the colliding nuclei. Geometrical properties of the collision, such as the number of participating nucleons and the number of binary nucleon-nucleon collisions, are deduced from a Glauber model with a sharp impact parameter selection and shown to be consistent with those extracted from the data. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.

Melting Hadrons, Boiling Quarks

“…A numerical model connects the artificially created reaction classes with the mean number of participants N part that contributed. For further details for the LHC work we refer to the recent ALICE review of their approach [211].…”

Measurement of the D0 Meson Production in Pb–Pb and p–Pb Collisions

“…The uncertainties on the pp cross section normalization (3.5%) [38] and the average nuclear overlap function T AA (4.7%) were also included [16]. The contribution due to the 1.1% relative uncertainty on the fraction of the hadronic cross section used in the Glauber fit to determine the centrality class was obtained by estimating the variation of the D meson dN/dp T when the limits of the centrality classes are shifted by ±1.1%.…”

“…This observable is defined as the ratio of particle production measured in nucleus-nucleus to that expected from the protonproton spectrum scaled by the average number of binary nucleon-nucleon collisions N coll occurring in the AA collision. Using the nuclear overlap function, defined as the convolution of the nuclear density profiles of the colliding ions in Section 1.5.1, the nuclear modification factor of the transverse momentum distribution can be expressed as: 16) where the AA spectrum corresponds to a given collision-centrality class and T AA is the average nuclear overlap function for that centrality class (proportional to N coll ). Inmedium energy loss determines a suppression, R AA < 1, of hadrons at moderate-to-high transverse momentum.…”

“…The factor 2 in Equation (4.7) accounts for the fact that the D meson yields for Pb-Pb collisions were integrated over half of the full azimuth. The average value of the nuclear overlap function in the centrality class 30-50% is T AA = 3.87 ± 0.18 mb −1 [16]. dσ pp /dp T is the proton-proton reference cross section, that will be described in Section 4.3.…”

“…The centrality classes were related to the collision geometry by fitting the VZERO summed amplitudes by a model based on the Glauber approach for the geometrical description of the nuclear collision [15] complemented by a two-component model for particle production [16], following the procedure explained in Section 1.5.1. The centrality distribution of the collected events was flat in the 30-50% centrality class.…”