Centrality determination of Pb-Pb collisions at $\sNN = 2.76$ TeV with ALICE

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2.76 TeV
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A key parameter in the study of the properties of QCD matter at extreme temperature and energy density is the initial overlap region in nucleus-nucleus collisions, which is directly related to the impact parameter of the collision. Geometrical properties of the collisions, such as the number of participating nucleons and of binary nucleon-nucleon collisions, typically deduced from a Glauber model, are useful to compare data across different collision systems. This publication describes the methods used by the ALICE Collaboration to determine the fraction of inelastic Pb-Pb collisions measured at a center-of-mass energy of 2.76 TeV per colliding nucleon, and to determine the centrality of the collision by using either the multiplicity of produced charged particles in the barrel and forward detectors or the energy carried away by spectator nucleons.