Centrality determination using the Glauber model in Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV

This document describes the methods used by the ALICE Collaboration for determining the centrality of Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV at the LHC within the Glauber model. The half-density radius $R$ and the diffusivity $a$ describing the Xe-129 nucleus with a 2-parameter Fermi distribution have been derived from a recent electron scattering measurement for the Xe-132 nucleus. The deformation parameter $\beta$ has been derived by an interpolation between measured deformation parameters for the even-$A$ Xe isotopes. The inelastic nucleon-nucleon cross-section at $\sqrt{s_{\rm NN}} = 5.44$ TeV has been estimated by interpolation of pp data at different center-of-mass energies. The particle multiplicity per nucleon-nucleon collision has been parameterized by a negative binomial distribution (NBD) and a number of independently emitting sources (ancestors) given by a linear combination of $N_{\rm part}$ and $N_{\rm coll}$. The NBD parameters $\mu$ and $k$ and the ancestor parameter $f$ are then obtained by a fit to the amplitude distribution measured in the V0, which is a detector comprised of two scintillator arrays. Finally, the mean number of participants and collisions and the mean nuclear thickness function are derived for sharp centrality cuts in the simulated V0 amplitude distribution. These values are compared with those obtained by defining centrality classes with sharp cuts in impact parameter. In general the numbers of participants are very similar between both approaches for 0-80% centrality, while they differ considerably for the most peripheral 80-100% class.