Data-driven model for the emission of spectator nucleons as a function of centrality in Pb-Pb collisions at LHC energies

At colliders the Zero Degree Carimeters (ZDC) measure mainly the energy carried by ``free'' spectators, namely the fragments are not bound in fragments after the collision. The most commonly used event generators for nucleus-nucleus collisions do not include a reliable model to describe the formation of nuclear fragments. This prevents a realistic simulation of the signal detected in the Zero Degree Calorimeters (ZDC) to be correctly evaluated. For simulations of ALICE data collected during the LHC Run~2 period, a data-driven model has been implemented in AliRoot, to generate a number of spectator nucleons similar to the observed one. This note explains how the data-driven effective functions are calculated from data for spectator neutrons and protons, and the potential applications of this model.