Longitudinal fluctuations of the center of mass of the participants in heavy-ion collisions

A model for computing the probability density of event-by-event participant center-of-mass rapidity y(c.m.) is presented. The evaluations of the y(c.m.) distribution are performed for different collision energies and different centralities. We show that for certain conditions the rapidity distribution is described by a Gaussian with a variance determined mostly by the collision centrality. It is found that the width of the y(c.m.) distribution increases strongly for more peripheral collisions, while it depends weakly on the collision energy. Other theoretical estimates of rapidity distribution are presented and questions of interaction and separation between spectators and participants are discussed.