Multi-dimensional investigation of the pion pair-source in heavy-ion collisions with EPOS

Abstract: The purpose of femtoscopy is to unveil the space-time characteristics of heavy-ion collisions as mirrored in the correlation function of identical particles. We aim to augment such research by investigating the source function of particle creation based on a modern event generator, the EPOS package. It simulates high energy nuclear collisions and is proven to reliably reproduce the characteristics of real collisions. The source function has been almost always assumed to be Gaussian, but lately it is experimentally found that L\'evy-stable distributions provide a much more acceptable description. This paper focuses on the source shape that is reconstructed in a femtoscopic measurement, in particular, the event-by-event geometry as well as the deviation from spherical symmetry of the source in the EPOS simulation. Such an investigation tests some scenarios of the appearance of non-Gaussian source functions. We find that in such a realistic simulation, non-Gaussian sources indeed appear, even in a three-dimensional setting and without event averaging. However, our reconstructed L\'evy exponent (responsible for the long-range behavior of the source) is systematically larger than that observed in experiment; a discrepancy that might hint at further significance of this exponent.