Energy dependence of pion interferometry scales in ultra-relativistic heavy ion collisions

A study of energy behavior of the pion spectra and interferometry scales is carried out for the top SPS, RHIC and LHC energies within the hydrokinetic approach. The latter allows one to describe evolution of quark-gluon and hadron matter as well as continuous particle emission from the fluid in agreement with the underlying kinetic equations. The main mechanisms that lead to the paradoxical, at first sight, behavior of the interferometry scales, are exposed. In particular, a slow decrease and apparent saturation of Rout/Rside ratio with an energy growth happens due to a strengthening of positive correlations between space and time positions of pions emitted at the radial periphery of the system. Such en effect is a consequence of the two factors: a developing of the pre-thermal collective transverse flows and an increase of the initial energy density in the fireball.