Asymmetrical and Non-equilibrium Expansion of Pion Gas Created in Ultrarelativistic Heavy Ion Collision

Results of the computer simulation of dilepton production from the expanding pion gas created in ultrarelativistic Pb+Pb 160 GeV/n collisions are presented. Finite size of the expanding pion gas influences invariant mass spectrum of dileptons. Sensitivity of the shape of dilepton mass spectrum to the initial stage of the pion gas is discussed. Second order asymmetry in azimuthal distribution of dileptons is predicted for non-central collisions. Non-equilibrium nature of the pion gas expansion is found to exhibit itself in rapidity distribution of produced dilepton pairs.