KINETICS OF AN EXPANDING PION GAS AND LOW-MASS DILEPTON EMISSION

We study the space-time dependence of muon pair production from dense, nonequilibrium pion matter in the framework of a Boltzmann type trans port model which properly includes effects of Bose statistics. Startin g from an initially supersaturated pion gas with a large positive effe ctive chemical potential we find that this pion chemical potential sta ys approximately constant during the evolution. As a consequence, such a scenario leads to an increased dilepton yield near the kinematical threshold for muon pair production via pion annihilation. Depending on the lifetime of the dense hadron matter formed in relativistic heavy ion collisions, this component can be observed above the background of muon pairs from resonance decays and the rho, omega bump. In such a w ay the measurement of the dimuon invariant mass spectrum could help to understand the initial state and the dynamical evolution of ultrarela tivistic heavy ion collisions.