In Medium Modifications to Ρ from Finite Pion Density Effects and Dilepton Spectrum

The behavior of the pion dispersion relation in a pion medium is strongly modified by the introduction of a finite chemical potential associated to the finite pion number density. Such behavior is particularly important during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the pion number changing processes, driven by the strong interaction, can be considered to be frozen. We make use of an effective Lagrangian that explicitly respects chiral symmetry through the enforcement of the chiral Ward identities. The pion dispersion relation is computed through the computation of the pion self-energy in a non-perturbative fashion by giving an approximate solution to the Schwinger-Dyson equation for this self-energy. Given the strong coupling between ρ vectors and pions, we argue that the modification of the pion mass due to finite pion density effects has to be taken into account self-consistently for the description of the in-medium modifications of ρ’s. We finally study some possible consequences of finite pion density effects for the low-mass dilepton spectrum produced in relativistic heavy-ion collisions.