Transverse expansion and dileptons from the rho-peak in ultrarelativistic nuclear collisions

The authors calculate the transverse-mass distribution of lepton pairs in the {rho} - {omega} peak. An equation of state with a strong first order phase transition is assumed. They focus on the baryon-free central rapidity region, and couple a boost-invariant longitudinal expansion to a cylindrically symmetric transverse expansion to model the dynamics of central collisions. In the absence of transverse expansion, the fit temperature of the resulting dilepton spectra provided a good thermometer for the critical temperature. Assuming thermal equilibrium throughout the collision, they study the effect of transverse expansion on the spectra. They allow deviations from chemical equilibrium in the high-temperature phase by appropriately parametrizing the equation of state. The freeze-out dependence is also studied. They calculate dilepton production vs. transverse-mass as a function of the critical temperature for these different scenarios.