Muon pairs from In + In collision at energies available at the CERN Super Proton Synchrotron

Abstract

The NA60 Collaboration has extracted the inverse slope parameters, ${T}_{\mathrm{eff}}$ of the di-muon spectra originating from the In + In collisions at $\sqrt{{s}_{NN}}=17.3$ GeV for various invariant mass regions. They have observed that the inverse slope parameter as a function of invariant mass of the lepton pair drops beyond the $\ensuremath{\rho}$ peak. In the present work, first we reproduce the observed invariant mass and transverse momentum spectra of the muon pairs by taking into account the medium effects on the hadronic spectral function. Then we show that the slope parameters extracted from the transverse momentum distributions for various invariant mass windows can be explained by assuming the formation of a partonic phase initially which reverts to the hadronic phase through a weak first-order phase transition at a temperature ${T}_{c}\ensuremath{\sim}175$ MeV. It is observed that a scenario without the formation of a partonic phase does not reproduce the nonmonotonic behavior of the inverse slope parameter nontypical of radial flow.