Pi+ Pi- Emission in High-Energy Nuclear Collisions

Realistic vacuum $\pi\pi$ interactions are employed to investigate thermal $\pi^+\pi^-$ emission spectra from the late stages of heavy-ion reactions at ultrarelativistic energies. Hadronic in-medium effects, including many-body $\rho$-meson spectral functions used earlier to describe the dilepton excess observed at CERN-SPS energies, are implemented to assess resulting modifications in relation to recent measurements of $\pi^+\pi^-$ invariant-mass spectra by the STAR collaboration in $p$-$p$ and $Au$-$Au$ collisions at $\sqrt{s_{NN}}$=200 GeV. Statistical model estimates for the $\rho^0/\pi^-$ and $K^*/K$ ratios close to the expected thermal freezeout are also given.