Thermal photons from heavy ion collisions: A spectral function approach

We analyze the photon rates from a hadronic gas in equilibrium using chiral-reduction formulas and a density expansion. The chiral reduction is carried to second order in the pion density, which in principal includes all kinetic processes of the type X→πγ and X→ππγ. The resulting rates are encoded in the form of vacuum correlation functions, which are amenable to experiment. The hadronic rates computed in this work along with the known perturbative quark gluon plasma rates are integrated over the space-time evolution of a hydrodynamic model tuned to hadronic observables. The resulting yields are compared to the recent photon and low-mass dilepton measurements at the Super Proton Synchrotron and Relativistic Heavy Ion Collider. Predictions for the Large Hadron Collider are made.