Photon production in heavy-ion collisions at SPS energies

Abstract

Single photon spectra in heavy-ion collisions at SPS energies are studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. We separate the total photon spectrum into `background' arising from the radiative decays of $π^0$ and $η$ mesons, and the `themal' one from other sources. For the latter we include contributions from radiative decays of $ρ$, $ω$, $η'$, and $a_1$, radiative decays of baryon resonances, as well as two-body processes such as $ππ\to ργ$ and $πρ\to πγ$. It is found that more than 95% of all photons come from the decays of $π^0$ and $η$ mesons, while the thermal photons account for less than 5% of the total photon yield. The thermal photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper bound set by the experimental measurment of the WA80 Collaboration.