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 ${\pi }^0$ and $\eta$ mesons, and the `themal' one from other sources. For the latter we include contributions from radiative decays of $\rho$, $\omega$, ${\eta }^{\prime }$, and $a_1$, radiative decays of baryon resonances, as well as two-body processes such as $\pi \pi \to \rho \gamma$ and $\pi \rho \to \pi \gamma$. It is found that more than 95% of all photons come from the decays of ${\pi }^0$ and $\eta$ 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.

Comments: RevTeX, 21 pages, including 13 postscript figures