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 $\rho$, $\omega$, $\eta'$, 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.Comment: RevTeX, 21 pages, including 13 postscript figure

Hadronic production of thermal photons

“…In particular, we recall that baryonic sources are very important [16,30] for understanding the observed excess in low-mass (M ≤ 1 GeV) dilepton production in (semi-) central P b-Au collisions at both full (160 AGeV) [31] and lower (40 AGeV) [17] SPS energy. It is therefore mandatory to scrutinize the role of baryons in photon production, especially since most investigations thus far not revealed substantial contributions [32,33,34].…”