Low-energy thermal photons from meson–meson bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a Uem(1) gauge, we re-investigate photon bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions ππ→ππγ and πK→πKγ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau–Pomeranchuk–Migdal and in-medium effects on “σ” and ρ-meson exchanges are briefly discussed.