Low-energy thermal photons from meson–meson bremsstrahlung

Abstract

Within an effective hadronic model including electromagnetic interactions via a U em (1) gauge, we reinvestigate 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 finalstate 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.

Anisotropic flow of thermal photons as a quark-gluon plasma viscometer

“…Since the viscous damping effects is strongest during the initial stages of the QGP phase, their inclusion is expected to further reduce the photon elliptic and triangular flow. On the other hand, we are also ignoring hadronic emission processes that involve collisions between mesons and (anti-)baryons and baryon-induced modifications of the vector meson spectral functions [32], as well as meson-meson and meson-baryon bremsstrahlung processes [48,49]. Furthermore, our current hydrodynamic approach, with its implemented sudden transition from a thermalized liq- uid to non-interacting, free-streaming particles does not allow for emission of photons by (increasingly rare) collisions among the dilute hadrons after kinetic freeze-out (such collisions are included in the PHSD approach which yields better agreement with the experimental data [49]).…”

Universal parametrization of thermal photon rates in hadronic matter

“…We also revisit the problem of hadronic Bremsstrahlung, specifically for the most abundant ππ → ππγ channel, by extending the calculations of Ref. [28] to higher energies and providing pertinent parameterizations as well.…”

“…Processes of type πN → πN γ and N N → N N γ are included in the ρ spectral functions used in the fits above, Comparison of rates at T = 150 MeV from Bremsstrahlung (blue curve) [28], πKρK * a1 interactions (green curve) [23], πρ → πγ with ω t-channel exchange (red curve) [23], and in-medium ρ decays (black curve).…”

“…The pertinent rates have been calculated in Ref. [28] in an effective hadronic model for S-and P -wave ππ (and πK) scattering. Special care was taken in maintaining EM gauge invariance in the presence of hadronic form factors, and in going beyond the often times applied soft-photon approximation.…”

“…The Bremsstrahlung rate is similarly suppressed compared to that from mesonic matter (including both the πKρK * a 1 and the ω t-channel contributions) [23] 1 . We note in passing that the contribution from πK Bremsstrahlung amounts to about 20% of the ππ one [28]. We have parameterized the latter using the ansatz…”