Open Access

Direct photon emission from heavy-ion collisions has been calculated and compared to available experimental data. Three different models have been combined to extract direct photons from different environments in a heavy-ion collision: Thermal photons from partonic and hadronic matter have been extracted from relativistic, non-viscous 3+1-dimensional hydrodynamic calculations. Thermal and non-thermal photons from hadronic interactions have been calculated from relativistic transport theory. The impact of different physics assumptions about the thermalized matter has been studied. In pure transport calculations, a viscous hadron gas is present. This is juxtaposed with ideal gases of hadrons with vacuum properties, hadrons which undergo a chiral and deconfinement phase transition and with a system that has a strong first-order phase transition to a deconfined ideal gas of quarks and gluons in the hybrid model calculations with the various Equations of State. The models used for the determination of photons from both hydrodynamic and transport calculations have been elucidated and their numerical properties tested. The origin of direct photons, itemised by emission stage, emission time, channel and baryon number density, has been investigated for various systems, as have the transverse momentum spectra and elliptic flow patterns of direct photons. The differences of photon emission rates from a thermalized transport box and the hadronic photon emission rates that are used in hydrodynamic calculations are found to be very similar, as are the spectra from calculations of heavy-ion collisions with transport model and hybrid model with hadronic Equation of State. Taking into account the full (vacuum) spectral function of the rho-meson decreases the direct photon emission by approximately 10% at low photon transverse momentum. The numerical investigations show that the parameter with the largest impact on the direct photon spectra is the time at which the hydrodynamic description is started. Its variation shows deviations of one to two orders of magnitude. In the regime that can be considered physical, however, the variation is less than a factor of 3. Other parameters change the direct photon yield by up to approximately 20%. In all systems that have been considered -- heavy-ion collisions at E_lab = 35 AGeV and 158 AGeV, (s_NN)**1/2 = 62.4 GeV, 130 GeV and 200 GeV -- thermal emission from a system with partonic degrees of freedom is greatly enhanced over that from hadronic systems, while the difference between the direct photon yields from a viscous and a non-viscous hadronic system (transport vs. hydrodynamics) is found to be very small. Predictions for direct photon emission in central U+U-collisions at 35 AGeV have been made. Since non-soft photon sources are very much suppressed at this energy, experimental results should very easily be able to distinguish between a medium that is entirely hadronic and a system that undergoes a phase transition from partonic to hadronic matter. In the case of lead-lead collisions at 158 AGeV, the situation is not so clear. In central collisions, the complete direct photon spectra including prompt photons seem to favour hadronic emission sources, while the partonic calculations only slightly overpredict the data. In peripheral collisions at the same energy, the hadronic contribution is more than one order of magnitude smaller than the prompt photon contribution, which fits the available experimental data. A similar picture presents itself at higher energies. At RHIC energies, however, the difference between transport calculations and hadronic hybrid model calculations is largest. Hybrid model calculations with partonic degrees of freedom can describe the experimental results in gold-gold collisions at 200 GeV. The elliptic flow component of direct photon emission is found to be consistently positive at small transverse momenta. This means that the initial photon emission from a non-flowing medium does not completely overshine the emission patterns from later stages. High-pt photons dominantly come from the beginning of a heavy-ion collision and therefore do not carry the directed information of an evolving medium.