AbstractsPhysics

A systematic investigation of hard thermal photon spectra from central ultra­relativistic heavy ion collisions is presented with emphasis on the effects of bremsstrahlung processes in the quark­gluon plasma (QGP). Bremsstrahlung photon production in the quark­gluon plasma has recently been considered within the Braaten­Pisarski method in thermal QCD, where rates have been found that exhibit the same order in the coupling constants as those describing the lowest order processes, Compton scattering and q¯q­annihilation. The im­ pact of these bremsstrahlung photon production rates on the thermal photon spectra is studied systematically within a simple, well understood one­fluid hydrodynamical model that describes an only longitudinally expanding fireball (1+1 Bjorken scaling hydrody­ namics). A first­order phase transition is implemented in which QGP (simulated by an ideal massless parton gas of two­flavors) 'hadronizes' according to the Gibbs criteria and Maxwell construction into a hot hadronic gas (HHG) (simulated by an ideal massless pion gas). It is found that the bremsstrahlung processes enhance the thermal photon yield from the QGP by about one order of magnitude over the complete considered p ­range independent of the choice of the model parameters. This results in an enhancement of the total thermal photon yield which is most significant for parameter sets that support a highly contributing QGP phase. The influence of each model parameter on the thermal photon spectra is examined carefully and a thorough understanding of the model is ob­ tained. Experimental upper limits on direct photon production in fixed target 200 A.GeV S + Au collisions at the CERN SPS are also considered and used to extract upper limits for the initial temperature of the QGP, where the QGP bremsstrahlung processes are found to make a difference of about 15 to 20 MeV depending on the temperature at which the phase transition is assumed. In comparison with other theoretical studies, the impor­ tance of reaction features not described in the simple model are estimated and interesting elements for a future extension of this systematic investigation are identified, which will be of great interest in prospect of the upcoming experiments at the BNL RHIC and the CERN LHC.