Bremsstrahlung out of the Quark-Gluon Plasma

Abstract

A systematic investigation of hard thermal photon spectra from central ultra­relativistic heavy ion collisions is presented with emphasis on the effects ofbremsstrahlung processes in the quark­gluon plasma (QGP). Bremsstrahlung photon production in the quark­gluon plasma has recently been considered withinthe Braaten­Pisarski method in thermal QCD, where rates have been found that exhibit the same order in the coupling constants as those describing the lowestorder processes, Compton scattering and q¯q­annihilation. The im­ pact of these bremsstrahlung photon production rates on the thermal photon spectra isstudied systematically within a simple, well understood one­fluid hydrodynamical model that describes an only longitudinally expanding fireball (1+1 Bjorkenscaling 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 thebremsstrahlung processes enhance the thermal photon yield from the QGP by about one order of magnitude over the complete considered p ­rangeindependent of the choice of the model parameters. This results in an enhancement of the total thermal photon yield which is most significant for parameter setsthat support a highly contributing QGP phase. The influence of each model parameter on the thermal photon spectra is examined carefully and a thoroughunderstanding 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 SPSare also considered and used to extract upper limits for the initial temperature of the QGP, where the QGP bremsstrahlung processes are found to make adifference of about 15 to 20 MeV depending on the temperature at which the phase transition is assumed. In comparison with other theoretical studies, theimpor­ tance of reaction features not described in the simple model are estimated and interesting elements for a future extension of this systematic investigationare identified, which will be of great interest in prospect of the upcoming experiments at the BNL RHIC and the CERN LHC.