Photon emission in heavy ion collisions at the CERN SPS

We compute the thermal photon spectrum in the Pb+Pb collisions at the CERN SPS energy using thermal emission rates and a hydrodynamic description for the evolution of produced hot matter and compare our results with the measurements of the excess photons by the WA98 collaboration. Our results show that the measured photon spectrum can be reproduced with realistic initial conditions which take properly into account also the finite longitudinal size of the initial collision zone and which simultaneously describe well both the transverse and longitudinal hadron spectra. In the scenario with initial formation of QGP the recently calculated emission rate, complete to order α s , reproduces the measured spectrum. However, the experimental spectrum can also be reproduced in a purely hadronic scenario without transition to QGP state, but a high initial temperature, much over the values predicted for the phase transition temperature T c , is required.