Single photons from Pb + Pb collisions at CERN SPS: quark-gluon plasma versus hadronic gas

In a hydrodynamic model, we have analysed the direct photon data obtained by the WA98 collaboration in 158 A GeV Pb + Pb collisions. The transverse expansion of the system has been taken into account. Two scenarios have been considered: (i) the formation of quark-gluon plasma and (ii) the formation of hot hadronic gas. Both scenarios describe the data equally well. However, the hadronic gas scenario requires a very high initial temperature (∼300 MeV) and it is difficult to conceive of the existence of hadron gas at that high temperature. If, initially, the hadronic fluid possesses small radial velocity (0.2-0.3 c), the data are well explained in the hadronic gas scenario, with reasonable initial temperatures