Single photons from Pb plus 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 + Ph 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 (similar to300 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.