Recently the WA98 collaboration published data concerning the direct photon production in the reaction Pb(158 AGeV)+Pb [1]. Analog to our studies [2] of the dilepton spectra in similar reactions, we use a superposition of hard photon production processes and of the yields from thermalized matter in order to describe these data.
The contributions of the hard annihilation and Compton like diagrams within the framework of perturbative QCD are calculated using the event generator PYTHIA. The comparison of PYTHIA results and various direct photon data in proton induced collisions indicates that the shapes of the data are well described. Fig. 1 displays two examples of the comparison of our calculations and data.
The hard perturbative QCD contribution, calculated for pp collisions, is up-scaled such that the large-p_^ tail of the WA98 data is reproduced (cf. fig. 2). The thermal source is described by

dN d3p/E = Neff  5aasTeff2 12p2 ó õ 1 0  ds s2 ó õ +1 -1  dx e-A log é ê ë 1+ 2.912 4pas A ù ú û (1)

with A = [(p_^coshy (1-sv₀x))/(T_effÖ[(1-(sv₀)²)])], an effective temperature of T_eff = 170 MeV and a space-time normalization factor N_eff = 3×10⁴ fm⁴. These parameters are taken from our analysis [2] of dilepton data. Contrary to the SPS dilepton data, flow effects are visible in the photon p_^ spectra at values around 2 GeV. With a maximal radial expansion of v₀ = 0.3c of the radiating fireball a good description of the data is achieved (see fig. 2).

Fig. 1 A comparison of calculations of the hard photon production in proton-proton collisions with data from E704 [3] at Ös = 19.4 GeV (small p^ values) and from UA6 [4] at Ös = 24.3 GeV (large p^ values). For more details cf. [5].		Fig. 2 The data of the WA98 collaboration [1] compared with the hard direct photons (dashed curve) and the thermal contribution (thin curve). The sum of these contributions is depicted by the solid curve. A radial flow of v0 = 0.3c is assumed.

 IKH 06/19/01 © B. Kämpfer