Upper limit for thermal direct photon production in heavy-ion collisions at 60 and 200A·GeV

The production of direct photons has been investigated in reactions ofp and16O projectiles at 60 and 200A·GeV with C and Au nuclei. Photon and π0 spectra have been measured in the pseudorapidity range 1.5≦η≦2.1 for the transverse momentum region 0.4 GeV/c≦p T ≦2.8 GeV/c employing the lead-glass spectrometer SAPHIR. An upper limit of 15% at the 90% confidence level for the direct photon signal relative to the neutral pion production is obtained from the comparison of measured photon spectra with Monte Carlo simulations of the hadronic background based on the reconstructed yield of π0 and η mesons. Consequences for a possible phase transition to a quark-gluon plasma are discussed.