Recent results from the WA98-experiment

Abstract

Recent results of the WA98 experiment with p and Pb induced reactions at 158 A GeV are presented. The invariant yield of direct photons in central Pb + Pb collisions is shown as a function of transverse momentum. A significant direct photon signal is seen at p T >1.5 GeV/c. The results constitute the first observation of direct photons in ultrarelativistic heavy-ion collisions. Azimuthal γ-γ correlations at high p T were studied in search for a signal of jet-like structures. A clear indication for back-to-back correlations can be seen in p + A with a strong dependence on the p T of the photons and the size of the system. In Pb + Pb in plane elliptic flow has been observed. Results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected Pb + Pb collisions. In going from peripheral to medium central collisions there is a nuclear enhancement increasing with transverse mass similar to the Cronin effect, while for very central collisions this enhancement appears to be weaker than expected.