Observation of Direct Photons in Central158AGeVP208b+P208bCollisions

A measurement of direct photon production in ${}^{208}\mathrm{Pb}{+}^{208}\mathrm{Pb}$ collisions at $158A$ GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval $0.5<\mathrm{}{p}_T<4\mathrm{}\mathrm{GeV}/c$. A significant direct photon signal, compared to statistical and systematical errors, is seen at $p_T>1.5\mathrm{}\mathrm{GeV}/c$. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.