  The production of single photons in $Pb+Pb$ collisions at the CERN SPS as
measured by the WA98 experiment is analysed. A quark gluon plasma is assumed to
be formed initially, which expands, cools, hadronizes, and undergoes
freeze-out. A rich hadronic equation of state is used and the transverse
expansion of the interacting system is taken into account. The recent estimates
of photon production in quark-matter (at two loop level) along with the
dominant reactions in the hadronic matter leading to photons are used. About
50% of the single photons are seen to have a thermal origin. An addition of the
thermal and prompt photons is seen to provide a very good description of the
data. Most of the thermal photons having large transverse momenta arise from
the quark-matter, which contributes dominantly through the mechanism of
annihilation of quarks with scattering, and which in turn is possible only in a
hot and dense plasma of quarks and gluons. The results thus confirm the
formation of quark gluon plasma and the existence of this mechanism of the
production of single photons.
