Single photons from relativistic collisions of lead nuclei at energies available at the CERN Super Proton Synchrotron (SPS): A reanalysis

Abstract

We present a reanalysis of single photon production from a relativistic collision of lead nuclei at CERN SPS measured by the WA98 experiment. The refinements include use of isospin and shadowing corrected NLO pQCD treatment for prompt photon production using an optimized scale for factorization, renormalization, and fragmentation and use of hydrodynamics suited for noncentral collisions along with a well tested equation of state admitting a quark-hadron phase transition. A quantitative explanation of the data requires a large initial temperature (at a small formation time of about 0.2 fm/$c$) and a moderate increase in the prompt yield which could perhaps be attributed to the Cronin effect in nuclei. The data can also be explained using a moderate initial temperature (at a formation time of about 1 fm/$c$) with a very large $K$-factor multiplying the prompt yield. We show that different initial times give rise to different values for the elliptic flow parameter ${v}_{2}$ for thermal photons. We also argue that a measurement of ${v}_{2}$ for thermal photons could also distinguish between the scenarios with or without a phase transition.