Single photons from relativistic collision of lead nuclei at CERN SPS: A reanalysis
Abstract
We present a reanalysis of single photon production from relativistic collision of lead nuclei at CERN SPS measured by the WA98 experiment. The refinements include use of iso-spin 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 non-central 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 v2 for thermal photons. We also show that a measurement of v2 for thermal photons could also distinguish between the scenarios with or without a phase transition.