Single photons from relativistic collision of lead nuclei at CERN SPS: A reanalysis

TL;DR

This paper reanalyzes single photon production in lead nucleus collisions at CERN SPS, incorporating refined theoretical models and hydrodynamics to better understand initial conditions and phase transition effects.

Contribution

It introduces an improved NLO pQCD approach with shadowing corrections and hydrodynamic modeling to interpret photon data, exploring different initial temperatures and phase transition scenarios.

Findings

Data can be explained with high initial temperature and moderate prompt yield increase.

Different initial times affect the elliptic flow parameter $v_2$ for thermal photons.

Measuring $v_2$ can distinguish between phase transition scenarios.

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 $v_2$ for thermal photons. We also show that a measurement of $v_2$ for thermal photons could also distinguish between the scenarios with or without a phase transition.