Flash of photons from the early stage of heavy-ion collisions

Abstract

The dynamics of partonic cascades may be an important aspect for particle production in relativistic collisions of nuclei at CERN SPS and BNL RHIC energies. Within the parton-cascade model, we estimate the production of single photons from such cascades due to the scattering of quarks and gluons $q\stackrel{\ensuremath{\rightarrow}}{g}q\ensuremath{\gamma},$ quark-antiquark annihilation $q\overline{q}\ensuremath{\rightarrow}g\ensuremath{\gamma}$ or \ensuremath{\gamma}\ensuremath{\gamma}, and electromagnetic bremsstrahlung of quarks $\stackrel{\ensuremath{\rightarrow}}{q}q\ensuremath{\gamma}.$ We find that the latter QED branching process plays the dominant role for photon production, similarly as the QCD branchings $\stackrel{\ensuremath{\rightarrow}}{q}\mathrm{qg}$ and $\stackrel{\ensuremath{\rightarrow}}{g}\mathrm{gg}$ play a crucial role for parton multiplication. We conclude therefore that photons accompanying the parton-cascade evolution during the early stage of heavy-ion collisions shed light on the formation of a partonic plasma.