Flash of Prompt Photons from the Early Stage of Heavy-Ion Collisions

We briefly recall the tremendous strides in the studies of the parton cascade model made by Klaus Geiger. Next, we argue that photons may provide confirmation of several of these ideas. Thus we know that, copious internetted partonic cascades may develop in the wake of relativistic collisions of nuclei at CERN SPS and BNL RHIC energies, We use the parton cascade model to estimate the emission of single photons generated from such cascades due to the branching of quarks $q \to q \gamma$, scattering of quarks and gluons, and annihilation of quarks. The formation of a hot and dense partonic matter is shown to be preceded by an intense radiation of photons from the QED branching of quarks. This is similar to the QCD branching $q \to qg$ which along with the gluon multiplication ($g \to gg$) which is known to play a crucial role in the formation of the dense partonic plasma.Comment: 14 pages, including 4 figures. Contribution to Proceedings of ``Kay Kay Gee Day- Klaus Kinder-Geiger Memorial Workshop'' held at Brookhaven National Lab, October 23, 199

Scaling of single photon production in hadronic collisions

Scaling of single photon production in $pp$ and $p\overline{p}$ collisions is studied. It is empirically observed the available data scales $\sim \sqrt{s}/p_T^5$ for $x_T=2p_T/\sqrt{s} \leq 0.1$ and $\sim (\sqrt{s})^{3.3}/p_T^9$ for larger $x_T$. The NLO pQCD predictions for $pp$ collisions at $\sqrt{s}$ of 200 and 5500 GeV, relevant for RHIC and LHC energies are seen to closely follow this scaling behaviour. Implications for single photon production in heavy ion collisions are discussed.Comment: 4 pages including 5 figures. Figure 5 changed to include correct two-loop contributions. (Submitted to Eur. Phys. J. C.

Single Photons from Relativistic Heavy Ion Collisions and Quark-Hadron Phase Transition

The present status of theoretical expectations of studies of single photons from relativistic heavy ion collisions is discussed. It is argued that the upper limit of single photon radiation from S+Au collisions at CERN SPS obtained by the WA80 collaboration perhaps rules out any reasonable description of the collision process which does not involve a phase transition to quark gluon plasma. Predictions for single photons from the quark-matter likely to be created in collision of two lead nuclei at RHIC and LHC energies are given with a proper accounting of chemical equilibration and transverse expansion. Finally, it is pointed out that, contrary to the popular belief of a quadrilateral dependence of electromagnetic radiations ($N_\gamma$) from such collisions on the number of charged particles ($N_{ch})$, we may only have $N_\gamma \propto N_{ch}^{1.2}$.Comment: Invited Talk given at Third International Conference on Physics and Astrophysics of Quark Gluon Plasma, Jaipur, India, March 199

A Second Look at Single Photon Production in S+Au Collisions at 200 A⋅\cdotGeV and Implications for Quark Hadron Phase Transition

We reanalyze the production of single photons in S+Au collisions at CERN SPS to investigate: i) the consequences of using a much richer equation of state for hadrons than the one used in an earlier study by us; and, ii) to see if the recent estimates of photon production in quark-matter (at two loop level) by Aurenche et al. are consistent with the upper limit of the photon production measured by the WA80 experiment. We find that the measured upper limit is consistent with a quark hadron phase transition. The measured upper limit is also consistent with a scenario where no phase transition takes place, but where the hadronic matter reaches a density of several hadrons per unit volume; which is rather unphysical.Comment: 10 pages, including 3 figures; Submitted to Euro. Phys. Jour.