Electromagnetic Emission and Energy Loss in the QGP

I discuss why photon production from the Quark Gluon Plasma (QGP) presents an interesting problem, both experimentally and theoretically. I show how the photon emission rate can be computed under the simplifying assumption that the QGP fully thermalizes. The theoretical issues are very similar to those for jet energy loss; so it should be possible to treat them in a common formalism and relate the predictions of one phenomenon to those of the other. Submitted to: J. Phys. G: Nucl. Phys. 1. Photons as a deep probe The main goals of the RHIC experimental program are, to produce and to characterize the quark-gluon plasma (QGP). The plasma is defined as being a state of matter for which the density of partons (quarks and gluons) is so great that a description in terms of hadronic degrees of freedom is impossible, which is furthermore extensive and relatively close to equilibrium. The problem is that the QGP expands, cools, and hadronizes. Even after hadronization, hadrons continue to interact; much of the information about the initial state is therefore lost. For instance, the spectrum of hadronic final states which we observe at RHIC is well characterized by a thermal model with a temperature of only about 160 MeV, too low for a QGP but reasonable for the freezeout of a hadronic gas [1]. Therefore, the hadronic observables give only rather indirect information about the QGP phase of the collision. For this reason, it was proposed years ago to study “hard” probes, meaning particles which are produced early in the collision, deep within the QGP, and which then escape with little or no interaction, giving relatively direct information about the early stages of the QGP. Shuryak proposed photons as a promising direct probe [2], and they continue to be actively investigated [3]. The advantage of photons as a probe of the QGP is that any photons produced are almost sure to escape without re-interacting, so they give direct, rather than indirect or processed, information about the early stages of the heavy ion collision. The main disadvantage is that there are several sources of photons, with the concomitant problem of determining which photons arose from which source. Namely, there are • Prompt photons, those produced by the initial collisions of the partons which constitute the heavy ions being collided; Electromagnetic Emission and Energy Loss in the QGP 2 • QGP photons, those produced during scatterings and interactions within the quark gluon plasma; • Hadronic photons, those produced after the QGP has changed into a hadron gas but before it has spread out enough that mutual interactions cease; and • Decay photons, those produced in the decay of certain hadrons, chiefly π and η mesons, “long” after the collision has finished.