Electromagnetic recombination spectra at the quark-hadron phase transition

TL;DR

This paper investigates electromagnetic radiation emitted during the quark-hadron phase transition, proposing mechanisms that could explain observed photon yields and flow in heavy ion collisions.

Contribution

It introduces calculations of photon emission from quark coalescence and decay of excited states during hadronization, linking these to experimental observations.

Findings

Photon yield from quark coalescence is about 1% of pion yield.

Late-stage photon emission can carry significant elliptic flow.

Enhanced photon emission explains large observed gamma/pi ratios.

Abstract

When quarks hadronize, they accelerate. Because they carry electric charge, they must radiate light as they accelerate and hadronize. This is true not only in jets but also in heavy ion collisions, where a thermalized plasma of quarks and gluons cools into a gas of hadrons. First, direct emission of photons from two quarks coalescing into pions is calculated using the quark-meson model. The yield of final-state photons to pions is found to be about $e^2/g^2_{\pi qq}$, which is on the order of a percent. Second, the yield of photons from the decay of highly excited color singlets, which may exist ephemerally during hadronizaton, is estimated. Because these contributions occur late in the reaction, they should carry significant elliptic flow, which may help explain the large observed flow of direct photons at RHIC by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC). The enhanced emission also helps explain PHENIX's surprisingly large observed $\gamma/\pi$ ratio.