Photon Production

Directly produced photons are of special interest [12]. Since they only interact electromagnetically, they have a mean free path much larger than the size of the reaction volume. Also, there are no final state interactions, as with hadrons, which means that the photons provide a direct probe of the initial stages of the collision. The direct photons are generally divided into prompt photons and thermal photons. The prompt photons are produced in initial hard parton scatterings, while thermal photons are produced in the possible QGP phase and the hadron gas phase. An increase in the emission of thermal photons is expected from a QGP.

The low production rate and huge background from decays, e.g. $\pi^0\rightarrow\gamma\gamma$ and $\eta\rightarrow\gamma\gamma$, make the isolation of the prompt and thermal components delicate.