Photon production by a quark-gluon plasma

Photon or dilepton production by a hot plasma is expected to reflect rather cleanly the state of the system. Indeed, in the collision of two heavy nuclei, the typical size of the system is much smaller than the mean free path of particles that feel only the electromagnetic interactions, which enables photons and leptons to escape freely. However, it has been realized recently that those rates are non-perturbative quantities. In other words, even for an idealized situation where the QCD coupling constant is small, the rate at leading order receives contributions from an infinite set of diagrams. To keep the model as simple as possible, I assume in this talk that the temperature is much larger than all the quark masses (T ≫ mq), that the strong coupling constant at this temperature scale is extremely small (g ≪ 1), and that the system is in both thermal and chemical equilibrium. Under these conditions, thermal field theory seems to be the tool of choice to calculate the photon production rate, which is obtained by calculating the imaginary part of the retarded photon polarization tensor [1]: dN dtdV dωd3q ∝ ImΠμμret(ω,q) . (1)