Non-perturbative production rate of photons with a lattice quark   propagator: effect of vertex correction
  We analyze the production rate of photons from the thermal medium above the deconfinement temperature with a quark propagator obtained from a lattice QCD numerical simulation. The photon-quark vertex is determined gauge-invariantly, so as to satisfy the Ward-Takahashi identity. The obtained photon production rate shows a suppression compared to perturbative results. 
Taekwang Kim, Masayuki Asakawa and Masakiyo Kitazawa
nucl-th hep-ph