Sum rules at finite temperature and the limitations of kinetic particle-production models

Generalized dipole sum rules for photon production are derived for systems at finite temperature. These sum rules provide a stringent check on the validity of photon-production models. We inspect such models for the case of nuclear matter at finite temperature employed in a kinetic regime encountered in intermediate-energy nuclear collisions. We find photon-production rates which significantly exceed the limits imposed by the sum rules. This suggests that destructive coherence phenomena lead to a considerable quenching of the production rates over a wide range of photon energies. Thus the mechanism can no longer be understood as an incoherent addition of free NNγ production processes. Also the currently used production cross sections are seen to have the wrong behaviour at high photon energies. (orig.)