NEUTRINO ANNIHILATION IN HOT PLASMA

We consider neutrino annihilation in a heat bath, including annihilation via the photon. We show that the annihilation cross section has high and narrow peaks corresponding to a plasmon resonance. This yields an enormous enhancement factor of O(10(8)) in the differential cross section as compared with the purely weak contribution. We also evaluate numerically the thermally averaged neutrino annihilation rate per particle in the heat bath of the early universe to be [sigma(nu-nuBAR --> e+e-)] congruent-to 2.93G(F)2T2. we have accounted for the final-state blocking factors as well as for the fact that the center-of-mass frame of collisions is not necessarily the rest frame of the heat bath. Despite the resonances, electromagnetic processes represent only a minor effect in the averaged annihilation rate.