Neutrino-photon reactions in astrophysics and cosmology

At energies above the threshold for $W$ production the process $\nu \gamma \to l W^+$iscompetivewith$\nu \nu$ scattering at the same center of mass energies. In a cosmological setting, absorption of ultra high energy neutrinos by the microwave photon background is comparable to absorption by the neutrino background. In passing through matter, the process $\nu \to l{W}^{+}$ will occur in the coulomb field of nuclei. For iron, the interaction rate per nucleon is roughly 20% of the charge current cross-section. The related process, $\text{anti}{\nu }_e{e}^{-}\to \gamma W^{-}$ dominates $\text{anti}{\nu }_e{e}^{-}$ scattering for about a decade in energy above the resonance for $W$ production.