Neutrino-Photon Reactions in Astrophysics and Cosmology

At energies above the threshold for $W$ production the process $\nu \gamma \to {\mathrm{lW}}^{+}$ is competitive with $\nu \nu$ scattering at the same center of mass energies. In a cosmological setting, absorption of ultrahigh energy neutrinos by the microwave photon background is comparable to absorption by the neutrino background. In passing through matter, the process $\nu \to {\mathrm{lW}}^{+}$ 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 ${\overline{\nu }}_e{e}^{-}\to \gamma W^{-}$ dominates ${\overline{\nu }}_e{e}^{-}$ scattering for about a decade in energy above the resonance for $W$ production.