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Neutrino-photon reactions in astrophysics and cosmology

At energies above the threshold for $W$ production the process $νγ\to l W^+$ is competive with $νν$ 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 $ν\to l W^+$ 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, $\antiν_e e^- \to γW^-$ dominates $\antiν_e e^-$ scattering for about a decade in energy above the resonance for $W$ production.