NEUTRINO-PHOTON REACTIONS IN ASTROPHYSICS AND COSMOLOGY

At energies above the threshold for W production the process nu gamma --> lW(+) is competitive with nu nu scattering at the same center of m ass energies, In a cosmological setting, absorption of ultrahigh energ y neutrinos by the microwave photon background is comparable to absorp tion by the neutrino background. In passing through matter, the proces s nu --> lW(+) will occur in the Coulomb field of nuclei. For iron, th e interaction rate per nucleon is roughly 20% of the charge current cr oss section. The related profess <(nu)over bar>(e)e(-) --> gamma W- do minates <(nu)over bar>(e)e(-) scattering for about a decade in energy above the resonance for W production. [S0031-9007(97)05173-9].