Neutrino helicity flips via electroweak interactions and SN1987a

Electroweak mechanisms via which neutrinos may flip helicity and become sterile are examined in detail. Exact and approximate expressions for a variety of flip processes relevant in astrophysics and cosmology, mediated by W,Z, and γ exchange, including their interference, are derived. It is shown that within the context of the Standard Model with massive neutrinos, for νe→νe scattering, σZflip/σγflip ∼ 6 X 103, independent of particle masses and energies to a good approximation. It is also shown that using some general considerations and the fact that the observed anti νe burst from SN1987a lasted several seconds, these weak interaction flip cross-sections can be used to derive an upper limit on μ and τ neutrino masses of ∼ 40 keV. Finally, some other consequences for astrophysics in general and supernovae in particular are briefly discussed. 29 refs.; 47 schemes