Neutrino helicity flips via electroweak interactions

Electroweak mechanisms via which neutrinos may flip helicity 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 for both Dirac and Majorana neutrinos (with emphasis on the former). It is shown that in general flip and nonflip cross sections differ by more than just a multiplicative factor of m_ν²4E_ν² contrary to what might be expected and that this additional dependence on helicities can be significant. It is also shown that within the context of the standard model with massive neutrinos, for νe→νe scattering, σZᶠˡⁱᵖσ_γᶠˡⁱᵖ≈10⁴, independent of particle masses and energies to a good approximation. As an application, using some general considerations and the fact that the observed νₑ burst from SN 1987A lasted several seconds, these weak-interaction flip cross sections are used to rule out μ and τ neutrino masses above 30 keV. Finally, some other consequences for astrophysics in general and supernovae in particular are briefly discussed.