Dirac neutrinos and SN 1987A

Within the standard electroweak theory "wrong-helicity" neutrinos are produced in a nascent neutron star by "spin-flip" processes (at a rate proportional to $m_{\nu }^2$), freely escape, and can lead to an excessively rapid cooling of the newly born neutron star. Previous work, based upon the neutrino-nucleon spin-flip scattering process alone, has shown that the observed cooling of the neutron star associated with SN 1987A excludes a Dirac-neutrino mass greater than Å20 keV for either ${\nu }_e$, ${\nu }_{\mu }$, or ${\nu }_{\tau }$. We reexamine the emission of "wrong-helicity" Dirac neutrinos from SN 1987A and conclude that due to neutrino degeneracy and additional emission processes ($N+N\to N+N+\nu \overline{\nu }$, ${\pi }^{-}+p\to n+\nu \overline{\nu }$) the effect of a Dirac neutrino on the cooling of SN 1987A has been underestimated. While a precise Dirac-mass limit awaits the incorporation of our new rates into detailed numerical cooling models, we believe that the limit that follows from the cooling of SN 1987A is better, probably much better, than 10 keV. In particular, we believe that SN 1987A definitely excludes a 17-keV (purely) Dirac-mass neutrino that mixes with the electron neutrino at the 1% level.