Limits to radiative neutrino decay from SN 1987A

Abstract

We calculate limits to the properties of massive, unstable neutrinos using data from $\ensuremath{\gamma}$-ray detectors on the Pioneer Venus Orbiter (PVO) Spacecraft. The absence of a $\ensuremath{\gamma}$-ray signal in the PVO detector constrains the branching ratio to photons ${(B}_{\ensuremath{\gamma}})$, mass ${(m}_{\ensuremath{\nu}})$, and radiative lifetime $({\ensuremath{\tau}}_{\ensuremath{\gamma}}=\ensuremath{\tau}{/B}_{\ensuremath{\gamma}})$. For low-mass $(m\ensuremath{\lesssim}T\ensuremath{\sim}8\mathrm{}\mathrm{MeV})$ neutrinos decaying $\ensuremath{\nu}\ensuremath{\rightarrow}{\ensuremath{\nu}}^{\ensuremath{'}}\ensuremath{\gamma}$, ${B}_{\ensuremath{\gamma}}<3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ for ${m}_{\ensuremath{\nu}}\ensuremath{\tau}\ensuremath{\lesssim}{10}^{6}\mathrm{}\mathrm{keV}\mathrm{sec}$, and ${B}_{\ensuremath{\gamma}}<2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{m}_{\ensuremath{\nu}}\ensuremath{\tau}/\mathrm{keV}\mathrm{}\mathrm{sec}$ for ${m}_{\ensuremath{\nu}}\ensuremath{\tau}\ensuremath{\gtrsim}{10}^{6}\mathrm{}\mathrm{keV}\mathrm{sec}$; limits for high-mass neutrinos are somewhat weaker due to Boltzmann suppression. We also calculate limits for decays that produce $\ensuremath{\gamma}$ rays through the bremsstrahlung channel, $\ensuremath{\nu}\ensuremath{\rightarrow}{\ensuremath{\nu}}^{\ensuremath{'}}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$. With one exception, the PVO limits are roughly comparable to those from an analysis of data from the Solar Max Mission (SMM) Satellite (which observed at higher $\ensuremath{\gamma}$-ray energies but for a much shorter time). For neutrino mass states that are nearly degenerate, $\ensuremath{\delta}{m}^{2}{/m}^{2}\ensuremath{\sim}0.1\ensuremath{\ll}1$, our limits for the mode $\ensuremath{\nu}\ensuremath{\rightarrow}{\ensuremath{\nu}}^{\ensuremath{'}}\ensuremath{\gamma}$ become more stringent by a factor as large as ${m}^{2}/\ensuremath{\delta}{m}^{2}$, because more decay photons are shifted into the PVO energy window. For this same reason, SMM cannot constrain this case.