Constraints on radiative decay of the 17-keV neutrino from COBE measurements.

Abstract

It is shown that, for a nontrivial radiative decay channel of the 17-keV neutrino, the photons would distort the microwave background radiation through ionization of the Universe. The constraint on the branching ratio of such decays from Cosmic Background Explorer measurements is found to be more stringent than that from SN 1987A. The limit on the branching ratio in terms of the Compton $y$ parameter is ${B}_{\ensuremath{\gamma}}\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}{[\frac{{\ensuremath{\tau}}_{\ensuremath{\nu}}}{({10}^{11}sec)}]}^{0.45}{(\frac{y}{{10}^{\ensuremath{-}3}})}^{1.11}{h}^{\ensuremath{-}1}$ for an $\ensuremath{\Omega}=1$, ${\ensuremath{\Omega}}_{B}=0.1$ universe.