A new mechanism for neutrino (ν) decay in the Mikheyev-Smirnov-Wolfenstein (MSW) regime of weak mixing and small ν mass differences is pointed out. Even though electron-neutrinos (${\nu }_e$) in this regime are practically stable, in solar matter, conversion of the ${\nu }_e$ to a ‘‘heavier’’ flavor by the MSW effect can catalyze ν decay. MSW+ν decay into Majorons can lead to a strong solar antineutrino signal in proposed experiments, directly probing ν-Majoron couplings ∼700 times smaller than the present laboratory bound of $g^2$<4.5×${10}^{\mathrm{-}5}$.

• Received 21 September 1987

DOI:https://doi.org/10.1103/PhysRevD.38.1317