Neutrino decay catalyzed by the Mikheyev-Smirnov-Wolfenstein effect

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 (νₑ) in this regime are practically stable, in solar matter, conversion of the νₑ 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 g²4.5×{}10^-5.