New bounds on neutrino magnetic moments from stellar collapse.

Abstract

Magnetic or electric dipole moments of neutrinos are indicators for possible extensions of the standard weak-interaction model. Certain values were recently proposed to explain the time variation in the detected solar-neutrino flux and an extragalactic background of ionizing radiation. Here we give a detailed analysis of their implications for a stellar collapse. We find two independent new upper bounds of 1.5 x 10/sup -12/..mu../sub B/ and 6 x 10/sup -12/..mu../sub B/ on the dipole moment of electron neutrinos, improving the existing bounds by one to two orders of magnitude and ruling out the proposed solar-neutrino effect. For values approx. >10/sup -13/..mu../sub B/ the coherently enhanced dipole scattering on the large nuclei in a collapsing star would flip the trapped left-handed neutrinos into right-handed ones, which would escape in a highly energetic prebounce ..nu../sub e/ ''collapse burst.'' This also reduces the trapped lepton number of the collapsing star. We calculate the spectra of the emitted neutrinos and the associated lepton-number decrease. From the supernova 1987A up to 100 neutrinos from this additional output should have been detected in the Kamiokande II detector. The absence of such events imposes the upper bound ..mu../sub ..nu..//sub >//sub e//sub =/ //sub e//sub =/<6 x 10/sup -12/..mu../sub B/, providing an independent supplementary bound.« less