Neutrino spin-flip effects in collapsing stars.

We study the spin-flavor transitions of neutrinos, {bar {nu}}{sub {ital e}}-{nu}{sub {mu}}, {nu}{sub {ital e}-}{bar {nu}}{sub {mu}}, etc., in the magnetic fields of a collapsing star. For the neutrino mass squared difference {Delta}{ital m}{sup 2}{similar_to}(10{sup {minus}10}--10) eV{sup 2} the transitions take place in an almost isotopically neutral region of the star, where the effective matter density is suppressed up to 3--4 orders of magnitude. This suppression is shown to increase the sensitivity of the neutrino burst studies to the magnetic moment of the neutrino, {mu}, by 1.5--2 orders of magnitude, and for realistic magnetic field the observable effects may exist for {mu}{similar_to}(2--3){times}10{sup {minus}14}{mu}{sub {ital B}} ({mu}{sub {ital B}} is the Bohr magneton). In the isotopically neutral region the jumps of the effective potential exist which influence the probabilities of transitions. The experimental signatures of the spin-flavor transitions are discussed. In particular, in the case of direct mass hierarchy, the spin-flip effects result in a variety of modifications of the {bar {nu}}{sub {ital e}} spectrum. Taking this into account, we estimate the upper bounds on {mu}{ital B} from the SN 1987A data. In the isotopically neutral region the effects of the possible twist of the magnetic field on the way ofmore » neutrinos can be important, inducing distortion of the neutrino energy spectra and further increasing the sensitivity to {mu}{ital B}. However, if the total rotation angle is restricted by {Delta}{phi}{lt}{pi}, the absolute change of probabilities is small.« less