Plasma induced neutrino spin-flip in a supernova and new bounds on the neutrino magnetic moment

The neutrino chirality-flip process under the conditions of the supernova core is investigated in detail with the plasma polarization effects in the photon propagator taken into account in a more complete form than in earlier publications. It is shown in part that the contribution of the proton fraction of plasma is essential. New upper bounds on the neutrino magnetic moment are obtained: μν < (0.5 − 1.1) × 10 −12 μB from the limit on the supernova core luminosity for νR emission, and μν < (0.4− 0.6) × 10 −12 μB from the limit on the averaged time of the left-handed neutrino washing out. The best upper bound on the neutrino magnetic moment from SN1987A is improved by the factor of 3 to 7. 1 Neutrino spin-flip in the supernova core Nonvanishing neutrino magnetic moment leads to various chirality-flipping processes where the left-handed neutrinos produced in the stellar interior become the right-handed ones, i.e. sterile with respect to the weak interaction. A considerable interest to the neutrino magnetic moment arised after the great event of SN1987A, in connection with the modelling of a supernova explosion, where gigantic neutrino fluxes define in fact the process energetics. It means that such a microscopic neutrino characteristic, as the neutrino magnetic moment, would have a critical influence on macroscopic properties of these astrophysical events. Namely, the lefthanded neutrinos produced inside the supernova core during the collapse, could convert into the right-handed neutrinos due to the magnetic moment interaction. These sterile neutrinos would escape from the core leaving no energy to explain the observed luminosity of the supernova. Thus, the upper bound on the neutrino magnetic moment can be established. This matter was investigated by many authors. We will mainly focus on the paper by R. Barbieri and R. N. Mohapatra [1] which now looks as the most reliable instant constraint on the neutrino magnetic moment from SN1987A, according to [2]. The authors [1] considered the neutrino spin-flip via both νLe − → νRe − and νLp → νRp scattering processes in the inner core of a supernova immediately after the collapse. Imposing for the νR luminosity QνR the upper limit of 1053 ergs/s, the authors obtained the upper bound on the neutrino magnetic moment: μν < (0.2− 0.8) × 10 −11 μB . (1) However, the essential plasma polarization effects in the photon propagator were not considered in [1], and the photon dispersion was taken in a phenomenolical way, by inserting an ad hoc thermal mass into the vacuum photon propagator. ∗ e-mail: [email protected] † e-mail: [email protected]