Speaker
Takashi Yoshida
(Department of Astronomy, University of Tokyo)
Description
If neutrinos are Majorana particles and have finite transition magnetic moment with about 10−12μB, where μB is Bohr magnetons, a spin precession between a left-handed neutrino and a right-handed antineutrino with different flavor occurs in strong magnetic field. This is called resonant spin-flavor (RSF) conversion. Supernova (SN) neutrino event is one of the most promising neutrino events to find evidence for the RSF conversion. The magnetic field of Fe core of a presupernova star is evaluated to become about 1010 gauss. Observations of pulsars indicated that neutron stars have magnetic field of about 1012 gauss. Such a strong magnetic field in the innermost region of a SN ejecta would allow the flavor change by the RSF conversion.
Recent studies of SN explosions indicated that a SN forms a hot-bubble region above the neutrino sphere and that electron-mole fraction Ye in this region exceeds 0.5 in several seconds after the core collapse. We investigated the dependence of the RSF conversion and SN neutrino spectra on electron-mole fraction using a SN explosion model of a 15M⊙ star. The RSF conversion of nubar(e) $ nu(mu,tau) occurs at a high RSF resonance in normal mass hierarchy and Ye > 0.5 or inverted mass hierarchy and Ye < 0.5.
In other cases, the RSF conversion of e $ ¯μ, occurs. If SN neutrinos are affected by the RSF conversion and the Ye value in the hot-bubble region changes with time, largetime variation of the SN neutrino spectra would be observed. When the adiabaticity of a high RSF resonance changes by the shock passage, the SN neutrino spectra also change. In the poster presentation, we show the time variation of the event number ratio of low nubar(e) energy to high nubar(e) energy as the SN neutrino spectra.
We discuss the dependence of the time variation of the event number ratio on neutrino oscillation parameters and neutrino magnetic moment.
