Appearance of neutronization peak and decaying supernova neutrinos

Nonradiative neutrino decay, which is not satisfactorily constrained, possibly and significantly changes the detected neutrino signal from galactic supernova explosions. We focus on the appearance of a sharp peak due to a neutronization burst in the time profile; this phenomenon would occur if the original nu(e), produced at the neutrinosphere and becoming nu(2) or nu(3) at the stellar surface, decays into a lighter antineutrino state such as (nu) over bar (1) or (nu) over bar (2) coupled to (nu) over bar (e). This is due to the fact that the signature of the neutronization burst is common to all numerical simulations, contrary to the spectral energy distribution of each flavor neutrino and antineutrino, which is still under intense debate. Therefore, the appearance of a neutronization peak in the (nu) over bar (e) signal, if it were detected, would clearly indicate the nonstandard properties of neutrinos; the nonradiative neutrino decay would be one of the possible candidates. Using a newly developed formulation that includes flavor conversions inside the supernova envelope and neutrino decay during propagation in a vacuum, we calculate the expected neutrino signal at the detectors; the lifetimes of three modes tau(12), tau(13), and tau(23) are taken to be free parameters. We further introduce simple quantities, which represent a peak sharpness of the time profile and spectral hardness, and investigate the parameter dependence of these quantities. As the result, it is found that they are quite dependent on the relevant parameters, but it would be quite difficult to distinguish models using the signal obtained by the Super-Kamiokande detector; the future megaton-class detectors would have sufficient sensitivity. We also compare the neutrino decay model with another mechanism-i.e., resonant spin-flavor conversion-which also may give the appearance of a neutronization peak, and conclude that these two independent mechanisms give a very different signal and one can be distinguished from the other.