Research

Supernova Neutrino

Massive stars, which are more than 8 M_sun, can lead to core collapse at the final stage of the stellar evolution and then cause supernovae. Neutrinos corresponding to 99% of total kinetic energy of stars are emitted from core-collapse supernovae. They not only escape from the center, but support the explosion mechanism via neutrino heating and cause nucleosynthesis via neutrino reaction. Neutrinos are important observational ways as well as gravitational waves because phenomena inside supernovae can not be immediately observed by electro-magnetic counterparts. For example, we can obtain the inner information of supernovae via detected neutrino energy spectra because they depend on the temperature and density of emission source.

On the other hand, propagating neutrinos undergo neutrino oscillation and the flavor states are mixed. Observed neutrinos on the Earth are different from original states at the center of supernovae and we can not grasp the inner information from observation data at neutrino experiments. To understand the mechanism of neutrino oscillation completely is inevitable to order to extract information from data.

Collective Neutrino Oscillation

Neutrino oscillations have three types in supernovae: vacuum oscillation, matter oscillation, and collective oscillation. Collective oscillation is a peculiar phenomenon to events such as supernovae and neutron star mergers with dense neutrino gases. The dense neutrino gases cause the self-interaction and can lead to the flavor conversions. The occurrence region is mostly from 10km to 1000km inside supernovae, and the computational cost is much higher due to the oscillation wavelength on the order of cm. Therefore, the neutrino-neutrino interactions are more complicated compared than other two oscillations and we need to perform large-scale numerical simulations using computer clusters to grasp the behaviors correctly. Recently, the possibility has been reported that collective neutrino oscillation can be triggered inside the shock wave and it may enhance or weaken the neutrino-heating process. Therefore, it is critical to capture when, where, and how many neutrino flavors are present in core-collapse supernovae. Many papers have reported the possibility that collective oscillation can have large influence on supernova dynamics and this research field is a hot topic!!