Research

Supernova Neutrino

Massive stars, which are more than 8 M_sun, can lead to a core collapse at the final stage of the stellar evolution, and then cause core-collapse supernovae. Neutrinos corresponding to 99% of the total kinetic energy of stars are emitted from the central core. They not only escape from the center, but support the explosion mechanism via neutrino heating, where neutrinos play important roles in energy transfer from the hotter central core to the cooler outer region. 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 the original states created at the center of supernovae and we can not grasp the inner information from the observation data at neutrino detectors. To understand the mechanism of neutrino oscillation completely is inevitable to order to extract the exact information from data.

Collective Neutrino Oscillation

Neutrino oscillations have three types in supernovae: vacuum, matter, and collective oscillation. Collective oscillation is a peculiar phenomenon to events such as supernovae and neutron star mergers with dense neutrino fluxes. The dense neutrino gases cause their self-interactions 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. The neutrino self-interactions are much more complicated compared than the other oscillation types 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, neutrino flavor conversion is one of large uncertainties in the theoretical modeling of core-collapse supernovae, and 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!!