Research

Research Interests

I am interested in understanding how microscopic physics is connected to macroscopic physical phenomena, especially in astrophysical systems. Currently, I focus on neutrino flavor oscillations, a quantum mechanical effect that can arise in extreme astrophysical environments. I study how these oscillations appear in explosive phenomena such as core-collapse supernovae and binary neutron star mergers, and how they influence the physics occurring there. To explore these questions, I use a combination of large-scale numerical simulations on high-performance computers and theoretical approaches such as linear stability analysis. I develop my own codes when necessary to carry out these studies.

Supernova Neutrino

Massive stars with more than 8 M_sun undergo gravitational collapse at the end of their evolution, when their central core can no longer support itself against its own gravity. This collapse leads to a core-collapse supernova explosion. During this process, an enormous number of neutrinos are produced and emitted from the central region, carrying away about 99% of the total released energy. Because neutrinos are elementary particles that interact only through the weak interaction, they are usually very weakly coupled to matter, and most of them can pass through it almost freely. However, in the extremely dense environment near the center of the star, even neutrinos cannot escape easily. As a result, neutrinos play the role of transporting energy from the hotter central region to the cooler outer layers. This gives rise to the so-called neutrino heating mechanism, in which neutrino energy deposition helps revive the shock wave and drive it outward toward the stellar surface.

At the same time, neutrinos exhibit a phenomenon known as neutrino oscillation, in which their flavors mix as they propagate. In other words, the flavors of neutrinos can change while they are transporting energy outward from the central region. This changes how neutrinos interact with matter, allowing a microscopic quantum effect to influence macroscopic astrophysical phenomena.

Collective Neutrino Oscillation

In explosive astrophysical environments such as core-collapse supernovae and binary neutron star mergers, neutrino oscillations can be classified into three types: vacuum oscillation, matter oscillation, and collective oscillation. Among these, collective neutrino oscillation is a flavor conversion characterizing in astrophysical events where a large number of neutrinos are emitted. It is caused by self-interactions among the neutrinos themselves.

In recent years, it has been suggested that such nonlinear neutrino flavor conversion may occur inside the shock wave. If this is the case, it could affect the efficiency of neutrino heating. In binary neutron star mergers, neutrino flavor conversion may also influence the electron fraction of the matter ejected after the merger, and therefore have a significant impact on the production of heavy elements through the r-process. For these reasons, collective neutrino oscillations are a major source of uncertainty in theoretical modeling of core-collapse supernovae and binary neutron star mergers. Determining when, where, and to what extent each neutrino flavor is present is therefore an important problem in astrophysics. In this way, the possible impacts of collective neutrino oscillations on explosive astrophysical phenomena have been discussed in many studies, making this a very active area of research today.