RESEARCH
Neutrinos, cosmic-ray muons, low-background physics, and astrophysical neutrinos
I study solar neutrinos observed with Super-Kamiokande to investigate neutrino oscillations and the physics of the solar interior. A particular focus is a search for periodic neutrino-flux variations associated with solar g-mode oscillations.
Related publications
I investigate cosmic-ray muons in underground detectors, including their charge ratio, polarization, and intensity variations. I also study unstable isotopes produced by negative-muon capture on oxygen, relevant to background understanding in neutrino experiments.
Related publications
Rare-event searches require precise control of environmental radioactivity. I develop sensitive radon detectors, techniques for measuring radon in purified water, and methods for reducing radon using adsorption materials.
I search for neutrinos associated with supernovae, gravitational-wave events, solar flares, and other energetic astrophysical phenomena. Using timing and energy information, I study the connection between distant cosmic events and neutrino emission.
The following notes provide additional background and context for each research topic.
Super-Kamiokande observes solar electron neutrinos through neutrino-electron scattering and measures their energy spectrum and time variation with high precision. Improvements in background reduction and event selection at low energies make it possible to study spectral features, the day-night effect, and other information related to neutrino oscillations.
I also use the long-term solar-neutrino data set to investigate possible connections between neutrino flux variations and oscillations in the solar interior. In particular, I study whether g-mode oscillations expected in the deep solar core could modify local temperature or density and produce periodic signatures through changes in neutrino production or matter-enhanced neutrino oscillations. Because neutrinos emerge directly from the solar core, they provide an independent probe of regions that are difficult to access with electromagnetic observations.
Related papers: Phys. Rev. D 94, 052010 (2016), Phys. Rev. D 109, 092001 (2024), arXiv:2604.06535
I search for neutrinos associated with transient astrophysical phenomena such as supernovae, gravitational-wave events, and solar flares using Super-Kamiokande. Since neutrinos interact only weakly with matter, they can escape from dense regions of astrophysical sources and carry information that is complementary to photons and gravitational waves. They are therefore an important messenger in multi-messenger astronomy.
For gravitational-wave events, I have studied neutrino candidates in time windows around GW150914/GW151226 and GW170817. For solar flares, I have worked on methods for defining physically motivated neutrino search windows and on searches using data spanning multiple solar cycles.
Related papers: Astrophys. J. Lett. 830, L11 (2016), Astrophys. J. Lett. 857, L4 (2018), Solar Physics 295, 133 (2020), arXiv:2210.12948
When primary cosmic rays interact with nuclei in the atmosphere, secondary particles such as pions and kaons are produced, and their decays generate large numbers of muons. Sufficiently energetic muons reach Super-Kamiokande even though the detector is located underground. I use these events to study quantities such as the muon charge ratio and polarization, which provide information on parent-particle production and the development of atmospheric cosmic-ray showers.
Negative muons that stop in water can also be captured by oxygen nuclei and produce unstable isotopes. By measuring subsequent decays of isotopes such as 16N, 15C, 12B, and 13B, I study branching ratios for nuclear muon-capture reactions. These isotopes can also contribute to backgrounds in low-energy neutrino searches, linking cosmic-ray physics directly to neutrino-background studies.
Related papers: Phys. Rev. D 110, 082008 (2024), Phys. Rev. C 112, 064614 (2025)
Measurements of low-energy rare events, including solar neutrinos and dark-matter interactions, require environmental radioactivity to be suppressed and quantified as precisely as possible. Beta decays of 214Bi, a daughter of 222Rn, are an important background in this energy region. My work has included the development of a high-sensitivity 80-L radon detector, measurements of radon in the Super-Kamiokande buffer gas, and systems for measuring radon dissolved in purified water.
I also work on techniques to reduce radon itself. These studies include measurements of radon adsorption with activated carbon fibers in xenon and CF4, as well as improved radon-detector sensitivity using a larger PIN photodiode. The goal is to address both measurement and removal of radioactive backgrounds, with applications not only to neutrino experiments but also to other rare-event searches such as dark-matter experiments.
Related papers: PTEP 2015, 033H01, NIM A 867, 108-114 (2017), NIM A 977, 164297 (2020), PTEP 2020, 113H01, arXiv:2112.06614, PTEP 2022, 023H01
Fluorine (19F) is widely considered as a target nucleus for dark-matter searches because of its nuclear spin, while it also offers potentially useful nuclear reactions for MeV-scale electron neutrinos. I am investigating the basic feasibility of a new low-energy neutrino detector using fluorine-containing media, including possible applications to solar-neutrino detection. In addition to the prompt electron signal, nuclear states produced after neutrino absorption may provide prompt-delayed signatures that could help discriminate signal from background.
As a possible detector medium, I study CF4 and detector-relevant properties such as its scintillation light yield at low temperature and radon-removal performance in CF4 gas. The aim is to evaluate not only the neutrino interaction itself but also the light-production and low-background conditions required for a practical detector concept.
Related papers: JINST 16, P12033 (2021), PTEP 2022, 023H01
Precision neutrino and rare-event measurements require a consistent understanding of detector response, calibration, simulation, and environmental radioactivity in addition to the physics analysis itself. I therefore also work on detector-calibration methods, simulation-based response studies, and organization of radioactivity information that supports background modeling.
These activities are not isolated topics; they form common infrastructure across solar-neutrino, cosmic-ray-muon, and low-background analyses. Better control of detector systematics and backgrounds directly improves the sensitivity and reliability of the final physics measurements. Related research outputs are summarized on the Works page.