In 1987, Nakahata observed neutrinos from supernova 1987A. It is the first observation of neutrino signals outside the solar system. In 1988, Nakahata observed solar neutrino signals in the Kamiokande detector. The observed flux of Kamiokande was almost half of the expectation from the standard solar model and it confirmed the solar neutrino problem. It is the first observation of solar neutrinos using real time detector. Super-Kamiokande(SK) was started in 1996. Nakahata was responsible for the energy calibration of the SK detector. Nakahata setup a linear accelerator (LINAC) for the precise calibration of the detector and it enabled very accurate measurement of the solar neutrino flux. In 2001, the flux measured by SK was compared with the CC measurement of SNO and the oscillation of solar neutrinos was established. In 2002, Nakahata analyzed energy spectrum of B8 solar neutrinos and the time variation of the flux using SK detector. Combining these results with other solar neutrino experiments, it was found that the Large Mixing Angle is the solution of solar neutrino oscillations.
Current reseach objectives
- Search for energy spectrum distortion in B8 solar neutrinos It was found that the Large Mixing Angle (LMA) is the solution of solar neutrino oscillations. The LMA solution predicts that the energy spectrum of B8 solar neutrinos is distorted (oscillation probability decreases as energy increases). Nakahata is interested in measuring the spectrum distortion by the Super-Kamiokande detector.
- Search for neutrino burst If a supernova happens in our galaxy, about 10,000 events must be observed at Super-Kamiokande. It will reveal detailed machanism of supernova explosion. Nakahata is waiting for the next supernova.
- Search for supernova relic neutrinos Neutrinos from all past core-collapse supernovae (called "supernova relic neutrinos"(SRN)) must be traveling in the universe. The flux upper limit of SRN obtained by Super-Kamiokande is very close to theoretical redictions. Nakahata is improving the detection method for the real detection of SRN signals.
- Search for cold dark matter The most possible candidate of dark matter is Weakly Interaction Massive Parciles (WIMPs) such as eutralinos in Supersymmetric model. Nakahata is working for the construction of a liquid xenon detector for the direct detection of WIMPs.