Measurement of the branching ratio of ¹⁶N, ¹⁵C, ¹²B, and ¹³B isotopes through the nuclear muon capture reaction in the Super-Kamiokande detector

The Super-Kamiokande detector has measured solar neutrinos for more than 25 years. The sensitivity for solar neutrino measurement is limited by the uncertainties of energy scale and background modeling. Decays of unstable isotopes with relatively long half-lives through nuclear muon capture, such as N16, C15, B12, and B13, are detected as background events for solar neutrino observations. In this study, we developed a method to form a pair of stopping muon and decay candidate events and evaluated the production rates of such unstable isotopes. We then measured their branching ratios considering both their production rates and the estimated number of nuclear muon capture processes as Br(N16)=(9.0±0.1)%, Br(C15)=(0.6±0.1)%, Br(B12)=(0.98±0.18)%, Br(B13)=(0.14±0.12)%, respectively. The result for N16 has world-leading precision at present and the results for C15, B12, and B13 are the first branching ratio measurements for those isotopes.