Neutral current (NC) interactions of atmospheric neutrinos on oxygen form one of the major backgrounds in the search for supernova relic neutrinos with water-based Cherenkov detectors. The NC channel is dominated by neutrino quasielastic (NCQE) scattering off nucleons inside ${}^{16}\mathrm{O}$ nuclei. In this paper we report the first measurement of NCQE cross section using atmospheric neutrinos at Super-Kamiokande (SK). The measurement used 2,778 live days of SK-IV data with a fiducial volume of 22.5 kiloton water. Within the visible energy window of 7.5–29.5 MeV, we observed 117 events compared to the expected 71.9 NCQE signal and 53.1 background events. Weighted by the atmospheric neutrino spectrum from 160 MeV to 10 GeV, the flux averaged NCQE cross section is measured to be $(1.01\pm 0.17(\mathrm{stat}.{)}_{-0.30}^{+0.78}(\mathrm{sys}.))\times {10}^{-38}{\mathrm{cm}}^2$.

• Received 16 January 2019

DOI:https://doi.org/10.1103/PhysRevD.99.032005

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Published by the American Physical Society