The Measurement of Neutral-Current Quasi-Elastic Cross Section at Super-Kamiokande Using the T2K Neutrino Beam

We report the measurement of the neutrino-oxygen neutral-current quasi-elastic (NCQE) cross section by observing nuclear de-excitation gamma rays at Super-Kamiokande with the T2K neutrino beam. The de-excitation gamma ray events induced by NCQE reactions are selected from the T2K RUN1-3 data corresponding to 3.01×1020 protons on target.The selections use the event information to remove the background events which are from radioactive impurities at the detector wall, and the event selection achieves 69% of NCQE signal purity. In the 4∼30 MeV reconstructed energy region forty-three events are observed, compared with the expected number of 51.0. The neutrinooxygen NCQE cross section is measured as 1.55 × 10−38cm2 with a 68% confidence interval of (1.08, 2.34) × 10−38cm2, and the 68% confidence interval is consistent with the theoretical NCQE cross section of 2.01 × 10−38cm2. The Cherenkov angle distribution of the candidate events shows a discrepancy between the data and the MC expectation. The discrepancy might be due to the secondary gamma-ray production which can be reduced by possible improvements in near future.