Speaker
Description
In the diffuse supernova neutrino background (DSNB) search with massive water Cherenkov detectors such as Super-Kamiokande, Atmospheric neutrino–oxygen neutral-current quasi-elastic (NCQE) events are one of the main backgrounds. This background events are estimated by following models: the HKKM as the atmospheric neutrino flux, NEUT for the neutrino interaction, and the model of Ankowski et al. as the nuclear de-excitation gamma rays from the NCQE interaction below the energy of 16 MeV. As a result of this uncertainty, a large systematic error of 68-82 % is assigned to the estimated number of NCQE events in the present DSNB analysis. To reduce this systematic uncertainty, it is necessary to measure NCQE events experimentally using a new approach. Therefore, we plan a new experiment, BISCOTTEE (BISmuth-germanate Crystal base neutrinO (neutrOn)-nucleus scaTTEring Experiment), to measure NCQE events in short baseline. To carry out the BISCOTTEE, we have conducted beam tests as follows. First, the main background of NCQE is expected to be accidental events of accelerated neutrino-induced neutrons, and we have measured the neutron background at the J-PARC (T101 experiment). In this test, finite neutron events have been observed using a liquid scintillator. Then, the flux of the neutron background was preliminary determined to be 1.45×10^(-12) cm^2 s^(-1) POT^(-1) at 29.725×10^19 POT data taking. In addition, we have an opportunity to measure neutrino-BGO CC events at the same setup. We observed one event of the CC candidate, and it is consistent with the expectation of 1.4±0.3 events using 192 g BGO crystal. Second, it is important to observe recoil nuclei in the BISCOTTEE, and we have measured the quenching factor of BGO scintillation light yield for oxygen nuclear recoil. In the beam test, we use mono-energy (61.9 MeV) neutrons at RCNP facility, Japan. In setup, a BGO detector as active target was placed along the beam axis, and a liquid scintillation detector was positioned at the neutron scattering angle. To select the neutron-nuclei elastic scattering events in the BGO, we have analyzed using time-of-flight and pulse-shape discrimination. The quenching factors were determined by taking the ratio of the visible energy (electron recoil equivalent) to the energy of recoil nuclei for varied scattering angles. In this presentation, we will report on the details of setup and analysis for the test experiments. Then, we will show the prospects of BISCOTTEE.