Multiplicity of neutrons associated neutrino interaction

When neutrinos interact with nuclei, they sometimes emit neutrons in addition to charged particles. Detection of these neutrons is expected to be useful for improving the sensitivity of T2K neutrino oscillation analysis and for suppressing background events in the search for supernova relic neutrinos.
Understanding the multiplicity of neutrons associated with neutrino interactions is an important topic for utilizing neutrons in neutrino physics. Therefore, we measured the mean neutron multiplicity using neutron capture signals by hydrogen nucleus, and found a large discrepancy between the data and models.
The key to understanding these discrepancies is the large uncertainty in the number of neutrons produced as predicted by each interaction model. The actual neutrino interaction involves the production and re-scattering of nucleons and pions inside and outside the nucleus to produce additional neutrons, but the behavior of these interactions are different among the models due to a lack of understanding of nuclear physics. 
Approximately 13 tons of gadolinium sulfate octahydrate has been dissolved in SK since 2020. Gadolinium has the largest neutron capture cross section of any element in nature and is about 100,000 times more capable of capturing neutrons than hydrogen nucleus. This allows the current SK environment to measure neutron counts with higher neutron detection efficiency than previous studies in the pure water phase. My analysis will expand our understanding of neutron production by measuring the neutron number associated with T2K neutrinos using gadolinium.