Study of Charged-Current neutrino interactions on water with nuclear emulsion in the NINJA experiment

Neutrino-nucleus interaction is one of the major sources of the uncertainty for neutrino oscillation experiments. The NINJA experiment aims to measure neutrino-water interactions containing low-momentum secondary particles using a nuclear emulsion detector. Since the nuclear emulsion has sub-micron position resolution, it allows us to observe the interaction vertices clearly. Short proton tracks down to 200 MeV/c momentum are expected to be observed, which are hardly reconstructed in plastic scintillator based detectors. A series of test experiments has been carried out with prototype detectors at J-PARC. In a test run in 2017-2018, a 3 kg water target detector was exposed to anti-neutrino beam corresponding to 0.7 × 1021 POT (protons on target). We have successfully detected low momentum protons above 200 MeV/c threshold from neutrino-water interactions.