Speaker
Description
The NINJA experiment aims to study neutrino–nucleus interactions using nuclear emulsion detectors for high-resolution tracking of final-state particles. A key focus is the detailed study of neutrino interactions on water, providing input for water-based neutrino oscillation experiments such as T2K and Hyper-Kamiokande. As a first step toward the full-data analysis, we study reconstructed muon, proton, and charged-pion kinematics in 82 selected events on water. This corresponds to roughly 2% of the expected full NINJA $\nu_\mu$-beam data set, collected at 1.5° off-axis with an energy spectrum peaked at 0.7 GeV. The distributions generally agree with Monte Carlo predictions based on the interaction model used in the T2K oscillation analysis, while the proton angular distribution shows tension ($p=0.011$), with fewer forward-going protons than predicted. This marks the first study of proton kinematics in $\nu_\mu$ charged-current interactions on water with sensitivity down to about 200 MeV/$c$. The full-data analysis will provide substantially greater sensitivity to potential modeling issues.