First Measurement of Missing Energy due to Nuclear Effects in Monoenergetic Neutrino Charged-Current Interactions.

We present the first measurement of the missing energy due to nuclear effects in monoenergetic, muon neutrino charged-current interactions on carbon, originating from K^{+}→μ^{+}ν_{μ} decay at rest (E_{ν_{μ}}=235.5  MeV), performed with the J-PARC Sterile Neutrino Search at the J-PARC Spallation Neutron Source liquid scintillator based experiment. Toward characterizing the neutrino interaction, ostensibly ν_{μ}n→μ^{-}p or ν_{μ}^{12}C→μ^{-}^{12}N, we define the missing energy as the energy transferred to the nucleus (ω) minus the kinetic energy of the outgoing proton(s), E_{m}≡ω-∑T_{p}, and relate this to visible energy in the detector, E_{m}=E_{ν_{μ}}(235.5  MeV)-m_{μ}(105.7  MeV)+[m_{n}-m_{p}(1.3  MeV)]-E_{vis}. The missing energy, which is naively expected to be zero in the absence of nuclear effects (e.g., nucleon separation energy, Fermi momenta, and final-state interactions), is uniquely sensitive to many aspects of the interaction, and has previously been inaccessible with neutrinos. The shape-only, differential cross section measurement reported, based on a (77±3)% pure double-coincidence kaon decay-at-rest signal (621 total events), provides detailed insight into neutrino-nucleus interactions, allowing even the nuclear orbital shell of the struck nucleon to be inferred. The measurement provides an important benchmark for models and event generators at hundreds of MeV neutrino energies, characterized by the difficult-to-model transition region between neutrino-nucleus and neutrino-nucleon scattering, and relevant for applications in nuclear physics, neutrino oscillation measurements, and Type-II supernova studies.