“Hammer” events, neutrino energies, and nucleon-nucleon correlations

Abstract

Background: Accelerator-based neutrino oscillation measurements depend on observing a difference between the expected and measured rate of neutrino-nucleus interactions at different neutrino energies or different distances from the neutrino source. Neutrino-nucleus scattering cross sections are complicated and depend on the neutrino beam energy, the neutrino-nucleus interaction, and the structure of the nucleus. Knowledge of the incident neutrino energy spectrum and neutrino-detector interactions are crucial for analyzing neutrino oscillation experiments. The ArgoNeut liquid argon time projection chamber (lArTPC) observed charged-current neutrino-argon scattering events with two protons back-to-back in the final state ("hammer" events) which they associated with short-range correlated (SRC) nucleon-nucleon pairs. The large volume MicroBooNE lArTPC will measure far more of these unique events. Purpose: Determine what we can learn about the incident neutrino energy spectrum and/or the structure of SRC from hammer events that will be measured in MicroBooNE. Methods: We simulate hammer events using two models and the well-known electron-nucleon scattering cross section. In the first model the neutrino (or electron) scatters from a moving proton, ejecting a π+, and the π+ is then absorbed on a moving deuteron-like np pair. In the second model the neutrino (or electron) scatters from a moving nucleon, exciting it to Δ or N*, which then de-excites by interacting with a second nucleon: ΔN -> pp. Results: The pion production and reabsorption process results in two back-to-back protons each with momentum of about 500 MeV/c, very similar to that of the observed ArgoNeut events. These distributions are insensitive to either the relative or center-of-mass momentum of the np pair that absorbed the π. In this model, the incident neutrino energy can be reconstructed relatively accurately using the outgoing lepton. The Δp -> pp process results in two protons that are less similar to the observed events. Conclusions: ArgoNeut hammer events can be described by a simple pion production and reabsorption model. The hammer events that will be measured in MicroBooNE can be used to determine the incident neutrino energy but not to learn about SRC. We suggest that this reaction channel could be used for neutrino oscillation experiments to complement other channels with higher statistics but different systematic uncertainties.