Hammer events, neutrino energies, and nucleon-nucleon correlations

TL;DR

This paper investigates neutrino-argon scattering events with back-to-back protons, called hammer events, to understand their relation to nucleon-nucleon correlations and their potential use in neutrino energy reconstruction.

Contribution

The study models hammer events using simple pion production and absorption models, showing their insensitivity to SRC and proposing their use in neutrino oscillation experiments.

Findings

Hammer events can be described by pion production and reabsorption models.

Neutrino energy can be reconstructed from outgoing leptons in these events.

Hammer events are not sensitive to short-range nucleon-nucleon correlations.

Abstract

Neutrino oscillation measurements depend on a difference between the rate of neutrino-nucleus interactions at different neutrino energies or different distances from the source. Knowledge of the neutrino energy spectrum and neutrino-detector interactions are crucial for these experiments. Short range nucleon-nucleon correlations in nuclei (SRC) affect properties of nuclei. The ArgoNeut liquid Argon Time Projection Chamber (lArTPC) observed neutrino-argon scattering events with two protons back-to-back in the final state ("hammer" events) which they associated with SRC pairs. The MicroBoone lArTPC will measure far more of these events. We simulate hammer events using two simple models. We use the well-known electron-nucleon cross section to calculate e-argon interactions where the e- scatters from a proton, ejecting a pi+, and the pi+ is then absorbed on a moving deuteron-like $np$ pair. We also use a model where the electron excites a nucleon to a Delta, which then deexcites by interacting with a second nucleon. The pion production model results in two protons very similar to those of the hammer events. These distributions are insensitive to the momentum of the $np$ pair that absorbed the $\pi$. The incident neutrino energy can be reconstructed from just the outgoing lepton. The Delta process results in two protons that are less similar to the observed events. ArgoNeut hammer events can be described by a simple pion production and reabsorption model. These hammer events 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.