Assessing the theory-data tension in neutrino-induced charged pion production: the effect of final-state nucleon distortion

TL;DR

This study investigates how final-state nucleon distortion affects neutrino-induced pion production cross sections, revealing a modest reduction but not resolving discrepancies with experimental data, highlighting the need for improved axial coupling measurements.

Contribution

It introduces a microscopic quantum mechanical approach using RDWIA to assess nucleon distortion effects in neutrino pion production, providing insights into existing experimental tensions.

Findings

Nucleon distortion reduces cross sections by up to 10%.

Shape of flux-averaged cross sections remains largely unchanged.

Discrepancies at low-Q^2 are not explained by nucleon distortion alone.

Abstract

Pion production on nuclei constitutes a significant part of the total cross section in experiments involving few-GeV neutrinos. Combined analyses of data on deuterium and heavier nuclei points to tensions between the bubble chamber data and the data of the MINER$\nu$A experiment, which are often ascribed to unspecified nuclear effects. To understand the origin of these tensions, a microscopic quantum mechanical framework is needed to compute nuclear matrix elements. We use the local approximation to the relativistic distorted wave impulse approximation (RDWIA) to assess the role of final-state nucleon distortion. To perform this comparison under conditions relevant to neutrino experiments, we compute cross sections for the MINER$\nu$A and T2K charged pion production datasets. The inclusion of nucleon distortion leads to a reduction of the cross section up to 10\%, but to no significant change in shape of the flux-averaged cross sections. Results with and without distortion compare favorably to experimental data, with the exception of the low-$Q^2$ MINER$\nu$A $\pi^+$ data. We point out that hydrogen target data from BEBC is also overpredicted at low-$Q^2$, and that the discrepancy is similar in shape and magnitude to what is found in comparison to MINER$\nu$A data. Including nucleon distortion alone cannot explain the overprediction of low-$Q^2$ cross sections measured by MINER$\nu$A. The similar overprediction of BEBC data on hydrogen means that it is impossible to ascribe this discrepancy solely to a nuclear effect. Axial couplings and their $Q^2$ dependence should ideally be derived from more precise data on hydrogen and deuterium.