# Weak Pion Production off the Nucleon

## TL;DR

This paper presents a comprehensive model for weak pion production off nucleons, incorporating background terms from chiral symmetry, and demonstrates improved data agreement and novel parity-violating effects due to interference effects.

## Contribution

It introduces a model including background terms alongside the Delta pole mechanism and re-fits the axial form factor to experimental data, revealing a smaller Delta contribution than previously assumed.

## Key findings

- Background terms significantly affect cross sections and angular distributions.
- Interference between mechanisms produces parity-violating effects linked to T-odd correlations.
- Model achieves better data description than Delta-only approaches.

## Abstract

We develop a model for the weak pion production off the nucleon, which besides the Delta pole mechanism (weak excitation of the $\Delta(1232)$ resonance and its subsequent decay into $N\pi$), includes also some background terms required by chiral symmetry. We re-fit the $C_5^A(q^2)$ form factor to the flux averaged $\nu_\mu p \to \mu^-p\pi^+$ ANL $q^2-$differential cross section data, finding a substantially smaller contribution of the Delta pole mechanism than traditionally assumed in the literature. Within this scheme, we calculate several differential and integrated cross sections, including pion angular distributions, induced by neutrinos and antineutrinos and driven both by charged and neutral currents. In all cases we find that the background terms produce quite significant effects and that they lead to an overall improved description of the data, as compared to the case where only the Delta pole mechanism is considered. We also show that the interference between the Delta pole and the background terms produces parity-violating contributions to the pion angular differential cross section, which are intimately linked to $T-$odd correlations in the contraction between the leptonic and hadronic tensors. However, these latter correlations do not imply a genuine violation of time reversal invariance because of the existence of strong final state interaction effects.