Angular distributions in electroweak pion production off nucleons: odd parity hadron terms, strong relative phases and model dependence

TL;DR

This paper thoroughly analyzes angular distributions in electroweak pion production off nucleons, highlighting parity violation effects, comparing models, and emphasizing the need for improved simulation assumptions in neutrino experiments.

Contribution

It provides a detailed comparison of microscopic models for pion production, investigates parity violation in angular distributions, and stresses the importance of accurate modeling for neutrino physics.

Findings

Parity violation arises from interference of non-real hadronic current contributions.

Significant anisotropies are observed in pion angular distributions.

Current Monte Carlo generators should incorporate microscopic model insights.

Abstract

The study of pion production in nuclei is important for signal and background determinations in current and future neutrino oscillation experiments. The first step, however, is to understand the pion production reactions at the free nucleon level. We present an exhaustive study of the charged-current and neutral-current neutrino and antineutrino pion production off nucleons, paying a special attention to the angular distributions of the outgoing pion. We show, using general arguments, that parity violation and time-reversal odd correlations in the weak differential cross sections are generated from the interference between different contributions to the hadronic current that are not relatively real. Next, we present a detailed comparison of three, state of the art, microscopic models for electroweak pion production off nucleons, and we also confront their predictions with polarized electron data, as a test of the vector content of these models. We also illustrate the importance of carrying out a comprehensive test at the level of outgoing pion angular distributions, going beyond comparisons done for partially integrated cross sections, where model differences cancel to a certain extent. Finally, we observe that all charged and neutral current distributions show sizable anisotropies, and identify channels for which parity violating effects are clearly visible. Based on the above results, we conclude that the use of isotropic distributions for the pions in the center of mass of the final pion-nucleon system, as assumed by some of the Monte Carlo event generators, needs to be improved by incorporating the findings of microscopic calculations.