Neutrino induced coherent pion production

TL;DR

This paper evaluates different models of neutrino-induced coherent pion production, examining their low momentum transfer behavior, and highlights limitations of common models like Rein-Sehgal compared to microscopic calculations.

Contribution

It provides a critical comparison of parameterizations of the $C_5^A(q^2)$ form factor and assesses the validity of the Rein-Sehgal model against microscopic calculations.

Findings

Moderate effects are difficult to observe with current experimental accuracy.

Rein-Sehgal model has significant weaknesses at low energies.

Models based on PCAC are unreliable for certain differential cross sections.

Abstract

We discuss different parameterizations of the $C_5^A(q^2)$ $N\Delta$ axial form factor, fitted to the old Argonne bubble chamber data for pion production by neutrinos, and we use coherent pion production to test their low $q^2$ behavior. We find moderate effects that will be difficult to observe with the accuracy of present experiments. We also discuss the use of the Rein-Sehgal model for low energy coherent pion production. By comparison to a microscopic calculation, we show the weaknesses of some of the approximations in that model that lead to very large cross sections as well as to the wrong shapes for differential ones. Finally we show that models based on the partial conservation of the axial current hypothesis are not fully reliable for differential cross sections that depend on the angle formed by the pion and the incident neutrino.