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Physical Review D

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Weak pion production off the nucleon

E. Hernández¹, J. Nieves², and M. Valverde²

¹Grupo de Física Nuclear, Departamento de Física Fundamental e IUFFyM, Universidad de Salamanca, E-37008 Salamanca, Spain.
²Departamento de Física Atómica, Molecular y Nuclear, Universidad de Granada, E-18071 Granada, Spain

Phys. Rev. D 76, 033005 – Published 23 August, 2007

DOI: https://doi.org/10.1103/PhysRevD.76.033005

Abstract

We develop a model for the weak pion production off the nucleon, which besides the delta pole mechanism [weak excitation of the $Δ (1232)$ resonance and its subsequent decay into $N π$ ], includes also some background terms required by chiral symmetry. We refit the $C_{5}^{A} (q^{2})$ form factor to the flux-averaged $ν_{μ} p \to μ^{-} p π^{+}$ 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.

Article Text

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