Form Factors of the Nucleon Axial Current

TL;DR

This paper uses a symmetry-preserving Poincaré-covariant quark+diquark model to predict the nucleon's axial and pseudoscalar form factors, providing parameter-free results that align with experimental and theoretical expectations.

Contribution

It offers the first parameter-free predictions for nucleon axial and pseudoscalar form factors using a Poincaré-covariant quark+diquark approach, including detailed flavor-separated quark contributions.

Findings

Predicted axial form factor $G_A$ fits a dipole form with $g_A=1.25(3)$ and $M_A=1.23(3) m_N.

Induced pseudoscalar charge $g_p^*=8.80(23)$ and ratio $g_p^*/g_A=7.04(22)$.

Strong diquark correlations suppress the $d$-quark component in the nucleon.

Abstract

A symmetry-preserving Poincar\'e-covariant quark+diquark Faddeev equation treatment of the nucleon is used to deliver parameter-free predictions for the nucleon's axial and induced pseudoscalar form factors, $G_A$ and $G_P$, respectively. The result for $G_A$ can reliably be represented by a dipole form factor characterised by an axial charge $g_A=G_A(0)=1.25(3)$ and a mass-scale $M_A = 1.23(3) m_N$, where $m_N$ is the nucleon mass; and regarding $G_P$, the induced pseudoscalar charge $g_p^\ast = 8.80(23)$, the ratio $g_p^\ast/g_A = 7.04(22)$, and the pion pole dominance Ansatz is found to provide a reliable estimate of the directly computed result. The ratio of flavour-separated quark axial charges is also calculated: $g_A^d/g_A^u=-0.16(2)$. This value expresses a marked suppression of the size of the $d$-quark component relative to that found in nonrelativistic quark models and owes to the presence of strong diquark correlations in the nucleon Faddeev wave function -- both scalar and axial-vector, with the scalar diquark being dominant. The predicted form for $G_A$ should provide a sound foundation for analyses of the neutrino-nucleus and antineutrino-nucleus cross-sections that are relevant to modern accelerator neutrino experiments.