Light-Front Transverse Nucleon Charge and Magnetisation Densities

TL;DR

This paper calculates light-front transverse charge and magnetisation densities of nucleons using two models, revealing detailed flavor separation and polarization effects consistent with experimental data.

Contribution

It provides a comparative analysis of three-body and quark-diquark models to predict nucleon transverse densities, highlighting flavor-specific and polarization-dependent features.

Findings

Valence u- and d-quark Dirac radii are nearly identical.

d-quark Pauli radius is about 10% larger than u-quark.

Transverse charge densities shift in polarized nucleons, breaking rotational symmetry.

Abstract

Nucleon elastic electromagnetic form factors obtained using both the three-body and quark + fully-interacting-diquark pictures of nucleon structure are employed to calculate an array of light-front transverse densities for the proton and neutron and their dressed valence-quark constituents, viz. flavour separations of the proton and neutron results. These two complementary descriptions of nucleon structure deliver mutually compatible predictions, which match expectations based on modern parametrisations of available data, where such are available. Amongst other things, it is found that transverse-plane valence $u$- and $d$-quark Dirac radii are practically indistinguishable; but regarding kindred Pauli radii, the $d$ quark value is roughly 10% greater than that of the $u$-quark. Moreover, magnetically, the valence $d$ quark is far more active than the valence $u$ quark, probably because it has much greater orbital angular momentum. Both pictures of nucleon structure agree in predicting that, in a polarised nucleon, the transverse-plane charge densities are no longer rotationally invariant. Instead, for a $+\hat x$ polarised nucleon, positive charge is displaced in the $+\hat y$ direction, with the opposite effect for negative charge.