Nucleon relativistic polarization and magnetization distributions

TL;DR

This paper investigates relativistic polarization and magnetization distributions inside a spin-1/2 particle using a quantum phase-space approach, comparing different decompositions and analyzing their light-front counterparts with nucleon data.

Contribution

It introduces a Sachs decomposition-based framework for polarization-magnetization tensors, providing a more natural picture and clarifying the relation to form factors in a relativistic context.

Findings

Relativistic distributions differ from non-relativistic ones.

Light-front magnetization is linked to Sachs magnetic form factor.

Application to nucleon data illustrates the framework.

Abstract

As a follow up of our work on the electromagnetic four-current, we study for the first time the relativistic polarization and magnetization spatial distributions inside a spin-$\frac{1}{2}$ target within the quantum phase-space approach. While the polarization-magnetization tensor is usually defined in terms of the Gordon decomposition of the electromagnetic four-current, a Breit frame analysis reveals that a physically simpler and more natural picture of the system arises when the polarization-magnetization tensor is instead defined in terms of a Sachs decomposition. Relativistic polarization and magnetization distributions for a moving target are compared with their light-front counterparts. In particular, we show that the genuine light-front magnetization distributions are defined in terms of Fourier transforms of the Sachs magnetic form factor, rather than in terms of the Pauli form factor as suggested earlier in the literature. We finally illustrate our results in the case of a nucleon using the electromagnetic form factors extracted from experimental data.