Meson Clouds and Nucleon Electromagnetic Form Factors

TL;DR

This paper clarifies the interpretation of nucleon electromagnetic form factors, revealing that the transverse charge densities derived from form factors show unexpected features like negative neutron charge density at the center and a pion cloud influence.

Contribution

It demonstrates that the two-dimensional Fourier transform of the form factor relates to transverse charge densities, challenging traditional non-relativistic assumptions and providing new insights into nucleon structure.

Findings

Neutron charge density is negative at the center.

Proton's central u quark charge density exceeds that of d quark by about 80%.

Evidence of a pion cloud in nucleon charge distributions.

Abstract

In contrast with common non-relativistic lore, the usual Sachs form factors are not the Fourier transforms of charge or magnetization densities. Instead, the two-dimensional Fourier transform of the electromagnetic $F_1$ form factor is the charge charge density of partons in the transverse plane. An analysis of the available data for neutron form factors leads to the result that the neutron charge density is negative at the center, and that the square of the transverse charge radius is positive. This contrasts with many expectations. Additionally, the use of measured proton form factors leads to the result that the proton's central $u$ quark charge density is larger than that of the $d$ quark by about 80%. The proton (neutron) charge density has a long range positively (negatively) charged component indicative of a pion cloud.