The electric and magnetic form factors of the proton

TL;DR

This paper presents a comprehensive analysis of proton electric and magnetic form factors derived from electron scattering data, revealing deviations from classical models and providing refined measurements of proton radii and two-photon exchange effects.

Contribution

It offers a new combined analysis of all available electron scattering data, improving the precision of proton form factors and radii, and introduces an empirical TPE correction.

Findings

Form factors deviate from the classical dipole model.

New measurements of proton electric and magnetic radii.

Empirical two-photon exchange correction derived.

Abstract

The paper describes a precise measurement of electron scattering off the proton at momentum transfers of $0.003 \lesssim Q^2 \lesssim 1$\ GeV$^2$. The average point-to-point error of the cross sections in this experiment is $\sim$ 0.37%. These data are used for a coherent new analysis together with all world data of unpolarized and polarized electron scattering from the very smallest to the highest momentum transfers so far measured. The extracted electric and magnetic form factors provide new insight into their exact shape, deviating from the classical dipole form, and of structure on top of this gross shape. The data reaching very low $Q^2$ values are used for a new determination of the electric and magnetic radii. An empirical determination of the Two-Photon-Exchange (TPE) correction is presented. The implications of this correction on the radii and the question of a directly visible signal of the pion cloud are addressed.