Weak radius of the proton

TL;DR

This paper reports that the proton's weak radius is significantly larger than its electric charge radius due to a large pion cloud, offering new insights into proton structure through parity-violating electron scattering.

Contribution

It provides the first precise measurement of the proton's weak radius, revealing a substantial difference from its charge radius and highlighting the role of the pion cloud.

Findings

Weak radius of the proton: 1.545 fm

Weak radius is 80% larger than charge radius

Large pion cloud contribution influences weak charge distribution

Abstract

The weak charge of the proton determines its coupling to the $Z^0$ boson. The distribution of weak charge is found to be dramatically different from the distribution of electric charge. The proton's weak radius $R_W= 1.545\pm 0.017$ fm is 80\% larger than its charge radius $R_{ch}\approx 0.84$ fm because of a very large pion cloud contribution. This large weak radius can be measured with parity violating electron scattering and may provide insight into the structure of the proton, various radiative corrections, and possible strange quark contributions.