On the pi pi continuum in the nucleon form factors and the proton radius puzzle

Abstract

We present an improved determination of the $\ddot{I}€\ddot{I}€$ continuum contribution to the isovector spectral functions of the nucleon electromagnetic form factors. Our analysis includes the most up-to-date results for the $\ddot{I}€\ddot{I}€\to \bar{N}N$ partial waves extracted from Roy-Steiner equations, consistent input for the pion vector form factor, and a thorough discussion of isospin-violating effects and uncertainty estimates. As an application, we consider the $\ddot{I}€\ddot{I}€$ contribution to the isovector electric and magnetic radii by means of sum rules, which, in combination with the accurately known neutron electric radius, are found to slightly prefer a small proton charge radius.

10 pages, 3 figures, spectral functions included as supplementary material; journal version