On the pi pi continuum in the nucleon form factors and the proton radius   puzzle
  We present an improved determination of the $\pi\pi$ continuum contribution to the isovector spectral functions of the nucleon electromagnetic form factors. Our analysis includes the most up-to-date results for the $\pi\pi\to\bar N N$ 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 $\pi\pi$ 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. 
M. Hoferichter, B. Kubis, J. Ruiz de Elvira, H.-W. Hammer, U.-G.   Mei{\ss}ner
hep-ph hep-lat nucl-ex nucl-th physics.atom-ph