Constraining off-shell effects using low-energy Compton scattering
  Off-shell effects in proton electromagnetic vertices can be constrained from their effects on known processes. In particular, parameters in models for the off-shell effects can be determined by fitting to the proton electric and magnetic polarizabilities measured in low-energy Compton scattering. There has been recent speculation that off-shell effects contribute enough energy to the muonic hydrogen Lamb shift to explain the discrepancy between muonic and electronic measurements of the proton radius. We find that the constraints discussed here make the off-shell effects about two orders of magnitude smaller than needed for this purpose. 
Carl E. Carlson (William and Mary) and Marc Vanderhaeghen (Johannes   Gutenberg University, Mainz)
physics.atom-ph hep-ph nucl-th