Nuclear Quasi-Elastic Electron Scattering Limits Nucleon Off-Mass Shell   Properties
  The use of quasi-elastic electron nucleus scattering is shown to provide significant constraints on models of the proton electromagnetic form factor of off-shell nucleons. Such models can be constructed to be consistent with constraints from current conservation and low-energy theorems, while also providing a contribution to the Lamb shift that might potentially resolve the proton radius puzzle in muonic hydrogen. However, observations of quasi-elastic scattering limit the overall strength of the off-shell form factors to values that correspond to small contributions to the Lamb shift. 
Gerald A. Miller, Anthony W. Thomas, and Jonathan D. Carroll
nucl-th hep-ph nucl-ex physics.atom-ph