Natural Resolution of the Proton Size Puzzle
  We show that off-mass-shell effects arising from the internal structure of the proton provide a new proton polarization mechanism in the Lamb shift, proportional to the lepton mass to the fourth power. This effect is capable of resolving the current puzzle regarding the difference in the proton radius extracted from muonic compared with electronic hydrogen. These off-mass-shell effects could be probed in several other experiments. 
G. A. Miller, A. W. Thomas, J. D. Carroll, and J. Rafelski
physics.atom-ph hep-ph nucl-ex nucl-th