Toward a 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 experiments. These off-mass-shell effects could be probed in several other experiments. A significant ambiguity appearing in dispersion relation evaluations of the proton polarizability contribution to the Lamb shift is noted.

Figure 1

Direct two-photon exchange graph corresponding to the hitherto neglected term. The dashed line denotes the lepton, the solid line the nucleon, the wavy lines denote photons, and the ellipse denotes the off-shell nucleon.