Problems with proton radii

As a consequence of the peculiar shape of the charge density ρ ( r ) –which is close to an exponential one–the value of the proton charge r m s -radius R c h determined from electron scattering data depends strongly on the density ρ ( r ) at large radii r . This density is poorly constrained by scattering data. Supplementing the (e, e) data with the large- r shape of ρ ( r ) resulting from the Fock components ( n + π , … ) which dominate the large- r behavior produces a more reliable value for R c h . The resulting radius agrees with the one we previously extracted from (e, e) and with the value determined from electronic Hydrogen, but disagrees with the one recently obtained from muonic Hydrogen. The origin of the discrepancy is not understood.