Argonne National Laboratory

New Measurement of the Size of the Proton

November 1, 2011

A recent measurement at JLab of the elastic scattering of electrons on the proton provides a new, high-precision value for the charge radius of the proton. The experiment was carried out under the leadership of Argonne National Laboratory's Medium Energy Physics group. The charge radius agrees with other recent extractions based on electron-proton scattering as well as with data from atomic physics measurements in hydrogen (Lamb shift). In contrast, the radius differs significantly from that measured in muonic hydrogen. In the absence of some error in the muonic hydrogen measurement or in its interpretation, a new interaction is required to resolve the discrepancy. More ยป