A measurement of the atomic hydrogen Lamb shift and the proton charge radius

The surprising discrepancy between results from different methods for measuring the proton charge radius is referred to as the proton radius puzzle. In particular, measurements using electrons seem to lead to a different radius compared with those using muons. Here, a direct measurement of the n = 2 Lamb shift of atomic hydrogen is presented. Our measurement determines the proton radius to be r(p) = 0.833 femtometers, with an uncertainty of +/- 0.010 femtometers. This electron-based measurement of r(p) agrees with that obtained from the analogous muon-based Lamb shift measurement but is not consistent with the larger radius that was obtained from the averaging of previous electron-based measurements.