The proton radius puzzle

Abstract

By means of pulsed laser spectroscopy applied to muonic hydrogen (μ- p) we have measured the 2S1/2F=1 - 2P3/2F=2 transition frequency to be 49881.88(76) GHz [1]. By comparing this measurement with its theoretical prediction [2, 3, 4, 5, 6, 7] based on bound-state QED we have determined a proton radius value of r p = 0.84184(67) fm. This new value differs by 5.0 standard deviations from the COD ATA value of 0.8768(69) fm [8], and 3 standard deviation from the e-p scattering results of 0.897(18) fm [9]. The observed discrepancy may arise from a computational mistake of the energy levels in μp or H, or a fundamental problem in bound-state QED, an unknown effect related to the proton or the muon, or an experimental error.