Muonic hydrogen spectroscopy: the proton radius puzzle

By means of pulsed laser spectroscopy applied to muonic hydrogen (μ¯p) we havemeasured the 2S (F=1)/(1/2) - 2P(F=2)/(3/2) transition frequency to be 49881.88(76)GHz.¹ By comparing this measurement with its theoretical prediction ^2-7 based on bound-state QED we have determined a proton radius value of rP =0.84184(67) fm. This new value differs by 5.0 standard deviations from the CODATA 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.