Muonic Hydrogen Spectroscopy: The Proton Radius Puzzle

By means of pulsed laser spectroscopy applied to muonic hydrogen (mu(-)p) we have measured the 2S(1/2)(F=1) - 2P(3/2)(F=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 r(p) = 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 mu p or H, or a fundamental problem in bound-state QED, an unknown effect related to the proton or the muon, or an experimental error.