The Lamb shift in muonic hydrogen and the proton radius

By means of pulsed laser spectroscopy applied to muonic hydrogen (μ− p) we have measured the 2S F=1 1/2−2PF=2 3/2 transition frequency to be 49881.88(76) GHz. By comparing this measurement with its theoretical prediction based on bound-state QED we have determined a proton radius value of rp=0.84184 (67) fm. This new value is an order of magnitude preciser than previous results but disagrees by 5 standard deviations from the CODATA and the electronproton scattering values. An overview of the present effort attempting to solve the observed discrepancy is given. Using the measured isotope shift of the 1S-2S transition in regular hydrogen and deuterium also the rms charge radius of the deuteron rd=2.12809 (31) fm has been determined. Moreover we present here the motivations for the measurements of the μ 4He+and μ 3He+2S-2P splittings. The alpha and triton charge radii are extracted from these measurements with relative accuracies of few 10−4. Measurements could help to solve the observed discrepancy, lead to the best test of hydrogen-like energy levels and provide crucial tests for few-nucleon ab-initio theories and potentials.