The Lamb shift in muonic hydrogen 1

The long quest for a measurement of the Lamb shift in muonic hydrogen is over. Last year we measured the 2S1/2 -2P3/2 energy splitting (Pohl et al., Nature, 466, 213 (2010)) in mp with an experimental accuracy of 15 ppm, twice better than our proposed goal. Using current QED calculations of the fine, hyperfine, QED, and finite size contributions, we obtain a root-mean-square proton charge radius of rp = 0.841 84 (67) fm. This value is 10 times more precise, but 5 standard deviations smaller, than the 2006 CODATA value of rp. The origin of this discrepancy is not known. Our meas- urement, together with precise measurements of the 1S-2S transition in regular hydrogen and deuterium, gives improved values of the Rydberg constant, R? = 10 973 731.568 160 (16) m -1 and the rms charge radius of the deuteron rd = 2.128 09 (31) fm.