International Conference on Exotic Atoms and Related Topics - EXA2017

Laser spectroscopy of muonic hydrogen [1,2] yielded a proton rms charge radius which is 4% (or ~6 sigmas) smaller than the CODATA value [3]. Also the deuteron charge radius from muonic deuterium [4] is 6 sigmas smaller than the CODATA value, but consistent with the smaller proton inside the deuteron. These smaller charge radii, when combined with precision measurements of the 1S-2S transitions in regular (electronic) hydrogen [5] and deuterium [6], yield a 6 sigmas smaller value of the Rydberg constant [7], compared to the CODATA value. In this talk I will report about a new measurement of the Rydberg constant from the 2S-4P transition in regular hydrogen performed in Garching [8], which supports the smaller, "muonic" value. I will also discuss the measurements in muonic hydrogen, deuterium, and helium, and the consequences for the "proton radius puzzle". [1] Pohl et al. (CREMA Coll.), Nature 466, 213 (2010) [2] Antognini et al. (CREMA Coll.), Science 339, 417 (2013) [3] Mohr et al. (CODATA 2014), Rev. Mod. Phys. 88, 035009 (2016) [4] Pohl et al. (CREMA Coll.), Science 353, 669 (2016) [5] Parthey et al., PRL 107, 203001 (2011) [6] Parthey et al., PRL 104, 233001 (2010) [7] Pohl et al., Metrologia 54, L1 (2017) [8] Beyer et al., submitted (2017).