Two-photon frequency comb spectroscopy of atomic hydrogen

We have performed two-photon ultraviolet direct frequency comb spectroscopy on the 1S-3S transition in atomic hydrogen to illuminate the so–called “proton radius puzzle,” and to demonstrate the potential of this method. The former is a significant discrepancy between data obtained with muonic hydrogen and regular atomic hydrogen that could not be explained within the framework of quantum electrodynamics. Combining our result f1S-3S = 2 922 743 278 665.79(72) kHz with a previous measurement of the 1S-2S transition frequency, we obtain new values for the Rydberg constant R_{infty} = 10 973 731.568 226(38) m^-1 and the proton charge radius r_{p} = 0.8482(38) fm. This result favors the muonic value over the world-average data as used in the most recent published CODATA 2014 adjustment