Two-photon frequency comb spectroscopy of atomic hydrogen.

Grinin, Alexey; Matveev, Arthur; Yost, Dylan C; Maisenbacher, Lothar; Wirthl, Vitaly; Pohl, Randolf; Hänsch, Theodor W; Udem, Thomas · Science · 2020

basic_science · Level V

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Abstract

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 proton radius puzzle 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. By combining our result [<i>f</i> <sub>1S-3S</sub> = 2,922,743,278,665.79(72) kilohertz] with a previous measurement of the 1S-2S transition frequency, we obtained new values for the Rydberg constant [<i>R</i> <sub>∞</sub> = 10,973,731.568226(38) per meter] and the proton charge radius [<i>r</i> <sub>p</sub> = 0.8482(38) femtometers]. This result favors the muonic value over the world-average data as presented by the most recent published CODATA 2014 adjustment.