New Measurement of the 1S−3S Transition Frequency of Hydrogen: Contribution to the Proton Charge Radius Puzzle

Abstract

We present a new measurement of the 1S-3S two-photon transition frequency of hydrogen, realized with a continuous-wave excitation laser at 205 nm on a room-temperature atomic beam, with a relative uncertainty of 9×10^{-13}. The proton charge radius deduced from this measurement, r_{p}=0.877(13) fm, is in very good agreement with the current CODATA-recommended value. This result contributes to the ongoing search to solve the proton charge radius puzzle, which arose from a discrepancy between the CODATA value and a more precise determination of r_{p} from muonic hydrogen spectroscopy.

Proton Charge Radius from Electron Scattering

“…The value of R from electron scattering (a recent compilation listed 0.879 ± 0.009 fm [2]) disagrees with the more precise value from muonic hydrogen, 0.8409 ± 0.0004 fm [3][4][5]; the comparison to the radius from electronic hydrogen [6,7] is not yet conclusive. This so-called "proton radius puzzle" has generated an extensive discussion ranging from a reevaluation of the uncertainties of R from the determination via electron scattering to understanding the difference in terms of new physics.…”

The proton radius: from a puzzle to precision

“…Again, (13) 2018 el. Lamb shift [17] 0.833 (10) 2019 el. Lamb shift [18] 0.831(7)(12) 2019 e − p scattering [19] a small radius in the range r p = 0.82 .…”

“…Still, the situation remained unsatisfactory as r p from the electronic Lamb shift was on the large side and there has been on-going debate about the extraction of the radius from electron scattering experiments. The situation changed, however, dramatically when three new experiments on the electronic Lamb shift [16,17,18], a novel measurement of electron-proton scattering at unprecedented small momentum transfer [19], and another dispersion-theoretical inspired analysis of electron scattering data [20] became available in the last few years, with the latter one just reinforcing the claims made by the Bonn-Mainz group since the mid 1990ties. With the exception of the Paris electronic Lamb shift measurement [17], all of these new determinations of r p consistently give a small proton radius.…”

“…The situation changed, however, dramatically when three new experiments on the electronic Lamb shift [16,17,18], a novel measurement of electron-proton scattering at unprecedented small momentum transfer [19], and another dispersion-theoretical inspired analysis of electron scattering data [20] became available in the last few years, with the latter one just reinforcing the claims made by the Bonn-Mainz group since the mid 1990ties. With the exception of the Paris electronic Lamb shift measurement [17], all of these new determinations of r p consistently give a small proton radius. Consequently, the newest addition of the CODATA compilation lists the proton charge radius as r p = 0.8414(19) fm [21], completely consistent with the value from muonic hydrogen and electron scattering data analysed using dispersion theory.…”

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

“…The proton polarizability contribution to the fine structure of light muonic atoms stems from the TPE diagram and is a substantial ingredient [21] in the proton radius puzzle, the 7σ discrepancy in the value of the proton charge radius extracted from hydrogen spectroscopy [22] and electron-proton (ep) scattering [23] on one hand, and muonic hydrogen [24,25] on the other hand. The situation stays confused with a small proton radius from electron scattering [26] and a large proton radius from hydrogen spectroscopy [27]. The hadronic uncertainty of the forward γZ-box correction has recently raised a significant interest [28][29][30][31][32][33][34][35][36][37][38][39][40] in the context of a precision determination of the weak mixing angle with parity-violating electron scattering [41,42].…”