Measurement of the $2$S$_{1/2}-8$D$_{5/2}$ transition in hydrogen

TL;DR

This paper reports a highly precise measurement of a specific hydrogen transition, leading to a proton radius and Rydberg constant that significantly differ from CODATA 2018 values, highlighting potential discrepancies in fundamental constants.

Contribution

First measurement of the hydrogen 2S$_{1/2}$-8D$_{5/2}$ transition with cryogenic atomic beam, providing data that challenge existing fundamental constant values.

Findings

Measured transition frequency with 2.6×10^{-12} uncertainty

Derived proton radius of 0.8584(51) fm, differing from CODATA

Identified a 3.1σ discrepancy with CODATA 2018 values

Abstract

We present a measurement of the hydrogen $2$S$_{1/2}-8$D$_{5/2}$ transition performed with a cryogenic atomic beam. The measured resonance frequency is $\nu=770649561570.9(2.0)$ kHz, which corresponds to a relative uncertainty of $2.6\times10^{-12}$. Combining our result with the most recent measurement of the $1$S$-2$S transition, we find a proton radius of $r_p=0.8584(51)$~fm and a Rydberg constant of $R_\infty=10973731.568332(52)$ m$^{-1}$. This result has a combined 3.1~$\sigma$ disagreement with the CODATA 2018 recommended value. Possible implications of the discrepancy are discussed.