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

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 $9\times {10}^{-13}$. The proton charge radius deduced from this measurement, $r_p=0.877(13)\mathrm{fm}$, 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.