Measurement of the 2S_1/2-8D_5/2Transition in Atomic Hydrogen

Spectroscopy of atomic hydrogen plays a vital role in precision tests of quantum electrodynamics, and in the determination of the Rydberg constant and proton charge radius. Here, we present a measurement of the 2S_1/2-8D_5/2 transition on a cryogenic beam of atomic hydrogen. The measured frequency is 770649561570.9(2.0) kHz, which represents a relative uncertainty of 2.6 x 10^-12 -- a three-fold improvement over the previous best measurement. We will discuss our characterization of the systematic effects with a focus on the ac- and dc-Stark shifts. Combining our measurement with the most recent measurement of the 1S-2S transition in atomic hydrogen, we extract a proton radius that has a combined 3.1σ disagreement with the most recent CODATA 2018 recommended value.