RF Measurement of the Hydrogen 2S Hyperfine Interval

We will discuss a recent measurement of the hydrogen 2S hyperfine interval using Ramsey spectroscopy and a cryogenic atomic beam. This measurement reduces the experimental uncertainty by a factor of approximately eight as compared with the previous most precise measurement, which used optical spectroscopy of the 1S-2S transition [N. Kolachevsky, et al., Phys. Rev. Lett. 102, 213002 (2009)]. A linear combination of the 1S and 2S hyperfine intervals gives the quantity D₂₁, which has been determined theoretically with high accuracy [Karshenboim and Ivanov, Eur. Phys. J. D 19, 13 (2002)]. Experimental comparisons of D₂₁ with theory provide a stringent test of fourth order bound-state QED, and can be used as a probe of beyond Standard Model physics.