Speaker
Description
Precision spectroscopy of atomic hydrogen is an important way to test bound-state quantum electrodynamics (QED), one of the building blocks of the Standard Model. In its simplest form, such a test consists of the comparison of a measured transition frequency with its QED prediction, which can be calculated with very high precision for the hydrogen atom. However, these calculations require some input in the form of physical constants, such as the Rydberg constant and the proton radius, both of which are determined to a large degree by (electronic and muonic) hydrogen spectroscopy itself. Therefore, the frequency of at least three different transitions needs to be measured in order to test QED. Furthermore, there are multiple recent, but discrepant measurements of the proton radius, so far precluding QED tests at the highest accuracy.
We have measured the 2S-6P transition in atomic hydrogen with a relative uncertainty of 0.7 parts per trillion (ppt), a six-fold improvement over our previous measurement of the 2S-4P transition [1]. This allows us to determine the proton radius and Rydberg constant with an uncertainty below the world-average CODATA-2018 values [2] and sufficient to distinguish between previous, discrepant values for the proton radius by more than 5 $\sigma$. Conversely, our measurement, in combination with [2-4], constitutes a test of bound-state QED with an accuracy below 1 ppt, making it one of the most precise tests of the Standard Model.
Here, we discuss the measurement and its analysis in detail, and present the unblinded results and their implications.
[1] A. Beyer, L. Maisenbacher, A. Matveev et al., Science 358, 79 (2017).
[2] E. Tiesinga et al., Rev. of Mod. Phys. 93, 025010 (2021).
[3] C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011).
[4] A. Antognini et al., Science 339, 417 (2013).
