Science 358, 79–85 (2017)

Seven years ago, measurements of muonic hydrogen — an exotic atom with a muon instead of an electron — yielded a radius of the proton significantly smaller than expected. The result was very puzzling, but follow-up measurements only confirmed it, deepening the mystery. Now Axel Beyer and colleagues have revisited the measurements of the proton radius using the 2s–4p transition frequency in regular hydrogen. The new result is in line with the values from muonic hydrogen spectroscopy and hence finds itself at odds with the accepted value of the proton radius.

Beyer et al. used a cryogenic beam of hydrogen atoms in one of the hyperfine ground states, which they excited to the 4p state. They measured the transition frequency from which they derived the proton radius and the value of the Rydberg constant, which also disagrees with the figures in the literature — by more than three standard deviations. Despite the excellent accuracy, the new values do not offer a solution for the proton puzzle, but rather call for more precision measurements.