They found the muon-orbited protons to be 0.84 femtometers in radius — 4% smaller than those in regular hydrogen, according to the average of more than two dozen earlier measurements. Which would mean protons really shrink in the presence of muons due to unknown physical interactions between protons and muons.
Hundreds of papers speculated about the possibilities.
But as Natalie Wolchover notes in Quanta, hopes that the “proton radius puzzle” would upend particle physics and reveal new laws of nature have now been dashed by more accurate measurements of the Lamb shift in muonic hydrogen published in Science.
Fig. 5. Summary of proton radius data. Shown are values for the proton RMS charge radius from our measurement, muonic hydrogen, CODATA 2014, and the measurements of Beyer et al. (18) and Fleurbaey et al. (19) combined with that of Parthey et al. (20). Also shown in gray is the value from Lundeen and Pipkin (6, 16). Bezginov et al., Science 365, 1007–1012 (2019) 6 September 2019