This is not directly related to measurements of the electron electric dipole moment, which I am working on. However, it is good news for the proton radius puzzle (which we discussed a few months ago). Now we have 5 recent experiments measuring r_p which all converge on the newer, smaller value of r_p = 0.84 fm:
- This new experiment, using electron-proton scattering (Jefferson Lab, 2019)
- Lamb shift measurement in atomic hydrogen (York University, 2019)
- Conventional hydrogen spectroscopy on the 2S-4P transition (MPQ, 2017)
- Muonic deuterium spectroscopy (MPQ, 2016)
- Muonic hydrogen (MPQ, 2010) - this was the initial experiment which started the proton radius puzzle
This new e-p result is impressive because previously, measurements using scattering techniques were all over the place:
(Taken from Figure 1 of Pohl et al. review, 2013 - before the newer results started coming in)
Now, there is still one holdout experiment which is refusing to converge to the smaller value: a 1S-3S hydrogen spectroscopy experiment in France. Also, looking at the above plot and seeing the past “reanalyses” of the old scattering measurements which shifted the measured value of r_p up to the previously assumed “correct” value of r_p = 0.88 fm raises my eyebrows. How much of the recent converging results was influenced by sociological factors? The Lamb shift measurement is particularly impressive to me because they took the data blinded - analyzing all the data without knowing the final value until the very end. This is to prevent experimental bias, such as looking for systematic errors until your value happens to converge to what is thought to be the “right” one. It’s not clear to me whether this newest e-p scattering experiment also blinded their data.
Still, it seems that in light of the recent convergence of most experiments using different measurements methods towards the smaller value of r_p = 0.84 fm, the official CODATA value (listed in the standard particle physics data books) might have to be changed.
The good news? The “proton radius puzzle” is now likely to be considered solved. The bad news? The fact that most experiments converge to a single value indicates that there likely wasn’t any new physics to explain the puzzle. The Standard Model still holds, frustratingly so, with its flaws and incompleteness.