Proton Radius Problem Still Solved

The charge radius of the proton (i.e. the radius of the proton measured from its electromagnetic properties) was measured in muonic hydrogen (i.e. a proton with an associated muon instead of an electron) and was found to be about 0.842 fm, a result 6.9 sigma (about 4%) smaller than the 0.878 fm measurement made with ordinary hydrogen that was the state of the art global average measurement of the proton charge radius as of the year 2010 which was the norm at the time of the muonic hydrogen measurement.

Later experiments found that measurements of the proton radius in ordinary hydrogen, conducted with improved methods, confirmed that the proton radius in ordinary hydrogen was consistent with that in muonic hydrogen, as the Standard Model would lead us to expect. The discrepancy had been due to errors in the older measurement of the charge radius of the proton in ordinary hydrogen which had been underestimated. As I explained in 2019:

In a September 19, 2018 post at this blog (about a year ago), entitled "Proton radius problem solved?", I stated: 

This would solve one of the major unsolved problems of physics (I have previously put this problem in the top twelve experimental data points needed in physics), called the "proton radius problem" or "muonic hydrogen problem" in which the radius of a proton in a proton-muon system appeared to be 4% smaller than measurements of the radius of a proton in ordinary hydrogen, contrary to the Standard Model. See previous substantive posts on the topic at this blog which can be found on November 5, 2013, April 1, 2013, January 25, 2013, September 6, 2011, and August 2, 2011.

Previously most measurements in ordinary hydrogen favored a larger value for the radius of a proton (the global average of these measurement was 0.8775(51) fm), while the new high precision and more direct measurement in muonic hydrogen found a smaller value for the radius of a proton of 0.84184(67) fm. Now, a new electron measurement devised to rigorously confirm or deny the apparent discrepancy measured a result that agrees with the muon-based measurements.

A new analysis of experimental evidence (also here) from ordinary hydrogen has likewise confirmed that the proton charge radius in ordinary hydrogen is the same as it is in muonic hydrogen.