A puzzling discrepancy

Subsequent measurements by various groups were inconclusive. For instance, in 2013, the same international team performed muon-based experiments that confirmed their 2010 value, producing a measurement of 0.84 femtometers for the proton’s radius, with a discrepancy of 7 sigma. Another experimental variation in 2016 involved replacing the electron with a muon in a deuterium atom—a heavier isotope of hydrogen, with a neutron as well as a proton and an electron. The idea was that the presence of a neutron would alter how electrons and muons perceive the proton’s charge. That, too, was in line with the 2010 result.

However, two experiments using regular hydrogen to measure the proton radius produced mixed results: A 2017 study also confirmed the 2010 result, while a 2018 measurement was in line with the larger value before the 2010 experiment. In 2019, York University scientists opted to make an electron-based measurement of the proton radius, in hopes of bringing the various conflicting results closer to a consensus. The result: Their measurement of 0.833 femtometers agreed with the smaller value from the 2010 study.

That brings us to the latest two papers, both of which involved experiments with hydrogen atoms in a vacuum chamber. They used lasers to control the electrons and measured the transitions between energies; this enabled them to infer the exact dimensions of the proton’s charge radius. Based on the combined results, the proton has a radius of about 0.84 femtometers, or less than 1 million-billionth of a meter, once again in keeping with the 2010 measurement that kicked off the debate.

“The proton radius should be a universal property; it should give the same result no matter how you look at it,” Juan Rojo, a physicist at Vrije University Amsterdam in the Netherlands, who was not involved in either experiment, told New Scientist. “This is why these two papers are quite nice, because they provide different perspectives to the same number.”