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Physical Chemistry

Downsizing the proton

Evidence mounts for a smaller radius

by Jyllian Kemsley
October 9, 2017 | A version of this story appeared in Volume 95, Issue 40

New measurements support a smaller proton.
Chart showing different values for proton radii with error bars.

Sources: International Council for Science Committee on Data for Science & Technology, 2014; Nature 2010, DOI: 10.1038/nature09250; Science 2017, DOI: 10.1126/science.aah6677

The radius of the proton has been puzzling scientists since 2010, when a study of muonic hydrogen, in which the hydrogen atom’s electron is replaced by a heavier muon, suggested that the proton’s radius is significantly smaller than the consensus value from other experiments—about 0.84 femtometers compared with 0.88 fm. New spectroscopic work on normal hydrogen now bolsters the 2010 result (Science 2017, DOI: 10.1126/science.aah6677). Scientists measure the proton radius using hydrogen spectroscopy or electron scattering. In the new work, a team led by Axel Beyer and Lothar Maisenbacher of the Max Planck Institute of Quantum Optics tightly controlled hydrogen spectroscopy experimental conditions to yield a proton radius of 0.8335 fm. The researchers note that both the 2010 value and the new value fall within the error ranges of some individual earlier experiments. The discrepancy occurs when those other, earlier values are combined into a recommended standard radius by the International Council for Science’s Committee on Data for Science & Technology (CODATA). Within their experimental errors, the 2010 muonic hydrogen and latest standard hydrogen experimental values do not overlap with the CODATA radius. The new hydrogen spectroscopy data also yield a value for the related Rydberg constant, 10,973,731.568076 m–1, that is outside the error margins of the CODATA value, 10,973,731.568508 m–1.


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