Physicists Think They've Resolved the Proton Size Puzzle
Freeman writes:
https://arstechnica.com/science/2026/04/physicists-think-theyve-resolved-the-proton-size-puzzle/
There has been considerable debate among physicists over the last 15 years about conflicting measurements of the charge radius of a hydrogen atom's proton
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The discrepancy hinted at possible exciting new physics. Now the debate seems to be winding down with the latest experimental measurements, described in two recent papers published in the journals Nature and Physical Review Letters, respectively. And the evidence has tilted in favor of a smaller proton radius and against new physics.
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As previously reported, most popularizations discussing the structure of the atom rely on the much-maligned Bohr model, in which electrons move around the nucleus in circular orbits. But quantum mechanics gives us a much more precise (albeit weirder) description.
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Hydrogen atoms are the simplest nuclei, with a single proton orbited by an electron, so that's typically what physicists have used for their experiments to measure the proton's charge radius. For a long time, the accepted value was .876 femtometers-a "world average"
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Muon spectroscopy measurements first caused the problem back in 2010. Physicists at the Max Planck Institute of Quantum Optics used muonic hydrogen, replacing the electron orbiting the nucleus with a muon, the electron's heavier (and very short-lived) sibling.
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The physicists expected to measure roughly the same radius for the proton as prior experiments, only with less uncertainty. There should be no difference (other than mass and lifetime) between the electron and the muon, theoretically. Instead, they measured a significantly smaller proton radius of 0.841 femtometers, 0.00000000000003 millimeters smaller, well outside the established error bars. It was five standard deviations from the value obtained by other methods.
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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.
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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.
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That brings us to the latest two papers, both of which involved experiments with hydrogen atoms in a vacuum chamber.
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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."
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this is disappointing for the discovery of new physics, but it is exciting that we are performing such stringent tests of the Standard Model.
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