Unraveling the Proton Radius Puzzle

This article was originally written and submitted as part of a Canada 150 Project, the Innovation Storybook, to crowdsource stories of Canadian innovation with partners across Canada. The content has since been migrated to Ingenium’s Channel, a digital hub featuring curated content related to science, technology and innovation.


In physics, there are fundamental quantities that have been so well studied that they are considered “known”. However, in the true spirit of science, researchers revisit these “known” quantities with new and improved techniques. From time to time, these new and enhanced methods show that maybe the “known” quantity wasn’t so well known after all.

The size of the proton is one such quantity. A proton is a subatomic particle found in atomic nuclei. After decades of experiments devoted to analyzing electron scattering and atomic spectroscopy, the radius of a proton was a widely accepted value within the global scientific community. Then in 2010, the Charge Radius Experiment with Muonic Atoms collaboration (CREMA) at the PSI Laboratory in Switzerland used a new technique for analyzing the proton radius, one that changed everything.

To measure the size of light nuclei, CREMA’s innovative experiment replaced the atom’s electron with a muon. A muon is an elementary particle similar to an electron, but about 200 times heavier and, unlike the electron, it only exists for about 2 millionths of a second. Muons orbit closer to the nucleus and are much more sensitive to nuclear size. CREMA found that the proton charge radius was in fact significantly smaller than previously thought. This new value differed by 7 standard deviations to the “well-known” value, meaning that there is less than a 1-in-a-trillion chance that the result was a statistical fluke.

This astonishing discrepancy catalyzed an on-going discussion of possible explanations for the difference in the observed proton radius. In 2013, a Canadian theory group working at TRIUMF (Canada’s national laboratory for particle and nuclear physics and accelerator-based science) contributed to this international challenge by providing the most precise estimates to date of important nuclear corrections needed to study this discrepancy. Atomic nuclei have an intricate internal structure that must be taken into account when analyzing experimental results, such as those done by CREMA. TRIUMF’s theorists, working with Israeli collaborators, have aided experimentalists in their investigation of whether this discrepancy occurs in other light nuclei as well.

First results on the deuteron, a nucleus made from a proton and a neutron, indicate intriguing, new puzzles. As research continues, stay connected with TRIUMF to learn more.

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TRIUMF is Canada’s particle accelerator centre. From the hunt for the smallest particles in the universe to the development of new technologies, including next-generation batteries and medical isotopes, TRIUMF is pushing the frontiers in research to advance science, medicine, and business.