Proton Puzzle

A long-awaited experimental result has found the proton to be about 5% smaller than the previously accepted value. The finding has helped to prompt a redefinition of the particle’s official size and seems to spell the end of the “proton radius puzzle,” which has enthralled physicists since 2010.

The result, published recently in Nature, puts the particle’s radius at 0.831 femtometres. This measurement, together with a concurring one made using a different technique that was published in Science in September, has been known to experts since last year. The findings led the Committee on Data for Science and Technology (CODATA)—an organization that records the most up-to-date measurements of the fundamental constants of nature—to revise its handbook at the end of 2018, says Krzysztof Pachucki, a theoretical physicist at the University of Warsaw who chairs a CODATA task group. Although some researchers are still cautious, he thinks the latest papers have “definitely resolved the puzzle.”

Physicists use two main techniques to measure the size of the proton. One relies on how electrons orbit atomic nuclei. Because some electron orbits pass through the protons in the nucleus, the size of the protons affects how strongly the electrons bind to the nucleus. Precise measurements of the differences between various electrons’ energy levels—a technique known as spectroscopy—therefore provide a way to estimate the proton’s radius. The second technique involves hitting atoms with a particle beam and seeing how those particles scatter off the nuclei.

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