https://www.scientificamerican.com/article/how-big-is-the-proton-particle-size-puzzle-leaps-closer-to-resolution/

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 in Nature on 6 November, 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”.