How Big Is the Proton? Particle-Size Puzzle Leaps Closer to Resolution – Scientific American

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…