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.
Image: The CEBAF accelerator at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, was used to measure the radius of the proton. Credit: DOE's Jefferson Lab
