Atomic measurements add weight to recent work suggesting that the proton is significantly smaller than previously thought.
In 2010, researchers studied muonic hydrogen (in which the electron is replaced with a muon, a bigger particle that is also negatively charged), which allowed them to measure the nuclear radius much more accurately than is possible with ordinary hydrogen. The proton size they found was smaller than expected from previous measurements. To confirm the results, the same team, led by Randolf Pohl of the Max-Planck Institute for Quantum Optics in Garching, Germany, studied the nucleus of muonic deuterium, which contains a proton and a neutron. They calculated that the proton radius is about 5% smaller than previously measured — a similar result to that of 2010.
Several laboratories are redoing the measurements in ordinary hydrogen atoms to try to resolve the contradiction.