A large team made up of researchers from across the globe has repeated experiments conducted several years ago that showed a different radius for a proton when it was orbited by a muon as opposed to an electron—a finding dubbed the proton radius puzzle—using a deuterium nucleus this time and has found the same puzzle. In their paper published in the journal Science,the team describes the experiments they conducted, what they found and offer a few possible ideas to abet dispel the notion that the puzzle indicates that there may be some problems with the Standard Model.
Source: phys.org