Proton Radius Puzzle Deepens With New Measurement

‘The puzzle is that the proton — the positively charged particle found in atomic nuclei, which is actually a fuzzy ball of quarks and gluons — is measured to be ever so slightly larger when it is orbited by an electron than when it is orbited by a muon, a sibling of the electron that’s 207 times as heavy but otherwise identical. It’s as if the proton tightens its belt in the muon’s presence. And yet, according to the reigning theory of particle physics, the proton should interact with the muon and the electron in exactly the same way. As hundreds of papers have pointed out since the proton radius puzzle was born in 2010, a shrinking of the proton in the presence of a muon would most likely signify the existence of a previously unknown fundamental force — one that acts between protons and muons, but not between protons and electrons…’

Source: Quanta Magazine