How big is a proton? Unlike an electron or neutrino, which are fundamental particles that behave like points, a proton is a messy collection of quarks, gluons, and virtual particles that occupies what should be a measurable amount of space. And just how much space can be rather significant; as the authors of a new paper on the proton's size put it, "The proton structure is important because an electron in an S [ground] state has a nonzero probability to be inside the proton."
Within experimental error, various measurements of the proton's size have all put it about 0.88 femtometers (an fm is 10-15 meters). But a team of researchers, working at a particle accelerator in Switzerland, has found a different way of measuring the proton's size: put a muon—a heavy, unstable, relative of the electron—in orbit around a proton. The resulting atom, called muonic hydrogen, can be measured during the brief time it exists before the muon decays. Those measurements have produced a new, very high-precision value for the proton's radius. Just one small problem: it differs from the other measurements by nearly seven standard deviations.