Ars Technica: Hydrogen is the simplest of atoms, formed by a single electron orbiting a single proton. Because of its simplicity, hydrogen is useful for determining many basic characteristics of particles. One such characteristic is the size of the proton, which researchers have measured to be roughly 0.88 fm. An international team of researchers has applied the same electron scattering technique normally used to measure the size of the proton to a variant of hydrogen in which the electron is replaced by a muon. The muon shares many of the same characteristics as the electron, but is 207 times heavier. Because the technique’s precision depends on mass, the muon measurements were more precise. The researchers were surprised, however, to discover that the resulting measurement of 0.84 fm was not only more precise than the value derived from electron scattering, but also smaller by more than seven standard deviations. What is causing the discrepancy is not known. But if muons interact with protons in a different way than electrons do, entirely new physics could be at work.