The finding that electrons and muons aren’t produced equally in certain particle decays may hint at a crack in the standard model.
What is the difference between an electron and a muon? The obvious answer is mass: the muon is about 200 times heavier than the electron. But as far as the standard model of particle physics is concerned, the electron and the muon, which are both leptons, behave the same. More specifically, the way they (and the third type of lepton, the tau particle) interact with other particles, either via the electromagnetic force or the weak force, is identical….
… Read more at http://physics.aps.org/articles/v7/102