In the Standard Model of particle physics the muon, along with the electron and tau, are in a family of particles called leptons (lower left grid). Each lepton has a partner particle called a neutrino: the muon neutrino, electron neutrino and tau neutrino. Unlike their partners, neutrinos lack electric charge. Scientists have observed neutrinos morphing between their three types, and they have reason to believe that the charged leptons might do the same. Image: Fermilab