Long-standing tension in the Standard Model addressed

The best-known particle in the lepton family is the electron, a key building block of matter and central to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, the muon and the tau lepton, and together they are known as the three lepton flavors. According to the Standard Model of particle physics, the only difference between the siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lepton is about 17 times heavier than the muon. It is a remarkable feature of the Standard Model that each flavor is equally likely to interact with a W boson, which results from the so-called lepton flavor universality. Lepton flavor universality has been probed in different processes and energy regimes to high precision.