Lepton greatest rest mass4/16/2023 ![]() ![]() The Higgs field, on the other hand, distinguishes between fermions of different flavours and endows them with different masses – sometimes strikingly so. It directly follows from the assumption that the SM gauge group, SU(3) × SU(2) × U(1), is one and the same for all three generations of fermions. This “flavour universality” is deeply ingrained in the symmetry structure of the Standard Model (SM) and applies to both the electroweak and strong forces (though the latter is irrelevant for leptons). The three flavours of charged leptons – electron, muon and tau – are the same in many respects. These 12 elementary fermions are grouped into three generations of increasing mass. A similar picture evolved for the leptons: the electron and the muon were joined by the unexpected discovery of the tau lepton at SLAC in 1975 and completed with the three corresponding neutrinos. From the three types known at the time – up, down and strange – the list of quark flavours grew to six. In 1971, at a Baskin-Robbins ice-cream store in Pasadena, California, Murray Gell-Mann and his student Harald Fritzsch came up with the term “flavour” to describe the different types of quarks. If the effect strengthens as more data are gathered, possible explanations range from new gauge forces to leptoquarks. Recent experimental results hint that some electroweak processes are not lepton-flavour independent, contrary to Standard Model expectations. ![]()
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