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A Challenge to Lepton Universality

The finding that electrons and muons aren’t produced equally in certain particle decays may hint at a crack in the standard model.

These two Feynman diagrams represent the interactions that lead to a bottom quark decaying to a strange quark and a pair of oppositely charged leptons (an electron and an antielectron or a muon and an antimuon)—a “flavor-changing neutral current” process that is allowed in the standard model but is extremely unlikely. As far as the standard model is concerned, the interactions that produce the lepton pairs are the same for electron pairs and muon pairs. This idea, known as lepton universality, is being challenged by a new measurement by the LHCb collaboration.

These two Feynman diagrams represent the interactions that lead to a bottom quark decaying to a strange quark and a pair of oppositely charged leptons (an electron and an antielectron or a muon and an antimuon)—a “flavor-changing neutral current” process that is allowed in the standard model but is extremely unlikely. As far as the standard model is concerned, the interactions that produce the lepton pairs are the same for electron pairs and muon pairs. This idea, known as lepton universality, is being challenged by a new measurement by the LHCb collaboration.

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

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