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In the elastic scattering of neutrinos on atomic electrons, why can tau and muon neutrinos only interact via weak neutral currents but electron neutrinos can interact with W or Z bosons?

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In a charged current interaction the neutrino transforms into its partner lepton and a W particle. By partner lepton I mean the lepton of the same flavour. So for $\nu_e$ $e$ scattering we get:

neutrino electron scattering

(image from this article)

Muon and tau neutrinos cannot scatter in this way because it would require them to transform into an electron instead of a muon/tau.

Charged current interactions are possible between muon neutrinos and muons, and between tau neutrinos and taus, but in most situations there are lots of electrons but negligibly few muons and taus. Hence the only charged current interactions seen involve electron neutrinos.

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  • $\begingroup$ Presumably the question is motivated by the observables in SNO and similar experiments where the neutrinos are at MeV energy scales, so the situation is simple, If there is enough energy in the system heavy-flavor neutrinos can do charged current, as well, but the results are experimentally distinguishable. And to get quasi-elastic kinematics you need a lot of fixed-target frame energy. $\endgroup$ – dmckee --- ex-moderator kitten Apr 1 '18 at 16:37

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