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there! Why cannot consider the muon or the tau just like excited states of the electron (and similarly, with the (u,d) (c,s) (t,b) pairs? I guess that it is related to the absence of decays like

muon-> electron+photon

and similar stuff. But, is there any other reason?

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An electron could only be excited if it was a compound system. If it isn't a compound system it has no degrees of freedom for you to add energy to. Since we believe the electron is fundamental and not composite, a muon or tau cannot be an excitation of the electron. Likewise for the quarks.

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  • $\begingroup$ I understand that "phenomenological" reason, but I was thinking about other "more theoretical" reasons. Of course, decays muon-> electron + photon or similar have not been observed, so they are very suppresed by some big scale, but they are not impossible, aren't they? $\endgroup$
    – riemannium
    Feb 24, 2014 at 18:42
  • $\begingroup$ My understand is that $\mu \rightarrow e + \gamma$ is forbidden in the standard model. $\endgroup$
    – John Rennie
    Feb 24, 2014 at 19:55

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