Talk:Flavor changing neutral current

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[edit] Lepton number violation in FCNC

Doesn't the standard model feynman diagram for a tau changing into an electron violate lepton number. There is a tau neutrino changing into an electron via a W boson.

Yes. I think this is allowed if you have neutrino oscillation. Total lepton number is conserved, but individual lepton numbers are not. -- Xerxes 14:48, 14 July 2006 (UTC)
True, but the caption claims that this is an example of a Standard Model FCNC, and traditionally, by the Standard Model, people often exclude neutrino oscillations. Maliz 12:16, 18 October 2006 (UTC)
Another thing I thought of: You don't have to use taus here. You could draw the same diagram without recourse to neutrino mixing in the quark sector with tau→s, mu→d, and nu→u. Of course, tau looks cleaner, because for the quarks, you would have some other hadronic bits to confine your quarks. On the other hand, the depicted tau decay (although allowed) has never been observed, while similar hadronic ones have. -- Xerxes 15:08, 14 July 2006 (UTC)

Hi Xerxes, as mentioned above these examples are not good because they resort to lepton flavor violation instead of FCNCs. Can't you use a more classical example and one that has been observed? Originally the missing FCNCs in kaon decay were explained by introducing charm to cancel direct d->s transitions via the GIM mechanism. So a BSM diagram would show s -> FCNC_vertex -> d with a photon/Z0 radiating from the FCNC_vertex. And the SM diagram would be a loop-diagram like a penguin in B-physics: d fluctuates into a loop with a W on one side and u/c/t on the other. i think in the lepton sector you can't write such a diagram yet, because it hasn't been shown that theta_13 > 0 yet, which your picture shows. gbrandt 09:37, 2 February 2007 (UTC)