Weak interactions that change lepton flavor

[Sebastian]

Are there any $W^\pm$ or $Z^0$ interactions that change lepton flavor? For example, turn an electron into a muon or vice versa?

Thanks!

 

[tom.stoer]

Yes; the simplest example is the myon decay via a so-called flavor-changing charged current (via an exchanged W-boson); see http://en.wikipedia.org/wiki/File:Muon_Decay.png

There are no flavor-changing neutral currents at tree level in the standad model; see: http://en.wikipedia.org/wiki/Flavor-changing_neutral_current

 

[Sebastian]

Thanks!

I wonder, what is the meaning of "highly suppressed" when describing flavor-changing neutral currents? Do such interactions exist and are just rare, or are they completely forbidden?

Also, is it possible for a changed weak current to conserve flavor?

 

[tom.stoer]

Do you know perturbation theory, Feynman diagrams and the meaning of vertices? For each vertex in a diagram you get a power of a small quantity, so a diagram with a large number of vertices comes always with a high power of a small quantity <1 and therefore represents a small quantity.

Highly suppressed means that you can draw diagrams for allowed processes of flavor changing neutral currents, but that these diagrams contain a typically more vertices and therefore represent only a small correction to other processes.

 

[Sebastian]

I'm taking an introduction course in elementary particle physics (3rd year level). We did study Feynman diagrams, at least qualitatively. And I understood your explanation, thanks :)

Could you please answer my second question (is it possible for a changed weak current to conserve flavor)?

 

[Vanadium 50]

It is not possible for a charged current to conserve flavor, because it must couple to two different fields to couple with two fields of different charges. A photon can couple to u-ubar or d-dbar, but a charged boson needs to couple to u-dbar or d-ubar.

 

[Parlyne]

It is not possible for a charged current to conserve flavor, because it must couple to two different fields to couple with two fields of different charges. A photon can couple to u-ubar or d-dbar, but a charged boson needs to couple to u-dbar or d-ubar.

There are charged current contributions to processes like $e\nu_e \rightarrow e\nu_e$ or $ud \rightarrow ud$ scattering where overall flavor is conserved, even though flavor is not conserved at each vertex. (If you object to the second case as relying on free quarks, it can be though of as a proxy for $pn \rightarrow pn$.)

 

[Vanadium 50]

Yes, but that's not very helpful. Yes, technically you can have two interactions, each with flavor violation that cancels the other. But is that something likely to clarify the OP's understanding or add to the confusion?