Phenomenology of purely leptonic reactions

The standard model, the simple and beautiful gauge theory that unites weak and electromagnetic interactions, was developed in Section 4.2 only as far as the inclusion of leptons was concerned. Even in the limited realm of purely leptonic reactions it has a rich and predictive structure which we shall here confront with experiment. 

Let us write out in detail the relevant parts of the Lagrangian for these reactions. FVom (4.2.27) and (4.2.44) we have 

It will be useful to remember that in order to yield the correct n lifetime in lowest order one requires [see (4.2.30)] 

5 Lowest order tests of the SM in the leptonic sector where for each left-handed fermion doublet /l W3 Qf  

Because the SM has exact lepton universality, we have also 

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