Propagators lepton

Except for the small term Av(weak) coming from the weak interaction, and a small known contribution arising from hadronic contributions to the photon propagator, this expression arises solely from the electromagnetic interaction of two point-like leptons of different masses in their bound state. The present theoretical value is (24,25)... 

In a picture in which the weak interactions are mediated by the exchange of vector bosons we expect to see a characteristic dependence coming from the boson propagator. [For leptonic reactions we have already seen an explicit example of this, viz. (5.1.15).] Such behaviour ought soon to be seen as available values increase. 

Diagram (a) involves the direct decay J/ —> /. Diagrams (b) and (c) are not really independent. The J/ in (6) provides a correction to the bare photon propagator in (c), and the two contributions interfere with each other. In Fig. 11.9 / is any final state, but if it consists of just e "e or pT then only (6) and (c) occur since we always assume that e and p couple directly only to photons (aside, of course, from their weak interaction which is irrelevant here). The latter assumption is indeed corroborated by the data. Table 11.4 shows that the direct J/ decay into leptons is actually quite small. 

Finally, to complete our armoury, we need expressions for the cross-sections induced by antineutrinos as visualized in Fig. 15.7. The change required at the leptonic vertex was explained in Section 1.3.2. The propagator is unchanged, and at the hadronic vertex, for CC reactions, h+ —> h— For NC reactions the hadronic vertex is unchanged. 

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