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- Not sure why there is a (V+A) term in here.

Have a look at O5 & O6 in Eqtns(5.4) . Why is there a (V+A) ?

(V+A) contains the projection operator which projects out the right Weyl from a Dirac spinor.

As per the Feynman rules of electroweak theory, there is a (V-A) assigned to each (Dirac) spinor-W boson vertex because W only couple to left Weyl spinors.

In the corresponding penguin diagrams there is the quark-antiquark loop coupling to the W so shouldn't both vertices have the same (V-A) factor always ?

(V+A) contains the projection operator which projects out the right Weyl from a Dirac spinor.

As per the Feynman rules of electroweak theory, there is a (V-A) assigned to each (Dirac) spinor-W boson vertex because W only couple to left Weyl spinors.

In the corresponding penguin diagrams there is the quark-antiquark loop coupling to the W so shouldn't both vertices have the same (V-A) factor always ?