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Physics 253a5Where the shaded circle stands for some subgraph that evaluates to-imΓ. Notethat we can’t get this from a term likeimΓφ2in the Lagrangian. The problem isthat the Lagrangian has to be real, so such a term isn’t allowed. It turns out thatimaginary parts in the value of a graph come from loops where loop momentacan go on shell, so the following series of graphs would work (assuming someconditions on the masses of the various particles on the internal lines – that we’lllearn about)+++· · ·4.(a) Let’s go to the center of mass frame, wherekμ= (m,0,0,0).