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corresponding integral. The Feynman diagrams in this chapter are intended to emphasize this correspondence, therefore they do not specify the spin or mass of any given particle and only serve to indicate the flow of momenta through the diagram. 

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The iη pole prescription is related to causality and is implicitly included in all propagators. As we are in Minkowski space, so the momentum k has one temporal component k$_{0}$ and d − 1 spatial components k$_{i}$ . Thus the integral can be written as 

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The k$_{0}$ integral has poles at k$_{0}$ = ± $^{√}$k 2 i + m 2 ∓ iη , as shown in Fig. 2.2. The locations of the poles permit the integration contour to be shifted as shown in Fig. 2.2. Rather than integrating along the real k$_{0}$ axis, we perform a Wick rotation and integrate along the 

