Let's say we want to calculate the imaginary part of the following scalar diagram in φ3 theory: This amplitude is given by the expression iM=i5∫d4ℓ1(2π)4∫d4ℓ2(2π)41D1D2D3D4D1,
Answer
Going back to Cutkosky's original paper (http://aip.scitation.org/doi/10.1063/1.1703676), it is clear he derives his result via residue theorem, as QuantumDot pointed out in his comment. Therefore, it seems natural that the generalization of the Cutkosky's cutting rule would have to analogous to the formula for the residue of a pole of order higher than one. Explicitly, if the cut propagator is raised to the n-th power, we should substitute 1Dn→(−2πi)(−1)n−1(n−1)!δ(n−1)(D).
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