Sunday, 5 August 2018

quantum field theory - Cutkosky rules for a two-loop diagram


Let's say we want to calculate the imaginary part of the following scalar diagram in φ3 theory:enter image description here This amplitude is given by the expression iM=i5d41(2π)4d42(2π)41D1D2D3D4D1,

where Dk is the k-th denominator as in the figure. To take the imaginary part, we have to perform cuts according to Cutkosky. Here we have three possible cuts - we can cut the following lines : 12, 234, and again 12. Employing the cutting rules, for the cut k-th line we put 1Dk2πiδ(Dk)
and get M=2d41(2π)4d42(2π)4(2πi)2D3D4δ(D1)D1δ(D2)+d41(2π)4d42(2π)4(2πi)3D21δ(D2)δ(D3)δ(D4).
The problem is, of course, the first term. How to interpret the explicitly divergent term δ(D1)/D1?



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)n1(n1)!δ(n1)(D).

In case n=2, we would then have 1D2(2πi)δ(D).
While I have not yet checked this substitution rule in a real calculation, I suspect that it will hold.


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