Problem calculating the cross-section when there is a four particle vertex involved in the process
Hi,
I am trying to generate spin 0 magnetic monopoles from photon fusion. The process has three diagrams:
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For diagram 2 and 3, the coupling of photon-boson-boson is proportional to g. So the matrix element is proportional to g^2 and the cross-section varies as g^4. This part is correctly calculated by the MadGraph.
My problem lies with diagram 1. Here the coupling order of photon-
The couplings are defined here:
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Here GC_109 is the coupling of photon-boson-boson. GC_110 is the coupling of photon-
The coupling of photon-
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and it is used in the Lorentz structure of the VVSS in the lorentz.py:
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But since the cross-section from diagram 1 is only proportional to g^2, not g^4 as expected, I wanted to change the coupling by hand:
a. The coupling GC_110 is just the square of the previous value:
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b. The form factor is also squared (but not the Lorentz structure) inside the lorentz.py in this line:
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Now the cross-section coming from only diagram 1 is proportional to g^4. But unfortunately, the total cross-section of the process is off by a factor of 1.6 to 0.6 (depending on the mass of the monopole) from the theoretical prediction.
My questions are the following:
1. How do I tell MG that the coupling order of diagram 1 is 2, not 1 – so that MG knows that the matrix element will be proportional to g^2 for diagram 1?
2. I was wondering if squaring this value of GC_110 'by hand' disturbs MG’s calculation of interference term when it needs to consider all three diagrams of the process. Is there any way of fixing the coupling order of the four particle vertex in MG (the usual coupling order parameter in MG only filters the diagrams, it does nothing to the coupling value)?
I really appreciate your help. Thank you,
Arka
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