For the same process, why do different sentences produce different sections

Asked by zxy


I use the following two statements to display diagrams respectively. They generate the same Feynman diagram, but when running them, the decay width is different. Why?

The two statements are as follows:

generate t > b w+, ( w+ > ta+ vt )

generate t > b ta+ vt

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said :

In one case, you do request the W to be onshell and therefore a onshell cut is associated to the W to prevent it to be too far offshell.
Therefore the associated width is smaller than the second case where you do not ask for such cut.



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zxy (sduzxy) said (last edit ):

Thank you for your answer ! This Solved My Problem

I also have a related question:
How can we make the results of the second case the same as those of the first case?
In other words, in the second case, can I achieve my goal by changing some onshell cuts?

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Best Olivier Mattelaer (olivier-mattelaer) said :


The best is to switch syntax actually.
It is possible to increase the bwcutoff to increase the mass window in which the particle is consider onshell.
But note that this can have side effect (both on the efficiency side and on the physics side since increasing this parameter will impact the parton-shower.



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zxy (sduzxy) said (last edit ):

Thanks Olivier Mattelaer, that solved my question.