Different syntax for the same diagram leads to different cross-sections: Why?

Created by Olivier Mattelaer on on 2016-05-20

One typical questions is why they are difference in cross-section between
a) p p > w+, w+ > e+ ve
b) p p > w+ > e+ ve

They are two reasons for that

1) In the first syntax, the w+ is forced to be onshell and we therefore apply a onshell cut on this W+.
The onshell cut is the following: M_W - bwcutoff * \Gamma_W < M^*_M < M_W + bwcutoff * \Gamma_W
The parameter bwcutoff being part of the run_card (default=15)

2) The cut on the final state particle are by default de-activated for particle coming from a forced on-shell decay (syntax a).
If you want to allow the various pt cut/.... you need to set in the run_card "cut_decays" to True