Wildly inconsistent cross section in process specification
Hi,
So we are trying to properly study interference of our BSM signal with SM processes. As part of that we have been using squared coupling orders to specify the interfering diagrams (no problem so far).
For a start I'm just going to focus on our signal process.
We basically have two ways of specifying our signal process:
Fully specify the decay chain:
1) p p > j VLQ / p t t~ tp tp~ bp bp~ x x~ z h a, (VLQ > WW bb)
or restricting the decays with coupling orders:
2) p p > j w+ b~ / p t t~ tp tp~ bp bp~ x x~ z h a YWB^2==4
p p > j w- b / p t t~ tp tp~ bp bp~ x x~ z h a YWB^2==4
Unsurprisingly, the cross-sections are basically equivalent, no problem so far.
But the above processes have an un-decayed W in the final state, and as we have to pass it off to Pythia this is not great.
So to fix that, we can specify the final state to be fermions, for our two cases:
Fully specified decay chain:
3) p p > j VLQ / p t t~ tp tp~ bp bp~ x x~ z h a, (VLQ > WW bb, WW > ferm ferm)
restricting the decays with coupling orders:
4) p p > j ferm+ ferm+ b~ / p t t~ tp tp~ bp bp~ x x~ z h a YWB^2==4
p p > j ferm- ferm- b / p t t~ tp tp~ bp bp~ x x~ z h a YWB^2==4
The cross-section of 3 corresponds pretty well with 1 and 2 (the small difference can probably attributed to the W being forced to be on shell in the first case)
but the cross-section of 4 is less than half of what is expected, even though the feyman diagrams seem to be equivalent.
Also, in the case of 4 the ratio of hadronic to leptonic final states is changed.
Either I'm misunderstanding some part of how to specify the process correctly, or something is wrong in the way MadGraph calculates cross sections in this case.
To reproduce, the model, plus the cards for cases 1-4 can be found here:
Model: https:/
Cards: https:/
I tested these on MG 2.6.2, 2.6.3 and 3.0 (beta)
Cheers,
Ferdi
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