Question about zero cross section error with dim6top_LO_UFO model
Hello MG experts,
I have a question about an error I have run into while using the dim6top_LO_UFO model. When generating ttW samples, we noticed that the cross section did not seem to depend on the value of the Wilson coefficient cptb, though we expected to see dependence on this coefficient. To investigate this further, we tried to generate just the quadratic contribution from cptb (using DIM6==1 and setting only cptb to a non-zero value). The process is:
define p = p b b~
define w = w+ w-
generate p p > t t~ w FCNC=0 DIM6==1
However, when we tried to run this, we saw this error about 0 cross section:
Survey return zero cross section.
Typical reasons are the following:
1) A massive s-channel particle has a width set to zero.
2) The pdf are zero for at least one of the initial state particles
or you are using maxjetflavor=4 for initial state b:s.
3) The cuts are too strong.
Please check/correct your param_card and/or your run_card.
Zero result detected: See https:/
INFO:
quit
This was surprising to us, as we expected a non-zero contribution. When we tried the same setup except with ttZ instead of ttW, the process ran fine and gave a non-zero cross section. We also tried ttW with just the cpQ3 coefficient set to a non-zero value (we tried this coefficient because it seems to have the same ttW diagrams as cptb), and this ran fine and gave a non-zero cross section.
I am wondering if you could help me to understand what might be causing the 0 cross section error for ttW cptb? Maybe I am just missing some difference in how ttW and ttZ should be set up and run?
In case it is relevant, I am using MG version 2_9_2.
Thank you! - Kelci
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