QCD jet cut

Asked by Wang Daohan on 2020-12-08

Dear Madgraph experts:
I have two questions about QCDjet background cut. In my project, QCD background is very important. I generate p p > j j and the cross section is very large. I set that the four hardest jets pT > 50 in the run_card but the survey return zero cross section. If only the first two hardest jet pT are setted to be larger than 50. The cross section is correct. I was wondering why. And the second question is that how to set the WBF cut to cut the cross section. I hope you can give me some hints. Thank you very much.

Best wishes
Daohan

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2020-12-08
Last query:
2020-12-08
Last reply:
2020-12-08

Hi,

All cuts in the run_card are done at parton level (i.e. before showering/hadronization/...)
Therefore if you run
generate p p > j j
it does not make sense to put a cut on the four hardest jet (since you have only two "jet" --actually single quark or gluon--)
I do not know what the code does if you put a cut on the fourth jet in this case (either it accept all events or reject all, looks like this is the second case according to your email)

> If only the first two hardest jet pT are setted to be larger than 50.

By construction you will have ptj1 = ptj2 by conservation of energy (again you are at parton level)
so in this case, it is better to simply use "ptj" as a cut (using ptj1min, ptj2min are less efficient)

 > And the second question is that how to set the WBF cut to cut the cross section

You have some VBF cut in the run_card
xetamin and deltaeta

Cheers,

Olivier

> On 8 Dec 2020, at 06:20, Wang Daohan <email address hidden> wrote:
>
> New question #694401 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/694401
>
> Dear Madgraph experts:
> I have two questions about QCDjet background cut. In my project, QCD background is very important. I generate p p > j j and the cross section is very large. I set that the four hardest jets pT > 50 in the run_card but the survey return zero cross section. If only the first two hardest jet pT are setted to be larger than 50. The cross section is correct. I was wondering why. And the second question is that how to set the WBF cut to cut the cross section. I hope you can give me some hints. Thank you very much.
>
>
>
> Best wishes
> Daohan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Wang Daohan (dorhand) said : #2

Thanks Olivier Mattelaer, that solved my question.