WW+Njets cross section

Asked by Lailin Xu on 2017-11-02

Hi,

I was trying to generate WW+Njets at LO with MG5 (2.4.3). I generated 0j, 1j and 2j separately and I got the following cross section numbers, which seem very strange to me:

WWos (W+W- + 0j)
04:26:36 Cross-section : 67.71 +- 0.1663 pb
WWosNp1 (W+W- + 1j)
04:27:03 Cross-section : 82.77 +- 0.2237 pb
WWosNp2 (W+W- + 2j)
04:30:27 Cross-section : 511.1 +- 1.266 pb

As you can see, with more jets, the cross section increases.
I put my LHE files for the 3 processes below:
https://cernbox.cern.ch/index.php/s/NtljvouFaz6YeOf

Could you please take a look and let me know if I did anything wrong?
Thank you!
Lailin

Question information

Language:
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Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Solved by:
Lailin Xu
Solved:
2017-11-02
Last query:
2017-11-02
Last reply:
2017-11-02

Hi,

1) You set your b mass to zero,
So the W+W- + 2j is actually dominated by the W+W- + 2b contribution coming from the tt~ sample.
Explaining the huge boost in cross-section of that process.
If you want to compute that cross-section with matching/merging you should prevent the top to be on shell (you can use the “$ t t~” syntax for that)

2) For the 0 and 1 j, this sounds more reasonable, especially since you do not have apply (yet?) any of the method in order to remove the double counting between those two samples. After having apply your CKKW-L method (which seems to be the one that you have setup to use later) you will have more physical result for those cross-section.

Cheers,

Olivier

> On Nov 2, 2017, at 10:53, Lailin Xu <email address hidden> wrote:
>
> New question #660246 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/660246
>
> Hi,
>
> I was trying to generate WW+Njets at LO with MG5 (2.4.3). I generated 0j, 1j and 2j separately and I got the following cross section numbers, which seem very strange to me:
>
> WWos (W+W- + 0j)
> 04:26:36 Cross-section : 67.71 +- 0.1663 pb
> WWosNp1 (W+W- + 1j)
> 04:27:03 Cross-section : 82.77 +- 0.2237 pb
> WWosNp2 (W+W- + 2j)
> 04:30:27 Cross-section : 511.1 +- 1.266 pb
>
> As you can see, with more jets, the cross section increases.
> I put my LHE files for the 3 processes below:
> https://cernbox.cern.ch/index.php/s/NtljvouFaz6YeOf
>
> Could you please take a look and let me know if I did anything wrong?
> Thank you!
> Lailin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Lailin Xu (xlltoade) said : #2

Hi Olivier,

Many thanks for your quick reply!
I didn't realize the ttbar contribution for WW+2j. That makes a lot of sense!
Yes, my plan is to use CKKW-L merging.

I'll use the "$ t t~" syntax to remove the tt~ contribution.

Thanks,
Lailin