big interference term

Hi Chirstophe,

1) Which model did you use (and did you use the same model for the three syntax).
I’m asking since your first process use the EWK coupling which does not exists in the SM, and because you do not use such coupling in the tow other syntax.

2) Which scale did you use? For such computation you should not use the default dynamical scale of MG5.

3) I do not observe any problem of the SM.
For the following, I used everything as default but the dynamical_scale_choice that I set to 3 (i.e. HT/2)
QCD=2 QED=4 -> 501.23
QCD=0 -> 4.82
QED=2 -> 496.38
QED^2==6. -> 0.31

Note that if you have f2py installed in your machine, you will automatically have the numbers for the various dynamical scale implemented in MG5.

Cheers,

Olivier

> On 3 Feb 2017, at 17:27, Christophe Roland <email address hidden> wrote:
>
> New question #450740 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/450740
>
> Dear developers,
>
> I am trying to look a the interference effect for this process:
> generate p p > e- ve~ j j QCD=2 EWK=4
> add process p p > e+ ve j j QCD=2 EWK=4
>
> between pure EWK production:
> generate p p > e- ve~ j j QED^2==8
> add process p p > e+ ve j j QED^2==8
>
> and QCD production:
> generate p p > e- ve~ j j QED^2==4
> add process p p > e+ ve j j QED^2==4
>
> The goal is check the interference for different renormalization and factorization scales, plotting ( QCD x EWK - QCD ) / EWK for the dijet mass distribution. It is my understanding the interference term should be small so that ( QCD x EWK - QCD ) / EWK would be close to one, but it is about 0.6, which shows a rather large negative interference term, but this shoudn't happen. It was done also with Sherpa, and with Sherpa this problem doesn't appears.
>
> Please tell what I could do/check to solve this problem.
>
> Thanks,
> Christophe Roland
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Oliver,

1) Yes, somehow I overlooked the fact that I am using EWK and not QED in the first sample although looking at the Feynman diagram it seems fine, but I will fix it. Everything is done in the Standard Model.

2) I did use the default scale of MG5. I will fix this too and try again.

Thank you for your help!

Cheers,
Christophe Roland

Hi Olivier,

It is much better now, but for high dijet mass (> 1500 GeV) we still see a big (negative) interference term. So, I have two questions:
1) Do you think this problem could be due to a NLO effect?
2) I am using MG5 version 2.3, do think it is worthwhile to try again with the latest version?

Thanks,
Christophe

Hi,

>It is much better now, but for high dijet mass (> 1500 GeV) we still see a big (negative) interference term.

The literature exists on such type of interference, so is it what you expect from the litterature?

>1) Do you think this problem could be due to a NLO effect?

Everything here is LO. So I would say no.

>2) I am using MG5 version 2.3, do think it is worthwhile to try again with the latest version?

Do not really see the point.

Cheers,

Olivier