Double counting in Drell-Yan production

Asked by Stephanie Baines on 2018-02-22

Hello,

I am producing Drell-Yan corss sections in Madgraph. It seems when I generate j j > a > e+ e-, madgraph counts twice the cross section I calculate from the Feynman rules analytically because it generates all processes twice
u u~ > a > e+ e-
u~ u > a > e+ e-
d d~ > a > e+ e-
d~ d> a > e+ e-
s s~ > a > e+ e-
s~ s > a > e+ e-
c c~ > a > e+ e-
c~ c > a > e+ e-

I don't understand why this is correct.
If one considers the cross section
 e+ e- > a > e+ e- which is identical bar a factor for charge and colour, the total cross section as given by Madgraph is not doubled to include e- e+ > a > e+ e-.

Which is physically correct if I want to know what would be produced by proton-proton collisions?
Thank you!
Best
Stephanie

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2018-02-22
Last query:
2018-02-22
Last reply:
2018-02-22

Hi

u u~ initial state and u~ u initial state are not identical.

since the direction of the initial particle are not identical
in the first case the u is coming from the proton going to the positive z direction
and in the second the u is coming rom the proton going to the negative z direction

The cross-section of both are obviously identical but the differential distribution can be assymetric so we need to sum those two to have a symmetric contribution.

Obviously if you run for tevatron, then the contribution would not even be identical (due to the PDF)

Cheers,

Olivier

> On 22 Feb 2018, at 15:57, Stephanie Baines <email address hidden> wrote:
>
> New question #664813 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/664813
>
> Hello,
>
> I am producing Drell-Yan corss sections in Madgraph. It seems when I generate j j > a > e+ e-, madgraph counts twice the cross section I calculate from the Feynman rules analytically because it generates all processes twice
> u u~ > a > e+ e-
> u~ u > a > e+ e-
> d d~ > a > e+ e-
> d~ d> a > e+ e-
> s s~ > a > e+ e-
> s~ s > a > e+ e-
> c c~ > a > e+ e-
> c~ c > a > e+ e-
>
> I don't understand why this is correct.
> If one considers the cross section
> e+ e- > a > e+ e- which is identical bar a factor for charge and colour, the total cross section as given by Madgraph is not doubled to include e- e+ > a > e+ e-.
>
> Which is physically correct if I want to know what would be produced by proton-proton collisions?
> Thank you!
> Best
> Stephanie
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Thanks Olivier Mattelaer, that solved my question.