# Double counting in Drell-Yan production

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

- 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:/

>

> 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.