QED order in WW scattering

Hi Andrew,

In fact that should all have the same cross-section.
Indeed they are no (tree-level) diagram with QED=5. QED=6,…QED=8
In the three cases you have exactly 774 diagrams. (i.e. the same diagram)

So I'm in fact worry about the QED=6 case.
My guess is that for QED=6 you ask for
w+ > l+ vl (repect w- > l- vl~)
this will explain why you have exactly a factor 4.

Cheers,

Olivier

On Aug 24, 2013, at 5:31 PM, Andrew Levin <email address hidden> wrote:

> New question #234572 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/234572
>
> I have calculated the cross sections for the following processes:
>
> mg5>generate p p > w+ w+ p p QED=4 QCD=0, w+ > e+ ve
> mg5>add process p p > w- w- p p QED=4 QCD=0, w- > e- ve~
>
> mg5>generate p p > w+ w+ p p QED=6 QCD=0, w+ > e+ ve
> mg5>add process p p > w- w- p p QED=6 QCD=0, w- > e- ve~
>
> mg5>generate p p > w+ w+ p p QED=8 QCD=0, w+ > e+ ve
> mg5>add process p p > w- w- p p QED=8 QCD=0, w- > e- ve~
>
> The cross sections are:
>
> For QED = 4: 0.003345 ± 5.7e-06
>
> For QED = 6: 0.01335 ± 2.3e-05
>
> For QED = 8: 0.003345 ± 5.7e-06
>
> Can someone please explain how the cross section for QED = 8 and QED = 4 are the same, but they are smaller than the cross section for QED = 6? I thought this was a maximal order, so I would expect the cross section to increase with QED order, unless there is some very weird interference.
>
> Thanks.
>
> Andrew
>
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