Cross section increasing after adding deltaeta cut

[Marc Thomas]

Hi,

In MG5_aMC_v2_2_3, I'm using a modified version of the SM in the unitary gauge, with a rescaling of the hWW and hZZ couplings.

For a 50 TeV per beam collision, for the process, u u~ > u u~ h h h,
using the default run_card.dat (other than the 50 TeV beam energies) gives,
cross section = 0.0003895 pb

If I then add the cut,
4 = deltaeta (previously 0 = deltaeta)
without changing any other values, I get
cross section = 0.0004431 pb

So adding a cut on rapdidity INCREASES the cross section!

This occurs with both a 'nn23lo1' and 'cteq6l1' pdf, and regardless of whether the renormalisation and factorisation scales are dynamic or fixed to MZ. A similar thing happens if I replace the up quarks with charms.

However if I don't use PDFs ( 0 = polbeam1, 0 = polbeam2 ), the problem does not occur.

I'm at a loss to explain what's going on or how to fix it.
I don't know if this is a problem with MadGraph or my model?
Any advice would be appreciated.

Thanks,
Marc

[Marc Thomas (mct1g11)]

Hi,

Is anyone able to help at all with this problem?

Thanks,
Marc

[Olivier Mattelaer (olivier-mattelaer)]

Hi,

This is likely to be a problem of the phase-space integrator which is not optimised for VBF like configuration (like you have).
We are discussing on how to fix this in the future.

But this is probably a long shot before we will have something improves for such situation.

Cheers,

Olivier

On 22 Apr 2015, at 16:58, Marc Thomas <email address hidden> wrote:

> Hi,
>
> Is anyone able to help at all with this problem?
>
> Thanks,
> Marc
>
> --
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1439121
>
> Title:
> Cross section increasing after adding deltaeta cut
>
> Status in MadGraph5_aMC@NLO Generator:
> New
>
> Bug description:
> Hi,
>
> In MG5_aMC_v2_2_3, I'm using a modified version of the SM in the
> unitary gauge, with a rescaling of the hWW and hZZ couplings.
>
> For a 50 TeV per beam collision, for the process, u u~ > u u~ h h h,
> using the default run_card.dat (other than the 50 TeV beam energies) gives,
> cross section = 0.0003895 pb
>
> If I then add the cut,
> 4 = deltaeta (previously 0 = deltaeta)
> without changing any other values, I get
> cross section = 0.0004431 pb
>
> So adding a cut on rapdidity INCREASES the cross section!
>
> This occurs with both a 'nn23lo1' and 'cteq6l1' pdf, and regardless
> of whether the renormalisation and factorisation scales are dynamic or
> fixed to MZ. A similar thing happens if I replace the up quarks with
> charms.
>
> However if I don't use PDFs ( 0 = polbeam1, 0 = polbeam2 ),
> the problem does not occur.
>
> I'm at a loss to explain what's going on or how to fix it.
> I don't know if this is a problem with MadGraph or my model?
> Any advice would be appreciated.
>
> Thanks,
> Marc
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/mg5amcnlo/+bug/1439121/+subscriptions

[Marc Thomas (mct1g11)]

OK, thanks Olivier for the clarification.

Cheers,

Marc

[Olivier Mattelaer (olivier-mattelaer)]

I will close this bug report for the moment.

Thanks,

Olivier