madspin and fixed order calculation

Asked by Oleksandr Zenaiev on 2018-01-25

Dear experts,
is it possible to generate events at NLO (not matched to parton shower) and decays with Madspin? When I set 'fixed_order=ON' the programs says "INFO: For coherence 'madspin' is set to 'OFF'", what is the reason for this? I would like to generate pp > t t~ at NLO with dileptonically decaying top quark pair.
Thanks,
Sasha.

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2018-01-25
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2018-01-25

Hi Sasha,

The main reason is that it is not possible to generate unweighted event in that configuration (and obviously those event can not be passed to a parton-shower).
Therefore we do not generate any event file in that mode by default and propose to only do plotting instead.

Now one mode of the NLO output is indeed to create a (huge) LHE file to allow to use standard tools of plotting.
I actually played in the past with that lhe file to be able to link it to madspin but this is not public version.
Also my implementation was not covering the "standard" madspin but only one of the special mode of madspin (it would be very difficult to do so on the default mode of madspin).

So this is technically possible and mainly waiting for physical motivation and to find someone willing to implement such feature

Cheers,

Olivier

> On 25 Jan 2018, at 17:07, Oleksandr Zenaiev <email address hidden> wrote:
>
> New question #663585 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/663585
>
> Dear experts,
> is it possible to generate events at NLO (not matched to parton shower) and decays with Madspin? When I set 'fixed_order=ON' the programs says "INFO: For coherence 'madspin' is set to 'OFF'", what is the reason for this? I would like to generate pp > t t~ at NLO with dileptonically decaying top quark pair.
> Thanks,
> Sasha.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Oleksandr Zenaiev (zenaiev) said : #2

Hi Olivier,
thanks for answering my question. I was looking specifically at the fixed-order mode to be able to use aMCfast interface to create tables which allow quick convolution with another PDFs and can be used in PDF fits, but aMCfast can be used only in the fixed-order mode.
Best regards,
Sasha.

Hi,

I do not know the details on aMCfast to be honest, but do you really need the full offshell effect and spin correlation?
My guess is that the easiest would be that in the analysis (or interface to aMCfast) one decay the particle via simple kernel like it is done inside parton-shower.

Cheers,

Olivier

> On 25 Jan 2018, at 22:07, Oleksandr Zenaiev <email address hidden> wrote:
>
> Question #663585 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/663585
>
> Oleksandr Zenaiev posted a new comment:
> Hi Olivier,
> thanks for answering my question. I was looking specifically at the fixed-order mode to be able to use aMCfast interface to create tables which allow quick convolution with another PDFs and can be used in PDF fits, but aMCfast can be used only in the fixed-order mode.
> Best regards,
> Sasha.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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