on-shell VB

Hi,

> Isn't it right that the later one should be smaller than the former one? how does it happened?
> thanks a lot!

That’s because of negative interference.

Cheers,

Olivier

> On Mar 15, 2016, at 06:27, Shyuan Guo <email address hidden> wrote:
>
> New question #288643 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/288643
>
> i am a beginer to MG. here is my problem:
> firstly, i generated the process
> p p > e+ e-
> at parton level, and the cross section is about 842.95 pb, then i generated process
> p p > z, z > e+ e-
> which resulted in a cross section about 1420 pb.
>
> Isn't it right that the later one should be smaller than the former one? how does it happened?
> thanks a lot!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

HI,

Actually the interference is indeed negative but not enough to explain such large difference.
So the True reason is that by default we do not apply the cut on the particle coming from on shell decay.
(see in the run_card.dat the parameter cut_decays)

Cheers,

Olivier

> On Mar 15, 2016, at 09:03, Olivier Mattelaer <email address hidden> wrote:
>
> Question #288643 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/288643
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi,
>
>> Isn't it right that the later one should be smaller than the former one? how does it happened?
>> thanks a lot!
>
> That’s because of negative interference.
>
> Cheers,
>
> Olivier
>
>> On Mar 15, 2016, at 06:27, Shyuan Guo <email address hidden> wrote:
>>
>> New question #288643 on MadGraph5_aMC@NLO:
>> https://answers.launchpad.net/mg5amcnlo/+question/288643
>>
>> i am a beginer to MG. here is my problem:
>> firstly, i generated the process
>> p p > e+ e-
>> at parton level, and the cross section is about 842.95 pb, then i generated process
>> p p > z, z > e+ e-
>> which resulted in a cross section about 1420 pb.
>>
>> Isn't it right that the later one should be smaller than the former one? how does it happened?
>> thanks a lot!
>>
>> --
>> You received this question notification because you are an answer
>> contact for MadGraph5_aMC@NLO.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

the cross section indeed decrease after implemented the decay cut, to about 600 pb. but i was still puzzled about that, i've checked the plots of pt of the final electron/positron when the decay cut was not implemented, it start from 0 which is contrary with the ptl cut in the run_card. why the ptl cut doesn't work here?

Hi,

Not sure to understand your question/remark.

I have done the following
generate p p > Z, Z > e+ e-
output
launch
done
set cut_decays T
and by looking at the automatic plot, I have indeed a minimal value for the pt.
So I do not understand your question.

Did you run a parton shower (or any other program) in addition of Madgraph?
Those cut do not use the cut defined at MadGraph level and this can explain what you observe.

Cheers,

Olivier

> On Mar 15, 2016, at 13:37, Shyuan Guo <email address hidden> wrote:
>
> Question #288643 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/288643
>
> Status: Answered => Open
>
> Shyuan Guo is still having a problem:
> the cross section indeed decrease after implemented the decay cut, to
> about 600 pb. but i was still puzzled about that, i've checked the plots
> of pt of the final electron/positron when the decay cut was not
> implemented, it start from 0 which is contrary with the ptl cut in the
> run_card. why the ptl cut doesn't work here?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Here was what i've done:
firstly, i generated the process
p p > z, z > e+ e-
output
launch
no modification on the run_card
after launch, checked the plot. i found the pt plot of final electron/ positron start from pt=0, while in the run_card, there was a ptl which set the minimum pt cut of charged lepton(by default set, it's 10GeV). My question was why the ptl does not work on the final electron/ positron here?(since the plot showed pt start from 0)

Hi,

This is because you have the parameter “cut_decays" on False
All particles coming from a forced onshell decay(like here your Z) are put in a special status and we do not apply various cut on those particles.
the pt cut is one of those.

Cheers,

Olivier
> On Mar 15, 2016, at 14:12, Shyuan Guo <email address hidden> wrote:
>
> Question #288643 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/288643
>
> Status: Answered => Open
>
> Shyuan Guo is still having a problem:
> Here was what i've done:
> firstly, i generated the process
> p p > z, z > e+ e-
> output
> launch
> no modification on the run_card
> after launch, checked the plot. i found the pt plot of final electron/ positron start from pt=0, while in the run_card, there was a ptl which set the minimum pt cut of charged lepton(by default set, it's 10GeV). My question was why the ptl does not work on the final electron/ positron here?(since the plot showed pt start from 0)
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

OK, thanks for solving my question.
but why the cut_cards in MG was set in such a strategy(i mean, put the decay products of a forced onshell particle in a special status)?
Thanks for your patience!

Thanks a lot! ^~^

Thanks Olivier Mattelaer, that solved my question.