Which parameters in run_card for NO matching?

Dear Maria,

Such process definition are just not physical without any kind of merging.
Since you will have a lot of double/triple counting in the cross-section by doing such computation.

But setting ickkw=0 and xqcut=0 will avoid to applied any kind of cut related to MLM matching/merging.
Which allow you to perform other type of merging related to the pardon-shower.

Cheers,

Olivier

> On Jul 8, 2016, at 17:47, Maria Giulia Ratti <email address hidden> wrote:
>
> New question #296147 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/296147
>
> Dear experts,
> I would like to compute the parton-level cross section for the direct pair-production of squarks in mssm, something on the line of:
>
> generate p p > susylq susylq~ $ go susyweak @1
> add process p p > susylq susylq~ j $ go susyweak @2
> add process p p > susylq susylq~ j j $ go susyweak @3
>
> I would like to do this only at the parton level for the moment - with no showering at all.
>
> Sorry if you find this question trivial but what are exactly the parameters in the run_card.dat that I have to set in order to be sure I am not performing any sort of matching procedure ?
> I think it must be ickkw=0; but I am not sure if this is enough.
>
> Many thanks,
> Maria Giulia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Olivier,

thanks for the reply. I am not sure I fully understand it though: do you mean that there will be some double/triple counting at the level of the matrix element itsself?

Thanks,
Maria Giulia

Dear Maria,

The cross-section computed are inclusive of any additional radiation.
This is why the pardon-shower does not change the cross-section (it only pass from inclusive to exclusive pattern).
So yes the double counting is already present at the level of the matrix-element.

Cheers,

Olivier

> On Jul 8, 2016, at 23:37, Maria Giulia Ratti <email address hidden> wrote:
>
> Question #296147 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/296147
>
> Maria Giulia Ratti posted a new comment:
> Dear Olivier,
>
> thanks for the reply. I am not sure I fully understand it though: do you
> mean that there will be some double/triple counting at the level of the
> matrix element itsself?
>
> Thanks,
> Maria Giulia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

===> The cross-section computed are inclusive of any additional radiation.

That's what I have studied in QCD. That as soon as you try to compute the O(alpha_S) correction to processes with quarks in the final state, you start getting soft and collinear divergences unless you sum over virtual and real correction at a given order.

I think I need to go a step back here.
If we take a simpler process, like e+e- > hadrons , at born level the cross section is the same as for e+e- > mu+mu- apart for the color factors.
If I generate the process e+e- > j j in MG I obtain diagrams with two quarks in the finale state. This should correspond to the Born level approximation of e+e- => jets , right?

So now if I generate the process e+e- > j j j in MG , does this represent the O(alpha_S) correction of e+e- > hadrons ?
I don't think so; because if I look at the diagrams I don't see the virtual correction that is needed to restore the finiteness of the result and that would make the result inclusive.

So in which sense are cross sections computed in MG inclusive od any additional radiation ?

Thanks very much,
Maria Giulia

Dear Maria,

I should refer you to the Peskin/schroeder or your favorite book about QCD/ cross-section computation/ diffusion theory.
This email/website is not the correct format for such type of lecture.

Cheers,

Olivier

> On Jul 10, 2016, at 16:37, Maria Giulia Ratti <email address hidden> wrote:
>
> Question #296147 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/296147
>
> Maria Giulia Ratti posted a new comment:
>
> ===> The cross-section computed are inclusive of any additional radiation.
>
> That's what I have studied in QCD. That as soon as you try to compute
> the O(alpha_S) correction to processes with quarks in the final state,
> you start getting soft and collinear divergences unless you sum over
> virtual and real correction at a given order.
>
> I think I need to go a step back here.
> If we take a simpler process, like e+e- > hadrons , at born level the cross section is the same as for e+e- > mu+mu- apart for the color factors.
> If I generate the process e+e- > j j in MG I obtain diagrams with two quarks in the finale state. This should correspond to the Born level approximation of e+e- => jets , right?
>
> So now if I generate the process e+e- > j j j in MG , does this represent the O(alpha_S) correction of e+e- > hadrons ?
> I don't think so; because if I look at the diagrams I don't see the virtual correction that is needed to restore the finiteness of the result and that would make the result inclusive.
>
> So in which sense are cross sections computed in MG inclusive od any
> additional radiation ?
>
> Thanks very much,
> Maria Giulia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.