Gluon-fusion heft + jets with MG5

Asked by Nikolaos Rompotis

Dear Madgraph experts,

I have a naive question about gluon-fusion higgs production
with madgraph heft model.

I am trying to generate samples with a gluon-fusion produced
Higgs in a 2HDM and I have combined 2HDM and heft models.
The question is nevertheless irrelevant to the 2HDM part and concerns only heft.
I was wondering what is the difference between doing simply
(A)
generate g g > H
and
(B)
generate g g > H
add process g g > H j
(where j is a jet).

In particular I would like information about the following:
(1) I will use in the end Pythia for hadronization anyways and I will get
jets. If so what is the advantage of using (B) over (A)? If there is no
advantage, then is there any advantage of using Madgraph with (A) + Pythia for
hadronization vs using Pythia all the way through?
(2) Is it recommended to use gluon-fusion + jets under heft? I mean is it
known for instance that there is a huge difference if you compare it with
the full calculation, so we should avoid using heft?

Thank you very much for your time, it is highly appreciated.

Best Regards,

Nikos

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi Nikos,

The problem is that the shower (pythia) is perfect to generate soft jet (where the resubmation of the logarithm is important) but not hard jet (where the collinear approximation breaks down and it require the full matrix element). Matrix element generator have the exact opposite behavior (problem with soft jet).

So if you do this
> generate g g > H

pythia will generate all the radiation including the hard one, and therefore your hard radiation will be poorly simulated.

If you do:
> generate g g > H
> add process g g > H j
Then it is possible to tell pythia to not simulate hard radiation (from the g g > H sample) such that those radiation are correctly describe.
This is what we call the matching/merging.

You will find more information about matching/merging at the following page:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/IntroMatching
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching

Cheers,

Olivier

On Nov 14, 2013, at 9:01 AM, Nikolaos Rompotis <email address hidden> wrote:

> New question #239205 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/239205
>
> Dear Madgraph experts,
>
> I have a naive question about gluon-fusion higgs production
> with madgraph heft model.
>
> I am trying to generate samples with a gluon-fusion produced
> Higgs in a 2HDM and I have combined 2HDM and heft models.
> The question is nevertheless irrelevant to the 2HDM part and concerns only heft.
> I was wondering what is the difference between doing simply
> (A)
> generate g g > H
> and
> (B)
> generate g g > H
> add process g g > H j
> (where j is a jet).
>
> In particular I would like information about the following:
> (1) I will use in the end Pythia for hadronization anyways and I will get
> jets. If so what is the advantage of using (B) over (A)? If there is no
> advantage, then is there any advantage of using Madgraph with (A) + Pythia for
> hadronization vs using Pythia all the way through?
> (2) Is it recommended to use gluon-fusion + jets under heft? I mean is it
> known for instance that there is a huge difference if you compare it with
> the full calculation, so we should avoid using heft?
>
> Thank you very much for your time, it is highly appreciated.
>
> Best Regards,
>
> Nikos
>
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Revision history for this message
Nikolaos Rompotis (robotisnicolas) said :
#2

Hi Olivier,

thank you very much for this explanation. It clearly answers my first question.

What about my second question on gluon-fusion + jets under heft?

Kind Regards,

Nikos

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#3

Hi Nikos,

>
> What about my second question on gluon-fusion + jets under heft?

Sorry I forget about that question.
As stated here:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Models/HiggsEffective
(For this approximation to hold, not only should the Higgs mass be smaller than twice the top mass, also all other kinematic variables, such as the transverse momentum of the Higgs boson, should be smaller than 2mt.)

If you add jets in the final state, the same constraint on the approximation occurs.
Therefore for one additional jet, you can compute that the approximation breaks down if the energy of the jet is higher than Mt.

Cheers,

Olivier

On Nov 14, 2013, at 10:56 AM, Nikolaos Rompotis <email address hidden> wrote:

> Question #239205 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/239205
>
> Status: Answered => Open
>
> Nikolaos Rompotis is still having a problem:
> Hi Olivier,
>
> thank you very much for this explanation. It clearly answers my first
> question.
>
> What about my second question on gluon-fusion + jets under heft?
>
> Kind Regards,
>
> Nikos
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Revision history for this message
Nikolaos Rompotis (robotisnicolas) said :
#4

Hi Olivier,

thank you for your clear answer.

For me the pT of the Higgs and the pT of the jets is not
a big problem.

Nevertheless, the mass of the Higgs I want to be high,
from 200 - 1000 GeV.

So, do you suggest that I shouldn't use madgraph heft for that?
I am trying to understand how forbidding your statement is
and whether I should be looking for other generators or
I could still use madgraph knowing that it is not perfect.

Kind Regards,

Nikos

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#5

Hi,

> Nevertheless, the mass of the Higgs I want to be high,
> from 200 - 1000 GeV.

In that case, I would be worry to use the heft model (inside MG or any other generator).
I forward this email to an heft expert. Maybe he can help you on that topic.
which program/approximation to use.

Cheers,

Olivier

On Nov 14, 2013, at 11:36 AM, Nikolaos Rompotis <email address hidden> wrote:

> Question #239205 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/239205
>
> Status: Answered => Open
>
> Nikolaos Rompotis is still having a problem:
> Hi Olivier,
>
> thank you for your clear answer.
>
> For me the pT of the Higgs and the pT of the jets is not
> a big problem.
>
> Nevertheless, the mass of the Higgs I want to be high,
> from 200 - 1000 GeV.
>
> So, do you suggest that I shouldn't use madgraph heft for that?
> I am trying to understand how forbidding your statement is
> and whether I should be looking for other generators or
> I could still use madgraph knowing that it is not perfect.
>
> Kind Regards,
>
> Nikos
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Revision history for this message
Nikolaos Rompotis (robotisnicolas) said :
#6

Thank you very much,
I will be waiting to hear from them.

Kind Regards,

Nikos

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