# The best way to generate NLO event (ggH)

Asked by JamesB on 2017-12-05

Hello all,

I am generating the following process:

import model heft
generate p p > h j, h > l- vl~ l+ vl

I am struggling to work out at what order MadGraph calculates this event at (presumably leading order?). I would like to calculate the cross-section at next-leading-order, for which I think I need to specify a QCD value (although I'm not sure what). I have assumed QED is not important here as the processes do not involve photons - is this assumption correct?

eg. is the following sufficient?
generate p p > h j, l- vl~ l+ vl QCD=2

I thought if I specified a higher QCD value, there would be additional diagrams (eg. with trees and loops), but this does not appear to be the case.

Thanks for any help,
James

## Question information

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Last query:
2017-12-06
Last reply:
2017-12-06
 Olivier Mattelaer (olivier-mattelaer) said on 2017-12-05: #1

Dear James,

Let’s first forget the Higgs decay since they should not be part of the discussion.

This process is actually quite complex since you have a double perturbation theory in this case.
On one side you have the m_h/mt expansion and on the other side you have the more usual QCD expansion.

When you use the heft model, you put yourself to the infinite top mass limit, which is actually not valid when you have a hard jet.
In that limit, the top quark loop can be contracted to a single effective operator and your syntax correspond therefore to a tree-level diagram.
This is indeed a LO contribution.

The heft model does not contains the information requires to do NLO computation for that you have to download some FeynRules specific model (HC_NLO_X0_UFO.zip)
which allow to compute such process in the infinite top-mass at NLO in QCD. (read the realme of that model to know how to generate such process.

You can also decide to stay at LO accuracy in QCD but include the finite top quark limit by using the loop-induced module of MG5aMC:
(see arXiv:1507.00020 for the syntax)

You can also play with approximation. Some attempt to approximation the NLO result with finite top quark mass effect, are using the NLO method above and then use the loop-improved reweighing method
to re-introduce top mass effect. This produce quite accurate result. (See 1607.00763 for the technical details and reference within for physical method)

Cheers,

Olivier

PS: IN all those method, the decay chain syntax that you use in your example is not supported, and you should do the decay via the madspin module (with the option spinmode=none)

> On Dec 5, 2017, at 19:47, JamesB <email address hidden> wrote:
>
> New question #661423 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/661423
>
> Hello all,
>
> I am generating the following process:
>
> import model heft
> generate p p > h j, h > l- vl~ l+ vl
>
> I am struggling to work out at what order MadGraph calculates this event at (presumably leading order?). I would like to calculate the cross-section at next-leading-order, for which I think I need to specify a QCD value (although I'm not sure what). I have assumed QED is not important here as the processes do not involve photons - is this assumption correct?
>
> eg. is the following sufficient?
> generate p p > h j, l- vl~ l+ vl QCD=2
>
> I thought if I specified a higher QCD value, there would be additional diagrams (eg. with trees and loops), but this does not appear to be the case.
>
> Thanks for any help,
> James
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

 JamesB (jamesb8) said on 2017-12-06: #2

Dear Oliver,

Thank you very much for your answer. I think I will include the finite mass of the top quark for now. I'm still unsure on how to implement this though. Your paper says 'models can be generated via FeynRules thanks to the NLOCT package', but surely something already exists for simple top quark loops?

Regards,
James

 Olivier Mattelaer (olivier-mattelaer) said on 2017-12-06: #3

Hi James,

Since in that case, the model is just sm. You do not have to do anything.
You can just do “generate p p > h j QED=1 [QCD]”

Cheers,

Olivier

> On Dec 6, 2017, at 12:03, JamesB <email address hidden> wrote:
>
> Question #661423 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/661423
>
> Status: Answered => Open
>
> JamesB is still having a problem:
> Dear Oliver,
>
> Thank you very much for your answer. I think I will include the finite
> mass of the top quark for now. I'm still unsure on how to implement this
> though. Your paper says 'models can be generated via FeynRules thanks to
> the NLOCT package', but surely something already exists for simple top
> quark loops?
>
> Perhaps this?
> https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Models/ggHFullLoop#no1
>
> Regards,
> James
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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