MadGraph5_aMC@NLO

QED=99 QCD=99 and decays within madgraph

Asked by Alberto Gascón

Hi all,

I have to separate questions.
(1) I'm generating final states with t tbar t tbar. What's the practical difference between

g g > t t~ t t~ and
g g > t t~ t t~ QED=99 QCD=99

As far as I know, the QED and QCD options control the maximum allowed QED and QCD order of the process, but what I remember from QFT does not allow me to grasp, in practical terms, which one would be "better" (if any). In principle I don't intend to suppress or focus on any particular diagram. Similarly, I'm generating 4t final states within a 2 Higgs doublet model. What would be again the difference between

g g > t t~ h2, h2 > t t~ and
g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ QED=99 QCD=99

where QED...QCD... is included in any of the two possible places. Toying with Madgraph I realized I could insert the option in any (or both) , and the number of diagrams generated differs.

(2) As for the other question, I see that I could either decay the tops at madgraph level

generate g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > j j)
add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > j j)
add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > l- vl~)
add process g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > l- vl~)

and run pythia, or generate the process as g g > t t~ t t~ only and let them decay when I run pythia/pgs. Would both decays give the same results after running pythia in terms of branching ratios, pt distributions, etc...

I'm using mg5_aMC version 2.2.2

Cheers,
   Alberto

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

I would suggest that you look at the following tutorial:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/MGTalks/13_06_10_tutomg_tasi.pdf

For the first question, the answer is that if you do not specify the QCD/QED order we select the lowest QED order which has some diagram.

> (2) As for the other question, I see that I could either decay the tops at madgraph level
>
> generate g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > j j)
> add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > j j)
> add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > l- vl~)
> add process g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > l- vl~)
>
> and run pythia, or generate the process as g g > t t~ t t~ only and let them decay when I run pythia/pgs. Would both decays give the same results after running pythia in terms of branching ratios, pt distributions, etc…

MadGraph makes a better job, since it include full spin-correlation which is not done in pythia.
You should not expect huge difference in pt, but much more for angle observable.

Cheers,

Olivier

On 15 Jan 2015, at 10:36, Alberto Gascón <email address hidden> wrote:

> New question #260756 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/260756
>
> Hi all,
>
> I have to separate questions.
> (1) I'm generating final states with t tbar t tbar. What's the practical difference between
>
> g g > t t~ t t~ and
> g g > t t~ t t~ QED=99 QCD=99
>
> As far as I know, the QED and QCD options control the maximum allowed QED and QCD order of the process, but what I remember from QFT does not allow me to grasp, in practical terms, which one would be "better" (if any). In principle I don't intend to suppress or focus on any particular diagram. Similarly, I'm generating 4t final states within a 2 Higgs doublet model. What would be again the difference between
>
> g g > t t~ h2, h2 > t t~ and
> g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ QED=99 QCD=99
>
> where QED...QCD... is included in any of the two possible places. Toying with Madgraph I realized I could insert the option in any (or both) , and the number of diagrams generated differs.
>
> (2) As for the other question, I see that I could either decay the tops at madgraph level
>
> generate g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > j j)
> add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > j j)
> add process g g > t t~ t t~ , ( t > b w+, w+ > j j ) , (t~ > b~ w-, w- > l- vl~)
> add process g g > t t~ t t~ , ( t > b w+, w+ > l+ vl) , (t~ > b~ w-, w- > l- vl~)
>
> and run pythia, or generate the process as g g > t t~ t t~ only and let them decay when I run pythia/pgs. Would both decays give the same results after running pythia in terms of branching ratios, pt distributions, etc...
>
> I'm using mg5_aMC version 2.2.2
>
> Cheers,
> Alberto
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Alberto Gascón (albergascon) said : 		#2

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Alberto Gascón (albergascon) said : 		#3

Hi Olivier,

thanks, that helped a lot. However I have a different question now.

Is it the same writing

g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ (16 diagrams) and
g g > t t~ h2 , h2 > t t~ QED=99 QCD=99 (10 diagrams)

For both processes I obtain the same cross section. Does writing

g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ QED=99 QCD=99 (18 diagrams)

make sense? Can I set the order for each subprocess? When I run this last process I get a cross section which is very close
to twice the cross section of

g g > t t~ h2 , h2 > t t~ (9 diagrams)

Getting a factor 2 difference made me suspicious.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#4

Dear Alberto,

> Is it the same writing
>
> g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ (16 diagrams) and
> g g > t t~ h2 , h2 > t t~ QED=99 QCD=99 (10 diagrams)

No those are indeed different as can see by the number of diagram which is different in both case.

> . Does writing
>
> g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ QED=99 QCD=99 (18 diagrams)
>
> make sense? Can I set the order for each subprocess?

Yes this syntax make sense. The order are indeed define for each order.

> Getting a factor 2 difference made me suspicious.

It is difficult to answer your question without knowing about your model. So this is mainly guess based on the information that you provide.
Looks like that in your case some QED interaction are not negligeable and that you do not have a good hierarchy between QED and QCD.
In particular it looks like that the h2 t t~ has a coupling of the same order for QED order QCD.
So looks like the model should have set the QED and QCD hierarchy at the same level to prevent MG to remove by default the QED interaction.
This can be done in the file couplings_order.py of the UFO model.

Cheers,

Olivier

On 15 Jan 2015, at 14:41, Alberto Gascón <email address hidden> wrote:

> Question #260756 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/260756
>
> Alberto Gascón posted a new comment:
> Hi Olivier,
>
> thanks, that helped a lot. However I have a different question now.
>
> Is it the same writing
>
> g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ (16 diagrams) and
> g g > t t~ h2 , h2 > t t~ QED=99 QCD=99 (10 diagrams)
>
> For both processes I obtain the same cross section. Does writing
>
> g g > t t~ h2 QED=99 QCD=99 , h2 > t t~ QED=99 QCD=99 (18 diagrams)
>
> make sense? Can I set the order for each subprocess? When I run this last process I get a cross section which is very close
> to twice the cross section of
>
> g g > t t~ h2 , h2 > t t~ (9 diagrams)
>
> Getting a factor 2 difference made me suspicious.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.