avoid unwanted feynman diagrams in SMEFTsim

Asked by Neil Warrack

Hello experts,
I am new to MadGraph and I'm using the SMEFTsim model (http://feynrules.irmp.ucl.ac.be/wiki/SMEFT using this download: SMEFTsim_A_general_alphaScheme_UFO_v2.tar.gz) I am trying to generate p p > t t~ j j with one or two wilson coeffs set to a non-zero value. This produces lots and lots of unwanted Feynman diagrams that will not contribute to the event generation. Is there a way to avoid this?

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
Last query:
Last reply:

This question was reopened

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#1

Would that link help you?

Cheers,

Olivier
FAQ #2312: “FR Model much slower than build-in MG model. Why and how to fix?”.

Revision history for this message
Neil Warrack (nwarrack) said :
#2

Hi Oliver,

Thanks for the reply.

I'm not sure that what you suggest answers my question. However, I don't think I fully understood my question when I asked it (still figuring all this MadGraph stuff out - please forgive me).

I'm interested in looking at the process pp->tt~ through the eyes of a SMEFT. If I tell madgraph that I want a dileptonic decay by specifying the output like so:
pp -> b b~ l+ vl u~ d j j
then I suppose it will generate many diagrams that do not contain top quarks. Can I rephrase my question like this: What is the best way to avoid generating the diagrams that I don't want (ie diagrams without tops). Is it still by means of generating my own restrictions card?

Thanks again for your help. Sorry if this is a trivial question.

Revision history for this message
Neil Warrack (nwarrack) said :
#3

Adding a comment:
If this constitutes a new question then let me know and I can post it as one. Thanks.

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

Hi,

In this case, I guess that what you are looking for is the decay-chain syntax:

generate p p > t t~, (t > w+ b, w+ > l+ vl), ( t~ > w- b~ , w- > l- vl~)

Note that this syntax is associate to a breit-wigner cut to ensure that the top (and W) are "close" to their pole mass.
(You can change the cut in the run_card, but by default "close" means up to 15 times the width of the particle)

Cheers,

Olivier

Revision history for this message
Neil Warrack (nwarrack) said :
#5

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Neil Warrack (nwarrack) said :
#6

We were also worried about very off-shell tops too. But you've answered that concern too. Cheers!

Revision history for this message
Neil Warrack (nwarrack) said :
#7

Sorry to dig this up again. And thanks for any answers provided.
If I do this:
generate p p > t t~, (t > w+ b, w+ > l+ vl), ( t~ > w- b~ , w- > l- vl~)
with the SMEFT (mentioned in original post) and include new physics ie
generate p p > t t~ / a h z NP^2==1, (t > w+ b, w+ > l+ vl), ( t~ > w- b~ , w- > l- vl~)

I get an error. The error is: "Decay processes cannot specify squared orders constraints"

So, using the following to create, for example, a dilepton decay of a ttbar sytem which can include the new physics of the EFT model (i'm interested in new physics coupling to the top quark and so looking at BSM vertices with tops):
generate p p > t t~ > b b~ e+ e- ve ve~ / a h z u u~ d d~ c c~ s s~ b b~ NP^2==1

works, but if I want to get rid of the diagrams with w-bosons in the t-channel then how can I do that? If i use:
generate p p > t t~ > b b~ w+ w- / w+ w- a h z u u~ d d~ c c~ s s~ b b~ NP^2==1

and then pick out events with dilepton decays that I want then I must loose the four-fermion t-b-l-vl vertex which I want to keep - because it has a top in it.

So is there a way get MG to create some p p > t t~ > (dilepton) processes using an EFT model but have it not generate unwanted diagrams which I could define as all diagrams with BSM vertices that do not include a top?

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said :
#8

Hi,

This is an highly non trivial diagram filtering. That require that you wrote your own filtering (which is not trivial).
You should look at the following page:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-15
Which actually include an example, which is quite close to the one your are referring to.

> which I could define as all diagrams with BSM vertices that do not include a top?

If that statement was true, then the following syntax should have been enough:
generate p p > t t~ > b b~ e+ e- ve ve~ NP^2==1

Actually, did you try the following one:
generate p p > t t~ w+ w- > b b~ e+ e- ve ve~ NP^2==1
I doubt that it will fit what you want, but at the same time it is not clear to me what you are looking for.

Cheers,

Olivier

Revision history for this message
Neil Warrack (nwarrack) said :
#9

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Neil Warrack (nwarrack) said :
#10

Sorry for the very slow reply. Thanks for your help! I appreciate it!