avoid unwanted feynman diagrams in SMEFTsim

Asked by Neil Warrack on 2018-10-26

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?

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Would that link help you?


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

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.

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.


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)



Neil Warrack (nwarrack) said : #5

Thanks Olivier Mattelaer, that solved my question.

Neil Warrack (nwarrack) said : #6

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

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?


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:
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.



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