MadGraph5_aMC@NLO

Separating production and decay in ttgamma

Asked by Baptiste Ravina

Dear MadGraph experts,

In https://answers.launchpad.net/mg5amcnlo/+question/672405 , a question was asked about how to separate the emission of the photon in the ttgamma process between "production" (ISR and top-gamma coupling) and "decay" modes (emission from W boson, b-quark, or charged decay products of the W boson). While the production mode is trivially accessed as

generate p p > t t~ a

the decay mode was attempted as e.g.

generate p p > t t~, (t > mu+ vm b a), (t~ > e- ve~ b~)

but this (t > mu+ vm b a) syntax produces 4 diagrams: one for the photon being emitted off the W, b and muon, and one from the top (which would overlap with the production mode). It was suggested to use MadSpin for the decays of the tops, but due to the particular setup we are now studying (EFT model with reweighting), this is not possible.

Would the following syntax for the decay mode produce a meaningful result? e.g.

generate p p > t t~, (t > w+ b, w+ > mu+ vm a), (t~ > e- ve~ b~) @0
add process p p > t t~, (t > w+ b, w+ > mu+ vm, b > b a), (t~ > e- ve~ b~) @1
(+ similar lines for the decay of the tbar)

Thanks for your help!

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

The only true answer I can give you is I do not know.
You should check the literature to see how people are doing this.

This being said, I would indeed suggest to use MadSpin in the "onshell" mode to have a un-ambiguous separation between production and decay.

For reweighting, the best is likely to re-weight by the full
p p > b mu+ vm a e- ve~ b~

Cheers,

Olivier

Revision history for this message
Baptiste Ravina (bravina) said : 		#2

Thanks Olivier Mattelaer, that solved my question.

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