MadGraph5_aMC@NLO

Functionality of Pythia8 in MadGraph for leptonic final states

Asked by Nicko Angelo Rabang

Hi Olivier! It is well-known that Pythia8 simulates the corresponding showering and hadronization for final state quarks but what is the function of Pythia8 in MadGraph for leptonic final states? Does it simulate ISR and FSR by default?

Thanks!

Sincerely,
Angelo

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

Hi,

Obviously, you do not have ISR for final state lepton. FSR for final state lepton will be handle by pythia8 in the same was as for final state quark and has in that case, you have to make the (physics) choice to either include some radiation in MG or fully trust Pyhtia depending on how hard are the radiation.

For ISR, you have to do a physics choice as well. since you can either include the ISR within the PDF definition
(and potentially beamstrahlung as well) or include the radiation in the final state directly or just trust Pythia8.
One particullarly critical issue here is that if you are above a resonance, it is important to allow such ISR to hit the resonance and this will be modeled correctly only if the ISR is part of the PDF (or if you allow a hard radiation in the final state).

Cheers,

Olivier

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Nicko Angelo Rabang (nyckxxstark) said : 		#2

Thanks Olivier! The process that I am looking at is the radiative return, mu- mu+ > e- e+ a with a hard photon. Does the FSR simulation of Pythia8 has an effect for that process?

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Effect is a vague word, so yes I do hope so.
But the only way to be specific is to run twice with FSR turn ON and OFF and compare what it does mean.

Cheers,

Olivier

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Nicko Angelo Rabang (nyckxxstark) said : 		#4

Thanks Olivier Mattelaer, that solved my question.