MadGraph5_aMC@NLO

Changing parameters of a ufo model each time before generating a new event?

Asked by Jona Ackerschott

I wanted to ask if it is possible in madgraph to generate events, each with different the parameters in the used ufo model, effectively without needing to re-initialize the entire generation. Effectively this would allow one to marginalize the parton level distribution over some distribution of model parameters.

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

Hi,

Well it does depend of what you exactly mean.

We do perform running on alphas which is indeed a modification of the coupling at each phase-space point. Other type of running are also supported (like EFT running) and you can certainly hack the code to use those functionalities to change things even more.

Now one need to be very careful about what you are doing since modifying mass and width events by events is not allowed/not possible. Indeed since you want to generate event, this means that you need to do a phase-space integration, such phase-space integration is impossible to do with brute force Monte-Carlo method (but for the simple case like 2>2 process without resonances)
So we were force to use the knowledge of the function that we are integrating to design a fine tuned integrator.
Such integrator is using the mass, width, cuts and the feynman diagram of the process under consideration. So changing the mass and width might lead to side effect not handle correctly by the phase-space.

Cheers,

Olivier

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Jona Ackerschott (jackerschott) said : 		#2

I see. What I specifically have in mind would be marginalizing the generated parton-level distribution over some pre-determined distribution over the Wilson-Coefficients of a SMEFT UFO model. But it sounds like this is not easily possible without having some deeper knowledge about MadGraph/MC-generators and having to change some internal code?

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

If you would have expected a plug and play solution, then indeed the answer is simply NO.

> marginalizing the generated parton-level distribution over some pre-determined distribution over the Wilson-Coefficients

Ok this should be doable.

> But it sounds like this is not easily possible without having some deeper knowledge about MadGraph/MC-generators and having to change some internal code?

I would say that indeed you need to know
1) how a phase-space integrator is working (mainly only the theory not the implementation)
2) What kind of modification to the amplitude will be allowed due to your change of coefficient --important for one--
3) know a bit of Fortran to hack the file Source/MODEL/coupl2.f to change your couplings on the flight

Cheers,

Olivier

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Jona Ackerschott (jackerschott) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

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Mr Stephen (mrstephen12) said : 		#5

Changing parameters of a UFO model before generating a new event can add variety and realism to simulations or visualizations. By adjusting parameters such as speed, size, color, and trajectory, you can create different scenarios and behaviors for the UFO in each event.
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