MadGraph5_aMC@NLO

Get x-section value w/o generating events?

Asked by Noam

Hello MG'ers,
A dummies question,
Is there some way to obtain the x-section of a process (or a bunch of them) from MG without actually generating loads of events?
If not, what would typically be the minimum number of events to be generated to get a reasonable x-section estimation of a simple resonance-only process, if there's even such a typical number?
Thanks!
Noam

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Valentin Hirschi (valentin-hirschi) said : 		#1

At NLO, you can turn the fixed order mode ON and aMC@NLO will not store unweighted events but only return the cross-section (while possibly filling plots as instructed in the fixed order analysis).

At LO, no, you will always generate parton level events. However, the integration is performed in two parts; the survey and the refine. The survey computes the cross-section with a fixed default basic accuracy, irrespectively of the number of events specified; so even if you specify that you want to generate a single event, the integration will still take some time to perform the survey which will provide you with an estimate of the cross-section with some rough accuracy (typically 10% or better).

There is really no "typical number"; you have to figure out what accuracy you need and choose a number of events to generate accordingly. As a rule of thumb, the relative accuracy obtained with N events is 1/sqrt(N).
That being said, 1% (so 10K events) is customary since it is usually well within scale uncertainties.
Also depending on what you want to do (for example a model scan), you might be interested in using the reweighting module (https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Reweight) as it offers faster way to recompute cross-sections in certain cases.

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