MadGraph5_aMC@NLO

Narrow-Width Approximation for top pair production.

Asked by Saurabh Niyogi

This is regarding our confusion on how Madgraph uses Narrow-Width Approximation for final states induced by a pair of top quarks. We have obtained the following results :
p p > t t~ > b w+ b~ w- = 550 pb (This is independent of "bwcutoff")
p p > t t~, t > b w+, t~ > b~ w- = 529.5 pb which gradually increases as we increase "bwcutoff".
I must say that the diagrams in both the cases are exactly the same.
The first result is more accurate in terms of original top pair production, p p > t t~ = 554 pb

On the contrary, CalcHEP gives same numbers for all such entries which are almost equal to basic p p > t t~ cross section.

In this light, how do we understand the MadGraph numbers? At what limit in MadGraph all
three numbers are approximately the same, i.e., equal to the cross section for p p > t t~?

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Saurabh,

All those numbers are perfectly correct.
Please read https://answers.launchpad.net/madgraph5/+faq/2014
about the meaning of the BWcutoff and the reason why we have introduce such cut.

Note that this is only a cut, i.e. if you remove it (i.e. set it to 1000) you will recover the value that you get for p p > t t~ > b w+ b~ w-.

Cheers,

Olivier

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