MadGraph5_aMC@NLO

cross-section/graph interpretation

Asked by Shanmuka

Hi,

I am not sure how to interpret the separately listed cross-sections under results.

e.g. in Bhabha scattering (e+ e- > e+ e-), there is the s and t channel. Under results, two cross-sections are listed tabularly with two graphs, i.e. s and t channel graphs. So does one interpret the cross-section next to the graph corresponding to the s-channel as the sum of a pure s-channel process (ss*) + s t* process, whereas the other cross-section next to the t-channel graph is interpreted as the sum of a pure t-channel process (tt*) + t s* process?

Thanks for any ideas.

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Valentin Hirschi
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Best Valentin Hirschi (valentin-hirschi) said : 		#1

As you know, s- and t- channel separation is not physical, and neither are the two cross-sections tabulated in the HTML.

These two cross-section come from the integration structure of MadEvent which uses Multi-channeling techniques.
If we call the *sum* of all the s-channel diagrams S and the complementary sum of all the others T, then these two cross-sections correspond to the integration of

|S+T|^2 * [ |S|^2 / ( |S|^2 + |T|^2 ) ]

and

|S+T|^2 * [ |T|^2 / ( |S|^2 + |T|^2 ) ]

As you can see the multiplicative factor in squared bracket adds up to one and each of them enhances the contribution of the S-channel and T-channel respectively so that they can be separately and efficiently probed in each channel.

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Shanmuka (sshivash) said : 		#2

Thanks Valentin Hirschi, that solved my question.