MadGraph5_aMC@NLO

color flow in the events.lhe

Asked by BruceLee

Hi
I am confusing about colour flow tags in the MG5 output lhe events, i.e. ICOLUP(1,I) and ICOLUP(2,I).
Actually we can know the colour flow at the amplitudes level, we can get the colour flow for every diagram. But in the lhe events, for each event, there are two colour flow tags. That's to say, for each event, there is a relevant colour low?
Suppose, there are two different colour structure diagrams, Amp1 & Amp2, and they have different colour flows. We can know the colour flow of Amp1^2 and Amp^2, but what about the interface terms? What does the colour flow of the event mean?

Thanks

Cheers, Bruce

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

The definition of such quantity are defined here:
 E. Boos et al.,in proceedingsof theWorkshoponPhysicsat TeVColliders,LesHouches,France,21 May { 1 June2001[hep-ph/0109068]

Note that such information should be Leading color accurate. And it is Forbidden to include sub-leading color in the event record.

Cheers,

Olivier

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BruceLee (brucelee403) said : 		#2

Dear Olivier

I have already read the paper, I quote a paragraph from [hep-ph/0109068]:
" A specific choice of color flow for a particular event is often unphysical, due to interference effects. However, SHGs require a specific color state from which to begin the shower—it is the responsibility of the user process to provide a sensible choice for the color flow of a particular event."
So, can I suppose the colour flow of events is just a colour state to do the shower? And it has nothing to do with the color structure of the |AMP|^2 or the interference terms? Again, what does the colour flow of the event mean?
And I'm confused with the Leading colour and sub-leading colour.

Thanks

Cheers,
Bruce

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

Hi,

The color flow is indeed only used by Parton shower program as far as I know.
We use the amplitude expressed in the color flow representation to choose which one to write.
We indeed neglect interference between flow when sound such choices. Other codes ( or future version of the code) might choose that in a different way

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BruceLee (brucelee403) said : 		#4

Thanks Olivier Mattelaer, that solved my question.