MadGraph5_aMC@NLO

Cross Sections and graphs

Asked by Manuel

Hello, everyone:

This might seem like a silly question: Does MG do interference of different diagrams? If it does, then why does it quote cross sections for each individual Feynman Diagram I have?

For example, if do:

generate u g > u g

I have three diagrams: one with an s-channel u-quark, one with a t/u-channel u-quark and one with a t/u-channel gluon. All of these diagrams have same initial and final particles and thus they should interfere with each other.

And yet MadGraph, in the index.html file, in the "Results and Event Database" section, quotes values for the cross section for EACH diagram, and then adds them up and says that's the total cross section of the process.

The same happens if I do, say, p p > j j and analyze the g g > g g part of the process: different diagrams have quoted cross sections.

What does this mean? I tried looking at Manuals and Single-Diagram-Enhanced but nothing was explaining this.

Thanks a lot.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Manuel
Solved:
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

> Does MG do interference of different diagrams?

Sure

> If it does, then why does it quote cross sections for each individual Feynman Diagram I have?

Because this is not the cross-section of an individual Feynman Diagram.

> What does this mean? I tried looking at Manuals and Single-Diagram-Enhanced but nothing was explaining this.

Yes this is the individual number of the single-diagram-enhanced method.
You can see details on this talk:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/Lund2014/Lund_3_MCGeneration.pdf
page 48

Where you can see the definition of each of the element of the G directory and why when you make the sum you get the correct cross-section with the interference.

Cheers,

Olivier

You have a discussion on this topic here:
https://answers.launchpad.net/mg5amcnlo/+question/187387

On 21 Apr 2015, at 15:51, Manuel <email address hidden> wrote:

> New question #265594 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/265594
>
> Hello, everyone:
>
> This might seem like a silly question: Does MG do interference of different diagrams? If it does, then why does it quote cross sections for each individual Feynman Diagram I have?
>
> For example, if do:
>
> generate u g > u g
>
> I have three diagrams: one with an s-channel u-quark, one with a t/u-channel u-quark and one with a t/u-channel gluon. All of these diagrams have same initial and final particles and thus they should interfere with each other.
>
> And yet MadGraph, in the index.html file, in the "Results and Event Database" section, quotes values for the cross section for EACH diagram, and then adds them up and says that's the total cross section of the process.
>
> The same happens if I do, say, p p > j j and analyze the g g > g g part of the process: different diagrams have quoted cross sections.
>
> What does this mean? I tried looking at Manuals and Single-Diagram-Enhanced but nothing was explaining this.
>
> Thanks a lot.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Manuel (buenabad) said : 		#2

Thanks!! That was really helpful.