identifying the feynman diagram for a specific event

Associating a particular Feynman diagram to a given LesHouches partonic event makes no sense from a quantum physics standpoint.

I imagine you are instead asking about the relative contribution of a resonant s-channel contribution versus other non-resonant ones, which is indeed a reasonable question within the narrow-width approximation.

Whenever MG5aMC produce an event from the integration channel featuring the resonant particle, it will write it as an intermediate particle in the LesHouches event record if it is sufficiently onshell (i.e. threshold controlled by the parameter bwcut in the run_card.dat).

So one could in principle use the above to decide on the relative contribution from one to another. I suggest however that you use a more direct approach and independently generate the subprocess with and without the mediator resonance (use the / syntax to forbid particles, or the '... > forced_s-channel > ...' to force s-channels).
This will unambiguously tell you the relative contribution of the square of the resonant and non-resonant diagrams respectively, as well as the magnitude of their interference.

Hi,

> Associating a particular Feynman diagram to a given LesHouches partonic
> event makes no sense from a quantum physics standpoint.

Let me first stress the above point.
Obviously this can makes (partly) sense if you select gauge invariant subset of diagram.

So if you want to play with that, one option is to use the re-weighting method and to (re-)associate to the events generated
weights associated to the various matrix-element square (and/or interference term).
For details on how to do that please read
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Reweight

Cheers,

Olivier

> On Oct 4, 2016, at 02:17, Valentin Hirschi <email address hidden> wrote:
>
> Question #402735 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/402735
>
> Status: Open => Answered
>
> Valentin Hirschi proposed the following answer:
> Associating a particular Feynman diagram to a given LesHouches partonic
> event makes no sense from a quantum physics standpoint.
>
> I imagine you are instead asking about the relative contribution of a
> resonant s-channel contribution versus other non-resonant ones, which is
> indeed a reasonable question within the narrow-width approximation.
>
> Whenever MG5aMC produce an event from the integration channel featuring
> the resonant particle, it will write it as an intermediate particle in
> the LesHouches event record if it is sufficiently onshell (i.e.
> threshold controlled by the parameter bwcut in the run_card.dat).
>
> So one could in principle use the above to decide on the relative contribution from one to another. I suggest however that you use a more direct approach and independently generate the subprocess with and without the mediator resonance (use the / syntax to forbid particles, or the '... > forced_s-channel > ...' to force s-channels).
> This will unambiguously tell you the relative contribution of the square of the resonant and non-resonant diagrams respectively, as well as the magnitude of their interference.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hello Valentin and Olivier,
   Thanks for your suggestions. I am generating the subprocesses with ">" and "/" syntax now to help myself understanding more abut the model.