MadGraph5_aMC@NLO

What is inside one "vertex" in the '.hepmc' file?

Asked by Henry

This might be a pythia question, if so apologies. Also a strongly suspect that the answer to this is somewhere in https://arxiv.org/pdf/1106.0522.pdf but I'm having trouble locating it, again apologies.

Reading the output of a madgraph run in the hepmc file there are a number of vertices that wouldn't correspond to a single interaction like a feynman diagram vertex.

Like this one;
V -49 0 0 0 0 0 1 3 0
P 1 2212 0 0 6.4999999322807234e+03 6.5000000000000000e+03 9.3827000000000005e-01 4 0 0 -49 0
P 66 21 2.0869421984099348e-01 6.0904627753566774e-02 5.1174502998488282e+02 5.1174507616280954e+02 0 61 0 0 -22 2 1 507 2 501
P 84 2101 -1.0018651117902214e-01 3.3600458813266509e-01 5.6170040605700478e+03 5.6170041013889304e+03 5.7933000000000001e-01 63 0 0 -113 1 2 507
P 85 2 -1.0850770866197118e-01 -3.9690921588623190e-01 3.7125047328087715e+02 3.7125084797495583e+02 3.3000000000000002e-01 63 0 0 -115 1 1 501
V -50 0 0 0 0 0 1 2 0

A proton goes into the vertex and out comes 'u', 'ud', and 'g'. Presumably the proton split up then the up quark radiated the gluon.
It's all given as one vertex.

There are much longer examples, when 10+ particles are coming from a vertex. Sometimes lots of particles going into one vertex too. I think these examples might be from the hadronization step, so really would be a pythia question. That's not what is happening to the proton though.

Am I right in thinking that one vertex is holding more than one interaction? Roughly what governs how many interactions in one vertex?

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

Hi,

You should look at the hepmc paper for that.

But already the output of madgraph does not provide enough information to reconstruct a unique Feynman Diagram.
The same should happens for the PS as well.

Cheers,

Olivier

> On 2 Feb 2021, at 15:55, Henry <email address hidden> wrote:
>
> New question #695328 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695328
>
> This might be a pythia question, if so apologies. Also a strongly suspect that the answer to this is somewhere in https://arxiv.org/pdf/1106.0522.pdf but I'm having trouble locating it, again apologies.
>
> Reading the output of a madgraph run in the hepmc file there are a number of vertices that wouldn't correspond to a single interaction like a feynman diagram vertex.
>
> Like this one;
> V -49 0 0 0 0 0 1 3 0
> P 1 2212 0 0 6.4999999322807234e+03 6.5000000000000000e+03 9.3827000000000005e-01 4 0 0 -49 0
> P 66 21 2.0869421984099348e-01 6.0904627753566774e-02 5.1174502998488282e+02 5.1174507616280954e+02 0 61 0 0 -22 2 1 507 2 501
> P 84 2101 -1.0018651117902214e-01 3.3600458813266509e-01 5.6170040605700478e+03 5.6170041013889304e+03 5.7933000000000001e-01 63 0 0 -113 1 2 507
> P 85 2 -1.0850770866197118e-01 -3.9690921588623190e-01 3.7125047328087715e+02 3.7125084797495583e+02 3.3000000000000002e-01 63 0 0 -115 1 1 501
> V -50 0 0 0 0 0 1 2 0
>
> A proton goes into the vertex and out comes 'u', 'ud', and 'g'. Presumably the proton split up then the up quark radiated the gluon.
> It's all given as one vertex.
>
> There are much longer examples, when 10+ particles are coming from a vertex. Sometimes lots of particles going into one vertex too. I think these examples might be from the hadronization step, so really would be a pythia question. That's not what is happening to the proton though.
>
> Am I right in thinking that one vertex is holding more than one interaction? Roughly what governs how many interactions in one vertex?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Henry (disreputabledog) said : 		#2

Thanks Olivier Mattelaer, that solved my question.