MadGraph5_aMC@NLO

How can I add particles and lagrangians?

Asked by Yaroslav Basov

Hello!

I'm a beginner, so I can't do some things I need.

I need to add a particle and a vertice into sm model. I found a foulder ./models/sm and there are necessary files. But I can't understand what some of the parameters mean. There is a photon in particles.py:
a = Particle(pdg_code = 22,
             name = 'a',
             antiname = 'a',
             spin = 3,
             color = 1,
             mass = Param.ZERO,
             width = Param.ZERO,
             texname = 'a',
             antitexname = 'a',
             charge = 0,
             GhostNumber = 0,
             LeptonNumber = 0,
             Y = 0)
What is pdg_code? Why is spin 3 instead of 0 for a photon? Where can I find a tutorial with an explanation of this parameters and model addition guide?

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

Hi,

Here you have three strategies:
1) add your particle interactions at the Lagrangian level and in that case, you need to use tools like FeynRules.
2) check if someone did not create the model already -> check the list of public model on the FeynRules model database (https://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage) and/or use google.
3) edit the UFO model by hand, in that case the paper of reference to read is https://feynrules.irmp.ucl.ac.be/wiki/ModelDatabaseMainPage

For the specific point for the spin, the convention is 2s+1, since the spin of the photon is "1" (vector boson) --not 0 which is scalar-- and therefore 2*1+1 = 3

PDG_code is a data identifier of all particle which is an agreement between all the HEP software, so using the correct identifier will ensure that your particle is handle as it should. Convention for PDF code can be found here: https://pdg.lbl.gov/2007/reviews/montecarlorpp.pdf

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Yaroslav Basov (yabasov) said : 		#2

Thanks Olivier Mattelaer, that solved my question.