MadGraph5_aMC@NLO

Madgraph doesn't recognize the production of the same particle production on-shell

Asked by Kevin Alberto Urquía Calderón

I am working with a BSM model (it is not publicly available, if needed I can share the UFO files, it was presented in https://arxiv.org/abs/1401.3345).

I am trying to produce the same particle on shell twice. This is my process:

generate e- e+ > n4 n4 > e- u d~ e+ u~ d /h hm2 h+ h3 h2 w+

Here n4 is a Majorana particle, there is therefore no n4~, the particles excluded should be very heavy scalars or particles with suppressed interactions. But when I want to see the diagrams that would generate such a process, for some of the I have not generated two on-shell n4's. And I could see that the output code said:

Drawing Process: e- e+ > n4 > e- u d~ e+ u~ d WEIGHTED<=12 /h hm2 h+ h3 h2 w+ @1

So it was only producing one n4. I could mimic the same process using one muon instead, and n5 (n5 couples to muons, n4 to electrons, n6 to taus):

generate e- mu+ > n4 n5 > e- u d~ mu+ u~ d /h hm2 h+ h3 h2 w+

And now this one works, so it seems that there is a bug with MadGraph not taking in the same particle twice?

I am working with version 3.3.1

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

The syntax " > A > " is not designed for onshell production.
If you want onshell production then you can use a syntax like
generate e- e+ > n4 n4 / h hm2 h+ h3 h2 w+, n4 > e- u d~ , n4 > e+ u~ d

The syntax "> A >" is a syntax doing diagram filtering requesting A to be at least once in S-channel.
The syntax "> A A >" does not have any dedicated meaning, so you just ask twice to have at least one "A" in S-channel and therefore this is consider as a redundent request. This is consider as un "undefined" behavior. So, maybe in the future someone will implement a change of syntax and implement it as a diagram filtering requesting two S-channel propagator. But we are quite reluctant to implement diagram filtering for off-shell particle since this can lead of breaking gauge and lorentz invariance in a very easy way (typicaly without the user realizing it).

Cheers,

Olivier

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