Formation of subprocesses

Asked by Jure Drobnak on 2012-03-14


i stumbled upon an issue that might not be too important, but is bothering me anyway.
first i run:
generate p p > j j
output dijet

looking in the dijet/SubProcesses/ folder i find only 5 sub folders
which is a very nice and efficient grouping of diagrams from what i understand.

now my question is, how come i do not get the same nice grouping if take Feynrules program,
import the simple SM ( included therein and export it as a SM_UFO then do:

import model SM_UFO
generate p p > j j
output dijet2

now dijet2 folder contains about 40 subfolders of subprocesses.
my question is why the nice grouping of the diagrams was not performed in the same way
as when using the default mg5 SM?
is the the UFO issue or is written is such a way that this grouping is not possible?

thanks for the reply,
and all the best,

Question information

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Solved by:
Olivier Mattelaer
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Last reply:

Hi Jure,

the SM_UFO is not the same model as the default MG5 model.
The differences are off course quite small (for example the parametrization of the CKM matrix)
but some of them are quite critical for the grouping.

The grouping of the SubProcesses works only if the particles have the same mass.
Since they are all free parameter in the SM_UFO (is it?), the merging is not possible.

In the model that we use by default (import model sm):
- the mass of the u/d/s are set to zero (restriction performed at FR level)
- the mass of the c is set to zero via a MG5 restriction card. (./models/sm/restrict_default.dat)
(so if you do import model sm-full, the c quark will have a non zero mass)

So since all the quark masses are the same (and here equal to zero), we can merge the resultant subprocesses



Jure Drobnak (jure-drobnak) said : #2

Thanks Olivier Mattelaer, that solved my question.

Jure Drobnak (jure-drobnak) said : #3

Thanks a lot for the prompt reply!
Keep up the good work!