FR Model much slower than build-in MG model. Why and how to fix?

Created by Olivier Mattelaer on on 2013-07-17
Keywords:

Original question:

Hello,
I am not sure of this question belongs here or in feynrules. The problem is as follows
When generating a process such as p p > t t~ , I tried using the sm implementation of Madgraph and compared it to the Feynrules sm model file by generating a UFO.

I do not get a discrepancy between the two, however I notice that the Feynrules Sm takes longer to run.
This I think is because the various subprocesses for different flavoured quarks do not get grouped together. whereas all the quark intitiated processes do get grouped together for the Madgraph implementation of SM.

Is there some way in which I can make the Feynrules SM work identically to the one in Madgraph, i.e. how do I get madgraph to group the subprocesses ?

thanks

Kirtimaan

#########################################################
## Answer:
#########################################################

The problem is that FR has not maslless quarks by default, therefore since each quark has a different mass, MG can't merge the various contributions. The solution is to set all those mass to massless at the model level.

This can be done in FR via a restriction file
1) create a file myrestriction.rst
containing
M$Restrictions which is a replacement list for the zero parameters (see Massless.rst in the SM directory for an example).
2) load it after the model using the function LoadRestriction[FILE].

The same effect can be done via a MG5 restriction:
Here you have to define a file
restrict_XXXX.dat in the UFO model
this file should be a valid param_card.dat
(so you can create this file by doing: python write_param_card.py in the UFO directory, then rename the output file)
from this file.
1) all zero/one value are fixed in the model and can't be changed anymore
2) if a Block has two identical value. Those two value are merged to a single one
3) all couplings which are zero for this param_card are removed from the model.

To load this restricted model in mg5. you have to do:
import model MODELNAME-XXXX

if XXXX is default, then the import import model MODELNAME load automatically the restriction restrict_default.dat. The original model can then be retrieve by the command import model MODELNAME-full.

Cheers,

Olivier