Default propagator for SM

Hi,

For many model, it does make more sense to not specify the propagator within the model such that we can decide which propagator to use within MG5aMC.

Many choice of propagators are defined within the function
 def get_custom_propa(self, propa, spin, id):
of the file aloha/create_aloha.py
which in particular defines the gluon propagator if you select the axial gauge (line 481).
Those are relatively human readable since they were designed after the creation of the propagators.py feature of UFO model and therefore are based on that feature to simplify the implementation.

The default propagators for all other particles are not human readable (but more machine readable/efficient/less bug prone) and can be found at the end of the file aloha_object.py.
In the case of the gluon, the one used is the
VectorPropagatorMassless= lambda l1, l2, part: complex(0,-1) * Metric(l1, l2)
Which is still human readable.
Note that when those are used, the default denominator is always used as well.

Otherwise, nothing prevent you to update the sm to add the propagators features. Note that you might need to update two other files (that you can take from any bsm models):
- __init__.py
- object_library.py

also note that the example of propagator shipped typically within propagators.py does not correspond to the default value used in madgraph (they are a factor i missmatch if I'm correct)

Cheers,

Olivier