Quartic coupling diagram

Daer Doung,

You can easily do what you ask for, e.g. by simply adding an additional parameter to the SM model files, and let this parameter regulate the strength of the vertex in question. Copy the models/sm directory to models/sm_4v, and add in the sm_4v/parameters.py file:

fwwaa = Parameter(name = 'fwwaa',
                  nature = 'external',
                  type = 'real',
                  value = 1,
                  texname = '\\text{fwwaa}',
                  lhablock = 'MYPARAMS',
                  lhacode = [ 1 ])

then modify the relevant coupling (GC_28, as you can see from interaction V_9 in the vertices.py file) to

GC_28 = Coupling(name = 'GC_28',
                 value = 'fwwaa*complex(0,1)*gw**2*sw**2',
                 order = {'QED':2})

in couplings.py. You can then set the strength of the coupling relative to the SM directly in the param_card.dat (in the myparams block) after creating your process.

Note that you need to import the new model using
import model sm_mod-full
to avoid applying the default restriction card restrict_default.dat, which doesn't have the new myparams block (you can read about restriction cards in the MG5 release paper).

You can now set the strength of the new fwwaa parameter to 1000 or to 0, to get 4-vertex dominance or completely remove the 4-vertex.

Caveat: Note that the process will no longer be (QED) gauge invariant when you modify the relative strength of this vertex, which also means that we can't guarantee that the result will be Lorentz invariant. So you might want to think of some gauge-invariant way to modify the vertex instead, e.g. using a new heavy particle effective couplings to W and photon. You can test that your model is Lorentz invariant by using the "check" functionality inside MG5, as described in the MG5 release paper. The parameters used in the "check" are the default parameters set in the UFO parameters.py file.

All the best,
Johan