I wonder how the analytical formula of the Lorentz structure is calculated in the lorentz.py file in UFO format. Currently, I'm modifying the python files by hand to characterize our DarkSUSY model. The decay chains we consider is
@1: h -> n2 + n2,
@2: n2 -> n1 + gamma_d,
@3: gamma_d -> mu^+ + mu^-,
Basically, Higgs decays into two neutralinos n2(spin 1/2) that decay into dark neutralino n1 (spin 1/2) and dark photon gamma_d(spin 1). Dark photon finally decays into two muons with opposite charge.
So far I can simply determine the lorentz structures in @1,
FFS1 = Lorentz(name = 'FFS1',
spins = [ 2, 2, 1 ],
structure = 'Identity(2,1)')
FFV1 = Lorentz(name = 'FFV1',
spins = [ 2, 2, 3 ],
structure = 'Gamma(3,2,1)')
But I don't know how the Lorentz structure for vertex @2 is determined. Can someone point me to some materials or comment on how the lorentz structure component is calculated?
The current particles.py in UFO is here: https://github.com/weishi10141993/DarkSUSY_MC_MadGraph5_aMC-NLO/blob/master/Two_dimuon/particles.py#L257-L288
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