Adding SM_Loop_ZPolar_NLO model

• Requestor: Martina Javurkova
• Content: SM model with Z bosons in different polarization states
• Source: Provided by Richard Ruiz [rruiz@ifj.edu.pl], MadGraph author
• Paper: https://arxiv.org/abs/1912.01725
• JIRA: https://its.cern.ch/jira/browse/AGENE-2175

In this model, the Z is redefined to be: Z -> ZX + Z0 + ZT + ZA

• Z and ZX are the same things. It is just a field redefinition. The mass and width of ZX are MZ and WZ, just like in any other SM UFO.
• Z0 has a propagator built solely from longitudinal polarization vectors. See Eq. 3.34 of [1]. For all polarization vectors, mg5amc uses the HELAS convention, which is given in App A.2 of [2]. Literally, the propagator is PI_munu = -i eps_mu(lambda=0) * eps_nu(lambda=0) / (p^2 - M^2 + iWZ * MZ)
• ZT has a propagator built from summing coherently, i.e., at the matrix-element level, over the two transverse polarization vectors
• ZA has a propagator built solely from an auxiliary polarization vector given in Eq 3.36 of [1].

[1] https://arxiv.org/abs/1912.01725
[2] https://inspirehep.net/literature/336604