How to generate polarized particles at LO

We introduce a new syntax ((2.7.0) to handle the polarization state from initial and final particles: the {} flag
generate e+{L} e-{R} > w+{0} w-{T}

The argument in the {} can be any combination of the following one:

spin 1/2:
"L" or "+" for the left handed helicity
"R" or "-" for the right handed helicity

spin 1:
"0" for longitudinal polarization
"T" for transverse polarization
"A" for the auxiliary polarization (for offshell particle)

The syntax is also available in conjunction of decay-chain formalism in the following way:
generate p p> w+{0} w-{T}, w+ > e+ ve, w- > mu- vm~

For MadEvent, you can choose the frame in which the matrix-element is evaluated allowing you
to define uniquely what is L/R/T/0 via the run_card parameter "me_frame". The (no so smart) default is the partonic center of mass.

If you want to use reweighting feature to pass from a not polarised sample to a polarised one, the re-weighting interface allow you to define a lambda function that will select the particle to include in the boost. Here is an example:
change boost lambda part: part.pdg in [23, 24, -23]

When using this feature please cite: 1912.01725 , which is also where you will get more details on the feature.