feynlag.vertices.bilinear.four_fermion_feynman_rule

feynlag.vertices.bilinear.four_fermion_feynman_rule(coefficient, gammas=None, boson_tuple=())

FFFF rule: the scalar i × coefficient × ∏(boson multiplicities)!.

Unlike fermion_feynman_rule(), the two Dirac structures gammas = (Γ, Γ′) are not folded into the returned value: they act on two independent spinor chains, so multiplying them would wrongly invoke the Clifford algebra across chains (e.g. contract a shared γ^μ). They are carried alongside the coupling (on the extraction key / a Vertex’s meta), exactly as UFO keeps the Lorentz structure separate from the coupling value. gammas is accepted for API symmetry and validated only.