is the head of a polarization momentum with (incoming) momentum k. A slashed polarization vector (e1(k) slash) has to be entered as DiracSlash[Polarization[k]]. The internal representation for a polarization vector e1 corresponding to a boson with four momentum k is: Momentum[ Polarization[ k, I ] ]. With this notation transversality of polarization vectors is provided, i.e. , Pair[ Momentum[k], Momentum[ Polarization[k, I] ] ] yields 0. Polarization[k,-I] denotes the complex conjugate polarization. Polarization is also an option of various functions related to the operator product expansion. The setting 0 denotes the unpolarized and 1 the polarized case.