Polarization[k] is the head of a polarization momentum with momentum k.
A slashed polarization vector (\varepsilon_{\mu}(k) \gamma^\mu) has to be entered as GS[Polarization[k]].
Unless the option Transversality is set to True, all polarization vectors are not transverse by default.
The internal representation for a polarization vector corresponding to a boson with four momentum k is: Momentum[Polarization[k, I ]].
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.
Polarization may appear only inside Momentum. Outside of Momentum it is meaningless in FeynCalc.
The imaginary unit in the second argument of Polarization is used to distinguish between incoming and outgoing polarization vectors.
Pair[Momentum[k], Momentum[Polarization[k, I]]] corresponds to \varepsilon^{\mu}(k), i.e. an ingoing polarization vector
Pair[Momentum[k], Momentum[Polarization[k, -I]]] corresponds to \varepsilon^{\ast \mu}(k), i.e. an outgoing polarization vector
Overview, PolarizationVector, PolarizationSum, DoPolarizationSums.
Polarization[k]\text{Polarization}(k)
Polarization[k] // ComplexConjugate\text{Polarization}(k)
GS[Polarization[k]]\bar{\gamma }\cdot \overline{\text{Polarization}(k)}
GS[Polarization[k]] // StandardForm
(*GS[Polarization[k]]*)Pair[Momentum[k], Momentum[Polarization[k, I]]]\overline{k}\cdot \bar{\varepsilon }(k)