Gauge dependence of the gauge boson projector

The propagator of a gauge boson, like the massless photon or the massive vector bosons $W^\pm$ and $Z$ of the electroweak theory, can be derived in two different ways, namely via Green's functions (semi-classical approach) or via the vacuum expectation value of the time-ordered product of the field operators (field theoretical approach). Comparing the semi-classical with the field theoretical approach, the central tensorial object can be defined as the gauge boson projector, directly related to the completeness relation for the complete set of polarisation four-vectors. In this paper we explain the relation for this projector to different cases of the $R_\xi$ gauge and explain why the unitary gauge is the default gauge for massive gauge bosons.