Unitary gauge, as described in Chapter 5, has several disadvantages that make it inappropriate for most calculations that go beyond tree level in the perturbative expansion. One of these difficulties is that the spin-one propagator does not fall to zero for large momenta, p → ∞, thereby making the ultraviolet behavior of the theory appear to be worse than it really is. It is therefore usually more convenient to use in these computations a gauge in which the proper ultraviolet behavior is more manifest. A one-parameter family of such gauges is given by the Lorentz-covariant ξ-gauges.

Although no loop graphs are attempted in this book, the modification of the Feynman rules appropriate for ξ-gauges are included here for the sake of completeness. There are three new types of propagator that arise in ξ-gauge. The first of these is a modified spin-one boson propagator that was given in Chapter 5.

The particular cases ξ = 1 and ξ = 0 are respectively known as Feynman–'t Hooft Gauge and Landau Gauge. There are also two other types of unphysical particles that arise in the ξ-gauge graphs. These are the unphysical scalars and the Fadeev–Popov–DeWitt ghosts. Their role is to cancel the contributions within loops of the various unphysical components of the vector boson propagator. Neither the unphysical scalars nor the ghosts ever appear in external lines in scattering amplitudes.