Gauge-fixing parameter
= Gauge-fixing parameter
{title2=$\xi$}
A <gauge-fixing parameter> controls the weighting of a gauge condition, commonly through $-G^2/(2\xi)$. In a linear <covariant gauge>, $\xi=0$ denotes the transverse Landau limit and $\xi=1$ the Feynman choice. In a broken-phase $R_\xi$ gauge it also sets unphysical Goldstone and ghost <masses>. Gauge-invariant observables are independent of this parameter when all required contributions are included consistently.