In Peskin on page 514, when deriving the Faddeev-Poppov ghosts, they arrive at the full Lagrangian for Yang-Mills: $$ \mathcal{L} = -\frac{1}{4}F^2 + \frac{1}{2\xi} (\partial \cdot A^a)^2+\textrm{Fermion Term + Ghosts Term} $$ I'm wondering, what's the correspondence (i.e., what's the way to derive) between the value of $\xi$ and the gauge condition on $A$? For example, Landau gauge is $\xi\to 0$. What kind of gauge-condition equation does that produce on $A$ and how can we derive it?
Compare with this page on Wikipedia: http://en.wikipedia.org/wiki/Landau_quantization which says that Landau gauge is when $A$ is only along the $y$-axis and has component there equal to $B\,x$ where $B$ is a constant and $x$ is the coordinate.
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