= Dynamical gauge-fixing parameter
{title2=$\delta S/\delta\xi=-(\partial\cdot A)^2/(2\xi^2)$}
If a real nonzero field $\xi(x)$ appears in the Lorentzian density $-F_{ab}F^{ab}/4+(\partial_aA^a)^2/(2\xi)$ and is varied, its algebraic <Euler-Lagrange equation> is $-(\partial_aA^a)^2/(2\xi^2)=0$. Hence it imposes the <Lorenz gauge> condition on real classical fields. Varying $A_b$ gives $\partial_aF^{ab}-\partial^b[(\partial\cdot A)/\xi]=0$, including derivatives of $\xi$. This constrained field theory differs from holding the gauge parameter fixed in a <Gaussian functional integral>.
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