= Longitudinal gauge propagator contraction
{title2=$p^aG_{ab}=\xi p_b/p^2$}
For the <positive-sign covariant gauge-fixing inverse>, $p^aG_{ab}=\xi p_b/p^2$. It is not identically zero for fixed $\xi\ne0$ and is a nonzero vector at a nonzero non-null <four-momentum>. Null <momenta> require the chosen pole prescription, and individual zero components are possible. The <Landau gauge> limit suppresses this longitudinal part. The <Ward identity> removes longitudinal contributions from physical conserved-current amplitudes.
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