= Background gauge field
{title2=$B_\mu$}
A <background gauge field> is a nondynamical source for a <conserved current>, rather than a field integrated over in the <path integral>. For a <Dirac current> with source term $+B_\mu j^\mu$, the combined transformations $\psi\mapsto e^{i\alpha}\psi$ and $B_\mu\mapsto B_\mu+\partial_\mu\alpha$ preserve the <action>. Whether this also preserves the <generating functional> depends on the absence of the corresponding <gauge anomaly>.
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