= Gamma trace of a vector-spinor
{title2=$\chi=\gamma^\mu\Psi_\mu$}
= Gamma trace
{synonym}
The gamma trace contracts the vector index of a <Rarita-Schwinger field> with a <gamma matrix>, producing an ordinary spinor. In the massive free theory it vanishes by the field equations. In the massless theory a <gauge transformation> changes it by $\gamma^\mu\partial_\mu\epsilon$, allowing a gamma-traceless gauge for propagating modes. Together with transversality this removes the unwanted spin-one-half sector.
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