= Axial derivative coupling and parity
{title2=$(\partial_\mu\phi)j_5^\mu$}
For an ordinary <scalar field>, its derivative is a polar covector, while the <axial current> is a pseudovector. Their contraction is invariant under the <proper orthochronous Lorentz group> but changes sign under <parity>. If the field is a <pseudoscalar>, the extra transformation sign makes this coupling parity even. In four spacetime dimensions its <mass dimension> is five, so Lorentz invariance alone does not imply power-counting <renormalizability>.
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