= Unitary flavour symmetry of degenerate Dirac fields
{title2=$U(N)$}
For $N$ <Dirac fields> with common <mass>, $\mathcal L=\bar\Psi(i\not\partial-m\mathbf1_N)\Psi$ is invariant under constant complex-linear rotations $\Psi\mapsto U\Psi$ with $U\in U(N)$. A Hermitian flavour generator $T^A$ has <Noether current> $j_A^\mu=\bar\Psi\gamma^\mu T^A\Psi$. A nondegenerate <fermion mass matrix> restricts the symmetry to unitary matrices commuting with it. If the theory is entirely free and rotations into <charge-conjugate fields> are allowed, its <enhanced internal symmetry of free Dirac fields> can be larger than this subgroup.
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