= Chiral flavour symmetry of massless Dirac fields
{title2=$U(N)_L\times U(N)_R$}
The free kinetic terms of left- and right-handed <Weyl fields> are separately invariant under <unitary group> rotations in flavour space. For $N$ massless <Dirac fields>, the subgroup preserving common <fermion number> is $U(N)_L\times U(N)_R$. A common nonzero <Dirac mass term> couples the two <chirality> sectors and leaves its diagonal $U(N)$ subgroup. Gauge interactions can obstruct some of the classical <axial currents> through a <chiral anomaly>.
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