= V-A interaction
{c}
{title2=$\gamma^\mu(1-\gamma^5)=2\gamma^\mu P_L$}
The vector-minus-axial-vector charged weak current contains the left-chiral projector $P_L=(1-\gamma^5)/2$. It violates <parity> because <parity> interchanges left and right chiral fields. For a massless particle <chirality> fixes <helicity>, but a massive charged <fermion>'s <helicity> need not be pure even when the vertex selects a chiral field. Nuclear <beta decay> uses the nonrelativistic reduction of vector and axial nucleon currents, giving Fermi and Gamow-Teller operators in the allowed limit.
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