= Solution
<Parity> is full <spatial reflection>, $\mathbf x\mapsto-\mathbf x$, represented by a <parity operator> $\mathcal P$. A <polar vector>, such as <momentum>, reverses, whereas an <axial vector>, such as <spin>, does not. A state of definite <parity> has $\mathcal P|\psi\rangle=\eta_P|\psi\rangle$, with $\eta_P=\pm1$ in the usual convention. A multiparticle state's <parity> includes both the product of its <intrinsic parities> and its orbital spatial factor, for example $(-1)^L$ for relative <orbital angular momentum> $L$.
An interaction conserves <parity> when its dynamics are invariant under this transformation, equivalently $[H,\mathcal P]=0$ for a time-independent <Hamiltonian>. <Parity violation> means that a process and its fully reflected counterpart need not have the same <probability>. It is more than a left-right asymmetry in an unreflected laboratory apparatus.
The electromagnetic and strong sectors of the <Standard Model> conserve <parity> to the accuracy relevant here, taking the strong-sector <Yang-Mills theta term> to vanish. The <weak charged current> selects left-chiral fermions and right-chiral antifermions, as defined by <fermion chirality>; the reflected right-chiral charged current is not equally present. The neutral weak current also generally has different left- and right-chiral couplings. Consequently the weak sector violates <parity>; this does not mean that every individual weak decay must have opposite initial and final parity. For a massive particle, <helicity> and <fermion chirality> should be distinguished.
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