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We discuss the construction of the chiral Lagrangian for a light spin-1boson, here denoted as $X$, featuring both vector and axial-vector couplings tolight $u,d,s$ quarks. Focusing on $\Delta S = 1$ transitions, we show thatthere are model-independent tree-level contributions to $K^\pm \to \pi^\pm X$,sourced by Standard Model charged currents, which receive an $m^2_K / m_X^2$enhancement from the emission of a longitudinally polarized $X$. This flavourobservable sets the strongest to date model-independent bound on the diagonalaxial-vector couplings of $X$ to $u,d,s$ quarks for $m_X < m_K - m_\pi$,superseding the bounds arising from beam-dump and collider searches.

Flavour constraints on light spin-1 bosons within a chiral Lagrangian approach