Abstract We discuss the construction of the chiral Lagrangian for a light spin-1 boson, here denoted as X, featuring both vector and axial-vector couplings to light u, d, s quarks. Focusing on ∆S = 1 transitions, we show that there are model-independent tree-level contributions to K ± → π ± X, sourced by Standard Model charged currents, which receive an m K 2 / m X 2 $$ {m}_K^2/{m}_X^2 $$ enhancement from the emission of a longitudinally polarized X. This flavour observable sets the strongest to date model-independent bound on the diagonal axial-vector couplings of X to u, d, s quarks for m X < m K – m π , superseding the bounds arising from beam-dump and collider searches.
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