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

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 mK2/mX2 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 mX < mK – mπ, superseding the bounds arising from beam-dump and collider searches.