Low-energy flavour probes of light vector bosons

In this work, we construct the chiral Lagrangian for a light spin-1 boson \(X\) possessing both vectorial and axial couplings to the light Standard Model quarks \(u, d, s\). We then use it in order to describe the tree-level, model-independent contributions to the \(\Delta S = 1\) transition \(K^\pm \rightarrow \pi^\pm X\), which is induced by Standard Model charged currents and is possibly enhanced by the emission of a longitudinally polarized \(X\) boson. Such a flavour observable is then shown to set the best model-independent bounds on the diagonal axial couplings of \(X\) to light quarks in the mass range allowed by the decay kinematics, improving the currently available constraints from beam-dump experiments and collider searches.