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

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 $\Delta S=1$ transitions, we show that there are model-independent tree-level contributions to $K^{\pm }\to {\pi }^{\pm }X$, sourced by Standard Model charged currents, which receive an $m_K^2/m_X^2$ 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 $m_X<m_K-m_{\pi }$, superseding the bounds arising from beam-dump and collider searches.

Comments: 15 pages, 5 figures, 1 table