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

TL;DR

This paper develops a chiral Lagrangian framework for a light spin-1 boson with vector and axial-vector couplings to light quarks, deriving model-independent constraints from kaon decay processes that surpass previous bounds.

Contribution

It introduces a novel chiral Lagrangian approach for light spin-1 bosons and derives the strongest model-independent bounds on their axial-vector couplings from kaon decay data.

Findings

Tree-level contributions to $K^ ightarrow \\pi X$ are enhanced by $m_K^2 / m_X^2$.

Kaon decay constraints set the strongest bounds on axial-vector couplings for $m_X < m_K - m_\\pi$.

Bounds surpass those from beam-dump and collider experiments.

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 $m^2_K / m_X^2$ 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.