Low-energy flavour probes of light vector bosons

TL;DR

This paper develops a chiral Lagrangian framework for a light vector boson with both vector and axial couplings to light quarks, and uses it to derive new bounds from kaon decay processes.

Contribution

It introduces a model-independent chiral Lagrangian for a light spin-1 boson and applies it to derive improved bounds from kaon decay observables.

Findings

Sets the best model-independent bounds on axial couplings of the boson.

Improves constraints over existing beam-dump and collider experiments.

Analyzes $ ext{K}^ ightarrow ext{pi} X$ decay processes for new physics sensitivity.

Abstract

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.