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 ∆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\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {m}_K^2/{m}_X^2 $$\\end{document} 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.