Comprehensive ALP Searches in Meson Decays

Abstract: We present a comprehensive study of axion-like particles (ALPs) in meson decays, combining effective field theory and ultraviolet models within the open-source tool ALPaca. The analysis accounts for running and matching effects across energy scales, including non-perturbative QCD corrections via chiral perturbation theory. We discuss several benchmark models, both flavour-universal and non-universal, using the most up-to-date theoretical computations for ALP decays and branching ratios. Experimental signatures such as prompt, displaced, and invisible decays are included. A dedicated analysis of the Belle II anomaly in the decay $B^+ \to K^+ \nu \bar{\nu}$ is performed. Our results highlight the power of flavour observables in constraining ALPs and provide a versatile foundation for future searches.