$B\to K+invisible$ in a model with axion-like particles

Abstract

The localized excess of $B\to K+\text{invisible}$ events reported by Belle-II is commonly interpreted as a signal for $B\to Ka$, where $a$ is an axion-like particle (ALP). In these proceedings, we summarize two theoretical updates regarding the $b\to sa$ decay amplitude within a minimal UV-complete model for invisible ALPs, namely the DFSZ model. (i) Contributions from certain two-loop diagrams can dominate the one-loop ones and are thus relevant for phenomenology. (ii) Although not unique, a rare feature -- which we refer to as apparent non-decoupling -- emerges in the light effective theory. The renormalizable ALP interactions fail to capture this behavior and therefore are incomplete as low-energy effective descriptions.

Comments: Contribution to proceedings of European Physical Society Conference on High Energy Physics (EPS-HEP2025), 7-11 July 2025, Marseille, France