Daniel Marcantonio ; Phillip Urquijo

30 April 2024

Abstract: Recently, there has been great interest in so-called feebly interacting particles (FIPs) - particles beyond the Standard Model that have small couplings to Standard Model particles. FIPs can arise in several different new physics scenarios, and can be produced in decays of $B$ mesons. FIPs can act as portals between the visible and dark sectors, and therefore can decay invisibly. In this letter we report a search for new invisible particles $a$ in the channels $\Bpm \to ha$ where $h \in \pipm, \Kpm, D_{s}^\pm, p$, and $B^0 \to \Dzb a$ at a centre-of-mass energy of 10.58 \gev collected by the Belle experiment. No significant signal is observed, and 90\% confidence level on the branching fraction exclusion limits for these decays are derived as a function of the invisible mass, improving constraints by at least an order of magnitude. We also provide constraints on visibly-decaying long-lived particles, set limits on the axion-like particle coupling $g_{aW}$.

Keyword(s): ALPs ; Missing energy ; FEI