Displaced or invisible? ALPs from $B$ decays at Belle II

TL;DR

This paper explores the potential of Belle II to detect axion-like particles through invisible decay signatures, demonstrating significant sensitivity improvements over previous collider and fixed-target experiments for sub-GeV to heavier particles.

Contribution

It introduces a new search strategy for invisible axion-like particles from B decays at Belle II, analyzing complementarity of displaced and invisible signatures for various decay lengths.

Findings

Belle II can probe branching ratios down to 10^{-7} for invisible axion-like particles.

Sensitivity to small couplings can improve by up to two orders of magnitude.

Different experimental strategies are optimal depending on the particle's decay length and mass.

Abstract

At colliders, neutral long-lived particles can be detected through displaced decay products or as missing energy. Which search strategy is better depends on the particle's decay length just as on the detector properties. We investigate the complementarity of displaced and invisible signatures for the Belle II experiment. Focusing on axion-like particles $a$ produced from meson decays, we present a new search strategy for two-body decays $B^+ \to K^+ a, a\to E\!\!\!/$ with missing energy $E\!\!\!/$. With $50\,$ab$^{-1}$ of data, Belle II can probe light invisible resonances with branching ratio $\mathcal{B}(B^+\to K^+ a) \gtrsim 10^{-7}$ and decay length $c\tau_a \gtrsim 1\,$m. For axion-like particles, we expect the sensitivity of $B^+ \to K^+ E\!\!\!/$ to small couplings to improve by up to two orders of magnitude compared to previous searches at collider and fixed-target experiments. For sub-GeV particles, $B^+ \to K^+ E\!\!\!/$ at Belle II and searches at beam-dump experiments are most sensitive; for heavier particles, searches for displaced vertices at Belle II, long-lived particle experiments at the LHC, and future fixed-target experiments can probe the smallest couplings.