Probing dark sectors with long-lived particles at BELLE II

TL;DR

This paper proposes a novel search strategy at Belle II for light scalar particles with very weak interactions, focusing on their long-lived decay signatures and potential to outperform other experiments in sensitivity.

Contribution

It introduces a new method to detect long-lived light scalars in rare meson decays at Belle II, with improved sensitivity over existing experiments.

Findings

Belle II can probe scalars in the GeV range with couplings as small as 10^{-5}

The sensitivity surpasses that of FASER and NA62 for certain parameter spaces

Potential to detect invisibly decaying scalars via missing energy signatures

Abstract

We propose a new search for light scalar singlets in rare meson decays. For tiny interactions, the scalar is long-lived at detector scales and decays into displaced pairs of leptons or light mesons. We show that Belle II has a remarkable potential to probe scalars in the GeV range with couplings as small as $10^{-5}$. The predicted sensitivity is higher than at the long-baseline experiments FASER and NA62. We also investigate signatures of invisibly decaying scalars in rare meson decays with missing energy.