Forecasting dark showers at Belle II

TL;DR

This paper explores how Belle II can detect dark showers from dark sectors through displaced vertices, improving sensitivity over previous experiments and highlighting the complementarity between collider and intensity frontier searches.

Contribution

It introduces a new search strategy for dark showers at Belle II, compares sensitivities with other experiments, and updates constraints on related dark sector models.

Findings

Belle II can significantly improve dark shower detection sensitivity.

Displaced vertex searches outperform prompt resonance searches.

LHCb and Belle II are sensitive to different decay lengths, showing experimental complementarity.

Abstract

Dark showers from strongly interacting dark sectors that confine at the GeV scale can give rise to novel signatures at $e^+e^-$ colliders. In this work, we study the sensitivity of $B$ factory experiments to dark showers produced through an effective interaction arising from a heavy off-shell mediator. We show that a prospective search for displaced vertices from GeV-scale long-lived particles at Belle II can improve the sensitivity to dark showers substantially compared to an existing search at BaBar. We compare the sensitivity of searches for displaced signals to searches for promptly produced resonances at BaBar and KLOE and calculate sensitivity projections for a single-photon search at Belle II to invisible dark showers produced through an effective interaction. The underlying structure of the effective interaction can be resolved at higher-energy experiments, where the mediator can be produced on-shell. To study the resulting constraints, we update electroweak precision bounds on kinetically mixed $Z'$ bosons and reinterpret a search for low-mass di-muon resonances at LHCb in terms of dark showers. We find that LHCb and Belle II are most sensitive to different particle decay lengths, underscoring the complementarity of LHC and intensity frontier experiments.