A search for feebly-interacting particles in B decays with missing energy at Belle

We present a search for an invisible hidden-sector particle Xᵢₙᵥ, produced in B⁰ → D̄⁰ Xᵢₙᵥ and B^±→ h Xᵢₙᵥ decays, where h = π^±, K^±, Dₛ^±, p^±. The search is performed using e⁺ e⁻ collision data recorded with the Belle detector, corresponding to 711~fb⁻¹. No significant signal is observed. We set 90% confidence level upper limits ranging between 10⁻⁴ and 10⁻⁶ on the branching fraction B(B → h Xᵢₙᵥ) as a function of m_ Xᵢₙᵥ. Corresponding limits are set on B(B → hX)(X→γγ) for lifetimes cτX between $10~μ$m and 10 m. Many of these limits are the first direct constraints on their respective decays. Our results provide the most stringent exclusion limits to date on the branching fractions for all search channels, and exclude previously unexplored regions of parameter space relevant to several new physics models.