Measurements of electroweak penguin and lepton-flavour violating $B$ decays to final states with missing energy at Belle and Belle~II

The Belle and BelleII experiments have accumulated a data set of $1.2\mathrm{ab}^{-1}$ of $e^+e^- \to B\bar{B}$ collisions at the $Υ(4S)$ resonance. Owing to the clean event environment and well-constrained initial-state kinematics, these data are ideally suited for searches for rare electroweak penguin and lepton-flavour violating $B$ decays with missing energy from neutrinos. We report results on $b\to sν\barν$ processes and the interpretation, together with searches for $B\to K^{*0}τ^+τ^-$ and for the LFV decays $B^0\to K_S^0τ^\pm\ell^\mp$ and $B^0\to K^{*0}τ^\pm\ell^\mp$ ($\ell=e,μ$).

💡 Research Summary

The Belle and Belle II experiments have together accumulated an unprecedented 1.2 ab⁻¹ of e⁺e⁻ collisions at the Υ(4S) resonance, providing a uniquely clean environment for studying B‑meson decays with missing energy. This paper presents a comprehensive program targeting rare flavour‑changing neutral‑current (FCNC) processes of the type b → s ν ν̄ and lepton‑flavour‑violating (LFV) decays involving τ leptons.

The first analysis revisits the exclusive decay B⁺ → K⁺ ν ν̄, originally measured with a combination of inclusive and hadronic (Full Event Interpretation, FEI) tagging. By constructing a model‑agnostic two‑dimensional density n₀(x) in the space of the transformed BDT₂ output (η_BDT₂) and the reconstructed momentum‑transfer squared (q²_rec), the authors re‑weight the Standard‑Model (SM) signal simulation with the ratio σ₁/σ₀ for any new physics (NP) model. Within the Weak Effective Theory (WET) framework, they perform a Bayesian fit to the vector (C_V^L + C_V^R), scalar (C_S^L + C_S^R) and tensor (C_T) Wilson coefficients. The posterior peaks at (C_V ≈ 11.3, C_S ≈ 0, C_T ≈ 8.2) with 95 % credible intervals