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

TL;DR

The paper analyzes rare $B$ decays sensitive to physics beyond the Standard Model using $1.2~\mathrm{ab}^{-1}$ of $e^+e^-\to B\bar{B}$ data at the $\Upsilon(4S)$ from Belle and Belle II. It employs inclusive and hadronic $B$-tagging, including the FEI algorithm, to probe $b\to s\nu\bar{\nu}$ and LFV channels, and provides a model-agnostic reinterpretation of $B^{+}\to K^{+}\nu\bar{\nu}$ within the Weak Effective Theory, extracting constraints on Wilson coefficients. The analyses yield the strongest current limits on the inclusive $B\to X_s\nu\bar{\nu}$ and on $b\to s\tau^{+}\tau^{-}$ transitions, and set first limits on LFV decays $B^{0}\to K_S^{0}\tau^{\pm}\ell^{\mp}$ with competitive bounds on $B^{0}\to K^{*0}\tau^{\pm}\ell^{\mp}$, all in agreement with SM expectations where no signal is observed. Collectively, the results demonstrate Belle II’s capability to deliver world-leading constraints on rare missing-energy B decays and enable rigorous NP testing through model-agnostic reinterpretation frameworks.

Abstract

The Belle and Belle~II 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 $B^0\to K^{*0}τ^\pm\ell^\mp$ ($\ell=e,μ$).

Figures (6)

• Figure 1: Diagrams representing $b\to s$ transitions: SM contributions (a) and possible NP scenarios (b,c).
• Figure 2: (a) ITA binned joint number densities. (b) $B^+\to K^+\nu\bar{\nu}$ differential branching fraction prediction.
• Figure 3: (a) The marginalized posterior for the Wilson coefficients. Dashed black lines indicate the SM predictions, and the yellow lines correspond to the posterior modes. (b) Observed and predicted best-fit yields in the highest sensitivity bins.
• Figure 4: The bin index distribution after the fit, for data and histogram templates.
• Figure 5: Distributions of $\eta$ (BDT) in $\text{SR}$ for the four signal categories. The $B^0 \rightarrow K^{*0} \tau^+ \tau^-$ signal, fitted with a branching fraction of $[-0.15 \pm 1.01] \times 10^{-3}$ and scaled assuming a branching fraction of $10^{-2}$, is shown as a reference. The bottom panel shows the pull distributions.