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

TL;DR

This work surveys measurements of electroweak penguin and lepton-flavor-violating B decays with missing energy using Belle and Belle II data at the $\\Upsilon(4S)$ to test SM and NP. It combines tagging-based missing-energy analyses of $b \\to s\\nu\\bar{\\nu}$, $b \\to s\\tau\\bar{\\tau}$, and $b \\to s\\tau\\bar{\\ell}$ channels, including a first evidence for $B^+ \\to K^+\\nu\\bar{\\nu}$ and a stringent $B \\to X_s\\nu\\bar{\\nu}$ limit interpreted within the Weak Effective Theory via Wilson coefficients $C_{V_R}$, $C_S_R$, and $C_T_R$. It also reports searches for $B^0 \\to K^{*0}\\tau^+\\tau^-$ and for cLFV decays $B^0 \\to K_S^0\\tau\\ell$ and $B^0 \\to K^{*0}\\tau\\ell$, finding no significant signals but providing efficiency maps for reinterpretation and highlighting the ongoing competitiveness of $B$-factories. Together, these results probe NP scenarios linked to flavor anomalies, and the analysis framework enables quantitative reinterpretations with forthcoming data and refined methods for missing-energy decays.

Abstract

The Belle and Belle II experiments have collected a 1.2 ab$^{-1}$ sample of collisions at a center-of-mass energy corresponding to the $Υ(4S)$ resonance. These datasets, with low particle multiplicity and constrained initial state kinematics, are an ideal environment to search for rare electroweak penguin $B$ decays and lepton-flavor-violating $B$ decays to final states with missing energy from neutrinos. Results from $b \to sν\barν$ processes and their interpretation are presented. In addition, we provide an overview of the search for the $B \to K^{*0}τ^+τ^-$ decays and the lepton-flavor violating decays $B^0 \to K^{(*)0}τ^\pm\ell^\mp$, where $\ell$ is an electron or a muon.

Figures (3)

• Figure 1: $(b \to s\nu\bar{\nu})$ recent Belle II results. \ref{['fig:a']}: Reinterpretation reint of the $B^+ \to K^+\nu\bar{\nu}$ evidence with marginalized posterior for WET Wilson coefficients (real $C_{V_R,S_R,T_R}$ sums). Contours: 68/95% credible intervals; dashed: SM; dash-dotted: mode; dotted: symmetry axes for sample symmetrization. \ref{['fig:b']}: $B\to X_s\nu\bar{\nu}$ data and post-fit histogram templates in three $X_s$ mass regions, with bins representing optimized BDT output ranges. Backgrounds are separated into charged and neutral B decays, and four $e^+e^- \to q\bar{q}$ components ($q = u, d, s, c)$.
• Figure 2: Transformed BDT distributions for four signal categories, showing $B\bar{B}$ and $q\bar{q}\;(q=u,d,s,c)$ backgrounds, the $B^0 \to K^{0}\tau^+\tau^-$ signal, and a reference signal at $\mathcal{B}=10^{-2}$. From K*0tautau.
• Figure 3: Fits to the recoil mass in the $\bar{b} \to \bar{s}\tau^- e^+$ data for the \ref{['fig:a2']}: $K_S^0$ mode, where Belle and Belle II data are combined (from KStaul); \ref{['fig:b2']},\ref{['fig:b3']}: $K^{*0}$ mode, with a simultaneous fit to the two datasets (from K*0taul).