PubDB | BELLE2-PUB-PH-2025-038

Abstract	We present a search for the rare flavor-changing neutral-current decay $B^{0} \to K^{∗0} \tau^{+} \tau^{−}$ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb$^{−1}$ data sample recorded at the center-of-mass energy of the $\Upsilon(4S)$ resonance. One of the $B$ mesons produced in the $\Upsilon(4S)\to B^{0}\bar{B^{0}}$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{∗0}$ and two $\tau$ leptons of opposite charge. The $\tau$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $\rho$ meson, and additional neutrinos. We set an upper limit on the branching fraction of $\mathcal{B}(B^{0}\to K^{*0}\tau^{+}\tau^{-}) < 1.8 \times 10^{−3}$ at the 90% confidence level, which is the most stringent constraint reported to date.
Tags	Run 1
Working group	Radiative and Electroweak Penguin
Principal authors	@ Claudia Cecchi, Elisa Manoni, Stefano Moneta
Reference	published in PRL
Document	BELLE2-PUB-PH-2025-038
Links	arXiv:2504.10042,

Abstract

We present a search for the rare flavor-changing neutral-current decay $B^{0} \to K^{∗0} \tau^{+} \tau^{−}$ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb$^{−1}$ data sample recorded at the center-of-mass energy of the $\Upsilon(4S)$ resonance. One of the $B$ mesons produced in the $\Upsilon(4S)\to B^{0}\bar{B^{0}}$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{∗0}$ and two $\tau$ leptons of opposite charge. The $\tau$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $\rho$ meson, and additional neutrinos. We set an upper limit on the branching fraction of $\mathcal{B}(B^{0}\to K^{*0}\tau^{+}\tau^{-}) < 1.8 \times 10^{−3}$ at the 90% confidence level, which is the most stringent constraint reported to date.

Search for the $B0 \to K^{∗0} \tau^{+} \tau^{-}$ decay with an hadronic tagged approach