We present a search for the rare flavor-changing neutral-current decay $B^0 \to K^{\ast 0} τ^+ τ^-$ 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 $Υ(4S)$ resonance. One of the $B$ mesons produced in the $Υ(4S)\to B^0 \bar{B}^0$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{\ast 0}$ and two $τ$ leptons of opposite charge. The $τ$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $ρ$ meson, and additional neutrinos. We set an upper limit on the branching ratio of $BR(B^0 \to K^{\ast 0} τ^+ τ^-) < 1.8 \times 10^{-3}$ at the 90% confidence level, which is the most stringent constraint reported to date.
- - - - - - - - Overview of HEPData Record - - - - - - - -<br/><br/></ul><b>Post-fit yields:</b><ul><li><a href="159541?version=1&table=Postfit%20yields:%20fit%20variable">Fit variable $\eta(\rm{BDT})$</a></ul><b>Signal $q^{2}$:</b><ul><li><a href="159541?version=1&table=Generated%20$q^2$"> Generated $q^{2}$ distribution </a></ul><b>Signal selection efficiency:</b><ul><li><a href="159541?version=1&table=Selection%20efficiency"> Selection efficieny in signal region </a>
Observed yields and fit results in bins of $\eta(\rm{BDT})$ as obtained by the fit on the four signal categories, corresponding to an integrated luminosity of 365 fb$^{-1}$. The yields are shown for $B^0 \rightarrow K^{\ast 0}\tau\tau$ signal and the two background components ($B\bar{B}$ decays and $q\bar{q}$ continuum).
Distribution of the di-tau invariant mass squared $q^2$ assumed for the generated signal $B^0 \rightarrow K^{\ast 0}\tau\tau$ events.
We search for the rare decay $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ in a $362\ \rm{fb}^{-1}$ sample of electron-positron collisions at the $\Upsilon(4S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$ meson in $\Upsilon(4S) \to B\kern 0.18em\overline{\kern -0.18em B}{}$ events to suppress background from other decays of the signal $B$ candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ branching fraction of $\left[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})\right] \times 10^{-5}$ and $\left[1.1^{+0.9}_{-0.8}(\mathrm{stat}){}^{+0.8}_{-0.5}(\mathrm{syst})\right] \times 10^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ to be $\left[2.3 \pm 0.5(\mathrm{stat})^{+0.5}_{-0.4}(\mathrm{syst})\right]\times 10^{-5}$, providing the first evidence for this decay at $3.5$ standard deviations. The combined result is $2.7$ standard deviations above the standard model expectation.
- - - - - - - - Overview of HEPData Record - - - - - - - -<br/><br/></ul><b>Post-fit yields:</b><ul><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20fit%20variable%20(ITA)">Y(4S) fit variable (ITA)</a><li><a href="146803?version=1&table=Postfit%20yields%20off-resonance%3A%20fit%20variable%20(ITA)">Off-resonance fit variable (ITA)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20%24%5Ceta(%5Crm%7BBDT%7D_%7B2%7D)%24%20(ITA)">Y(4S) $\eta(\rm{BDT}_{2})$ (ITA)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20%24q%5E%7B2%7D%24%20(HTA)">Y(4S) $q^{2}$ (ITA)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20%24%5Ceta(%5Crm%7BBDT%7D_%7B2%7D)%24%20(ITA%2C%20signal-rich)">Y(4S) $\eta(\rm{BDT}_{2})$ (ITA, signal-rich)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20%24q%5E%7B2%7D%24%20%20(ITA%2C%20signal-rich)">Y(4S) $q^{2}$ (ITA, signal-rich)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20fit%20variable%20(HTA)">Y(4S) fit variable (HTA)</a><li><a href="146803?version=1&table=Postfit%20yields%20Y(4S)%3A%20%24q%5E%7B2%7D%24%20(HTA)">Y(4S) $q^{2}$ (HTA)</a></ul><b>Signal selection efficiency:</b><ul><li><a href="146803?version=1&table=Selection%20efficiency%20(ITA)">ITA</a><li><a href="146803?version=1&table=Selection%20efficiency%20(HTA)">HTA</a></ul><b>Likelihoods:</b><ul><li><a href="146803?version=1&table=Profile%20likelihoods%20(HTA)">HTA</a><li><a href="146803?version=1&table=Profile%20likelihoods%20(ITA)">ITA</a><li><a href="146803?version=1&table=Profile%20likelihoods%20(Combined)">Combined</a></ul><b>Branching fraction summary:</b><ul><li><a href="146803?version=1&table=Branching%20fraction%20summary">Branching fraction summary</a></ul></ul>HistFactory json with full statistical model for combined likelihood and minimal fitting example are included under Resources.<ul>
Observed yields and fit results in bins of the $\eta(\rm{BDT}_{2})\times q^2_{\mathrm{rec}}$ space obtained by the ITA simultaneous fit to the off- and on-resonance data, corresponding to an integrated luminosity of 42 and 362 fb$^{-1}$, respectively. The yields are shown individually for the $B^+ \rightarrow K^{+}\nu\bar\nu$ signal, neutral and charged $B$-meson decays and the sum of the five continuum categories. The yields are obtained in bins of the $\eta(\rm{BDT}_{2})\times q^2_{\mathrm{rec}}$ space.
Observed yields and fit results in bins of the $\eta(\rm{BDT}_{2})\times q^2_{\mathrm{rec}}$ space obtained by the ITA simultaneous fit to the off- and on-resonance data, corresponding to an integrated luminosity of 42 and 362 fb$^{-1}$, respectively. The yields are shown individually for the $B^+ \rightarrow K^{+}\nu\bar\nu$ signal, neutral and charged $B$-meson decays and the sum of the five continuum categories. The yields are obtained in bins of the $\eta(\rm{BDT}_{2})\times q^2_{\mathrm{rec}}$ space.
A search for the flavor-changing neutral-current decay $B^{+}\to K^{+}\nu\bar{\nu}$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The results are based on a data sample corresponding to an integrated luminosity of $63\,\mbox{fb}^{-1}$ collected at the $\Upsilon{(4S)}$ resonance and a sample of $9\,\mbox{fb}^{-1}$ collected at an energy $60\mathrm{\,Me\kern -0.1em V}$ below the resonance. A novel measurement method is employed, which exploits topological properties of the $B^{+}\to K^{+}\nu\bar{\nu}$ decay that differ from both generic bottom-meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of $B^{+}\to K^{+}\nu\bar{\nu}$ of $4.1 \times 10^{-5}$ is set at the 90% confidence level.
- - - - - - - - Overview of HEPData Record - - - - - - - - <br/><br/> </ul> <b>Post-fit yields:</b> <ul> <li><a href="130199?version=1&table=Postfit%20yields%20Y(4S)">Y(4S)</a> <li><a href="130199?version=1&table=Postfit%20yields%20off-resonance">Off-resonance</a> </ul> <b>Exclusion limit:</b> <ul> <li><a href="130199?version=1&table=Expected%20and%20observed%20Limit">Expected limit and observed limit</a> </ul> <b>Efficiency:</b> <ul> <li><a href="130199?version=1&table=Selection%20efficiency">Selection efficiency as a function of $q^{2}$</a> </ul>
Yields in on-resonance data and as predicted by the simultaneous fit to the on- and off-resonance data, corresponding to an integrated luminosity of 63 and 9 fb$^{−1}$, respectively. The predicted yields are shown individually for charged and neutral B-meson decays and the five continuum background categories. The leftmost three bins belong to the first control region (CR1) with BDT$_{2} \in [0.93; 0.95]$ and the other nine bins correspond to the signal region (SR), three for each range of BDT$_{2} \in [0.95; 0.97; 0.99; 1.0]$. Each set of three bins is defined by $p_{T}(K^{+}) \in [0.5; 2.0; 2.4; 3.5] \rm{GeV}/c^{2}$.
Yields in off-resonance data and as predicted by the simultaneous fit to the on- and off-resonance data, corresponding to an integrated luminosity of 63 and 9 fb$^{−1}$, respectively. The predicted yields are shown individually for the five continuum background categories. The leftmost three bins belong to the third control region (CR3) with BDT$_{2} \in [0.93; 0.95]$ and the other nine bins correspond to the second control region (CR2), three for each range of BDT$_{2} \in [0.95; 0.97; 0.99; 1.0]$. Each set of three bins is defined by $p_{T}(K^{+}) \in [0.5; 2.0; 2.4; 3.5] \rm{GeV}/c^{2}$.
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