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We report on a search for a resonance$X$decaying to a pair of muons in$e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}X$events in the$0.212–9.000\mathrm{GeV}/c^2$mass range, using$178{\mathrm{fb}}^{-1}$of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of$X$beyond the standard model: a$Z^{\prime }$vector boson in the$L_{\mu }-L_{\tau }$model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the$L_{\mu }-L_{\tau }$model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below$6\mathrm{GeV}/c^2$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the$L_{\mu }-L_{\tau }$model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.

We measure the tau-to-light-lepton ratio of inclusive$B$-meson branching fractions$R(X_{\tau /\ell })\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, where$\ell$indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed$B$meson and a charged lepton candidate from$189{\mathrm{fb}}^{-1}$of electron-positron collision data collected with the Belle II detector. We find$R(X_{\tau /\ell })=0.228\pm 0.016(\mathrm{stat})\pm 0.036(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of$R(X_{\tau /\ell })$.

We report a measurement of decay-time-dependent charge-parity ($CP$) asymmetries in$B^0\to K_S^0{K}_S^0{K}_S^0$decays. We use$387\times {10}^6$$B\overline{B}$pairs collected at the$\mathrm{\Upsilon }(4S)$resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the$CP$-violating parameters$S$and$C$from a fit to the distribution of the decay-time difference between the two$B$mesons. The resulting confidence region is consistent with previous measurements in$B^0\to K_S^0{K}_S^0{K}_S^0$and$B^0\to (c\overline{c})K^0$decays and with predictions based on the standard model.

We search for the rare decay$B^{+}\to K^{+}\nu \overline{\nu }$in a$362{\mathrm{fb}}^{-1}$sample of electron-positron collisions at the$\mathrm{\Upsilon }(4S)$resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying$B$meson in$\mathrm{\Upsilon }(4S)\to B\overline{B}$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$B^{+}\to K^{+}\nu \overline{\nu }$branching fraction of$[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})]\times {10}^{-5}$and$[{1.1}_{-0.8}^{+0.9}(\mathrm{stat}{)}_{-0.5}^{+0.8}(\mathrm{syst})]\times {10}^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay$B^{+}\to K^{+}\nu \overline{\nu }$to be$[2.3\pm 0.5(\mathrm{stat}{)}_{-0.4}^{+0.5}(\mathrm{syst})]\times {10}^{-5}$, 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.

We report results from a study ofB^±→ DK^±decays followed byDdecaying to theCP-even final stateK⁺K⁻and CP-odd final state$$ {K}_S^0{\pi}^0 $$$K_S^0{\pi }^0$, whereDis an admixture ofD⁰and$$ {\overline{D}}^0 $$${\overline{D}}^0$states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angleϕ₃. The results are based on a combined analysis of the final data set of 772×10⁶$$ B\overline{B} $$$B\overline{B}$pairs collected by the Belle experiment and a data set of 198×10⁶$$ B\overline{B} $$$B\overline{B}$pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be$$ \mathcal{A} $$$A$_CP+= (+12.5±5.8±1.4)% and$$ \mathcal{A} $$$A$_CP−= (−16.7±5.7±0.6)%, and the ratios of branching fractions to be$$ \mathcal{R} $$$R$_CP+= 1.164±0.081±0.036 and$$ \mathcal{R} $$$R$_CP−= 1.151±0.074±0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries$$ \mathcal{A} $$$A$_CP+and$$ \mathcal{A} $$$A$_CP−have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5^°<ϕ₃< 16.5^°) or (84.5^°<ϕ₃< 95.5^°) or (163.3^°<ϕ₃< 171.5^°) and 0.321 <r_B< 0.465.