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We report measurements of time-dependent $CP$asymmetries in $B^0\to K_S^0{\pi }^0\gamma$decays based on a data sample of $(388\pm 6)\times {10}^{6}B\overline{B}$events collected at the $\mathrm{\Upsilon }\left(4S\right)$resonance with the Belle II detector. The Belle II experiment operates at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$collider. We measure decay-time distributions to determine $CP$-violating parameters $S$and $C$. We determine these parameters for two ranges of $K_S^0{\pi }^0$invariant mass: $m\left(K_S^0{\pi }^0\right)\in (0.8,1.0)\mathrm{GeV}/c^2$, which is dominated by $B^0\to K^{*0}(\to K_S^0{\pi }^0)\gamma$decays, and a complementary region $m\left(K_S^0{\pi }^0\right)\in (0.6,0.8)\cup (1.0,1.8)\mathrm{GeV}/c^2$. Our results have improved precision as compared to previous measurements and are consistent with theory predictions. Published by the American Physical Society2025 

We describe a measurement of charge-parity ( $CP$) violation asymmetries in $B^0\to {\eta }^{\prime }{K}_{\mathrm{S}}^0$decays using Belle II data. We consider ${\eta }^{\prime }\to \eta (\to \gamma \gamma ){\pi }^{+}{\pi }^{-}$and ${\eta }^{\prime }\to \rho (\to {\pi }^{+}{\pi }^{-})\gamma$decays. The data were collected at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$collider between the years 2019 and 2022, and contain $(387\pm 6)\times {10}^6$bottom-antibottom meson pairs. We reconstruct $829\pm 35$signal decays and extract the $CP$violating parameters from a fit to the distribution of the proper-decay-time difference between the two $B$mesons. The measured direct and mixing-induced $CP$asymmetries are $C_{{\eta }^{\prime }{K}_{\mathrm{S}}^0}=-0.19\pm 0.08\pm 0.03$and $S_{{\eta }^{\prime }{K}_{\mathrm{S}}^0}=+0.67\pm 0.10\pm 0.03$, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with current world averages and standard model predictions. Published by the American Physical Society2024 

A<sc>bstract</sc> We report a determination of the CKM angleϕ₃, also known asγ, from a combination of measurements using samples of up to 711 fb⁻¹from the Belle experiment and up to 362 fb⁻¹from the Belle II experiment. We combine results from analyses ofB⁺→ DK⁺,B⁺→ Dπ⁺, andB⁺→ D^*K⁺decays, whereDis an admixture ofD⁰and$$ {\overline{D}}^0 $$ ${\overline{D}}^0$mesons, in a likelihood fit to obtainϕ₃= (75.2±7.6)^°. We also briefly discuss the interpretation of this result. 

The ratio of branching fractions $R\left(D^{*}\right)=\mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of $189{\mathrm{fb}}^{-1}$at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$collider. Data is collected at the $\mathrm{\Upsilon }\left(4\mathrm{S}\right)$resonance, and one $B$meson in the $\mathrm{\Upsilon }\left(4\mathrm{S}\right)\to B\overline{B}$decay is fully reconstructed in hadronic decay modes. The accompanying signal $B$meson is reconstructed as $\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$using leptonic $\tau$decays. The normalization decay, $\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell }$, produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields $R\left(D^{*}\right)={0.262}_{-0.039}^{+0.041}\left(\mathrm{stat}{)}_{-0.032}^{+0.035}\right(\mathrm{syst})$. This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society2024 

A<sc>bstract</sc> We present a measurement of the ratio$$ {R}_{\mu }=\mathcal{B}\left({\tau}^{-}\to {\mu}^{-}{\overline{\nu}}_{\mu }{\nu}_{\tau}\right)/\mathcal{B}\left({\tau}^{-}\to {e}^{-}{\overline{\nu}}_e{\nu}_{\tau}\right) $$ $R_{\mu }=B\left({\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }\right)/B\left({\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau }\right)$of branching fractions$$ \mathcal{B} $$ $B$of theτlepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKBe⁺e⁻collider. The sample has an integrated luminosity of 362 ± 2 fb⁻¹at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result,R_μ= 0.9675 ± 0.0007 ± 0.0036, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to theWboson inτdecays of 0.9974 ± 0.0019, in agreement with the standard model expectation of unity. 

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral $B$mesons produced in $\mathrm{\Upsilon }\left(4S\right)$decays. It improves previous algorithms by using the information from all charged final-state particles and the relations between them. We evaluate its performance using $B$decays to flavor-specific hadronic final states reconstructed in a $362{\mathrm{fb}}^{-1}$sample of electron-positron collisions collected at the $\mathrm{\Upsilon }\left(4S\right)$resonance with the Belle II detector at the SuperKEKB collider. We achieve an effective tagging efficiency of $(37.40\pm 0.43\pm 0.36\%)$, where the first uncertainty is statistical and the second systematic, which is 18% better than the previous Belle II algorithm. Demonstrating the algorithm, we use $B^0\to J/\psi K_{\mathrm{S}}^0$decays to measure the mixing-induced and direct $CP$violation parameters, $S=(0.724\pm 0.035\pm 0.009)$and $C=(-0.035\pm 0.026\pm 0.029)$. Published by the American Physical Society2024 

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. Published by the American Physical Society2024 

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R\left(X_{\tau /\ell }\right)\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, 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 $R\left(X_{\tau /\ell }\right)=0.228\pm 0.016\left(\mathrm{stat}\right)\pm 0.036\left(\mathrm{syst}\right)$, in agreement with standard-model expectations. This is the first direct measurement of $R\left(X_{\tau /\ell }\right)$. Published by the American Physical Society2024