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Using data samples of $988.4{\mathrm{fb}}^{-1}$ and $427.9{\mathrm{fb}}^{-1}$ collected with the Belle and Belle II detectors, we present a study of the singly Cabibbo-suppressed decays ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta$ , $\mathrm{\Lambda }{\eta }^{\prime }$ , and $\mathrm{\Lambda }{\pi }^0$ . We observe the decay  ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta$ and find evidence for the decay ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime }$ , with corresponding branching ratios determined to be $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta )/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(4.16\pm 0.91\pm 0.23)\%$ and $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime })/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(2.48\pm 0.82\pm 0.12)\%$ , respectively. We find no significant signal in the ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0$ decay mode and set an upper limit at the 90% credibility level of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0)/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})<3.5\%$ . Multiplying these ratios by the world-average branching fraction of the normalization channel, $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(1.43\pm 0.27)\%$ , we obtain the absolute branching fractions of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta )=(5.95\pm 1.30\pm 0.32\pm 1.13)\times {10}^{-4}$ , $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime })=(3.55\pm 1.17\pm 0.17\pm 0.68)\times {10}^{-4}$ , and an upper limit at the 90% credibility level on the absolute branching fraction of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0)<5.2\times {10}^{-4}$ . The quoted first and second uncertainties are statistical and systematic, respectively, while the third uncertainties arise from the branching fraction of the normalization mode. These results are consistent with most theoretical predictions and further the understanding of the underlying decay mechanisms. 

Abstract A series of data samples was collected with the Belle II detector at the SuperKEKB collider from March 2019 to June 2022. We determine the integrated luminosities of these data samples using three distinct methodologies involving Bhabha (), digamma (), and dimuon () events. The total integrated luminosity obtained with Bhabha, digamma, and dimuon events is (426.88 ± 0.03 ± 2.61) fb⁻¹, (429.28 ± 0.03 ± 2.62) fb⁻¹, and (423.99 ± 0.04 ± 3.83) fb⁻¹, where the first uncertainties are statistical and the second are systematic. The resulting total integrated luminosity obtained from the combination of the three methods is (427.87 ± 2.01) fb⁻¹. 

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 

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 report measurements of thee⁺e⁻→$$ B\overline{B} $$ $B\overline{B}$ ,$$ B{\overline{B}}^{\ast } $$ $B{\overline{B}}^{\ast }$ , and$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$ cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct oneBmeson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$ threshold, thee⁺e⁻→$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$ cross section increases rapidly. This may indicate the presence of a pole close to the threshold. 

A<sc>bstract</sc> We present the result of a search for the charged-lepton-flavor violating decayτ⁻→μ⁻μ⁺μ⁻using a 424 fb⁻¹sample of data recorded by the Belle II experiment at the SuperKEKBe⁺e⁻collider. The selection ofe⁺e⁻→τ⁺τ⁻events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a 90% confidence level upper limit of 1.9×10⁻⁸on the branching fraction of theτ⁻→ μ⁻μ⁺μ⁻decay, which is the most stringent bound to date. 

We measure the branching fraction of the decay $B^{-}\to D^0\rho (770{)}^{-}$ using data collected with the Belle II detector. The data contain 387 million $B\overline{B}$ pairs produced in $e^{+}{e}^{-}$ collisions at the $\mathrm{\Upsilon }\left(4S\right)$ resonance. We reconstruct $8360\pm 180$ decays from an analysis of the distributions of the $B^{-}$ energy and the $\rho (770{)}^{-}$ helicity angle. We determine the branching fraction to be $(0.939\pm 0.021\left(\mathrm{stat}\right)\pm 0.050\left(\mathrm{syst}\right))\%$ , in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two. Published by the American Physical Society2024 

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 

A<sc>bstract</sc> We present an analysis of the processe⁺e⁻→π⁺π⁻Υ(nS) (wheren= 1, 2, or 3) reconstructed in 19.6 fb⁻¹of Belle II data during a special run of the SuperKEKB collider at four energy points near the peak of the Υ(10753) resonance. By analyzing the mass distribution of theπ⁺π⁻Υ(nS) system and the Born cross sections of thee⁺e⁻→π⁺π⁻Υ(nS) process, we report the first observation of Υ(10753) decays to theπ⁺π⁻Υ(1S) andπ⁺π⁻Υ(2S) final states, and find no evidence for decays toπ⁺π⁻Υ(3S). Possible intermediate states in theπ⁺π⁻Υ(1S,2S) transitions are also investigated, and no evidence for decays proceeding via the$$ {\pi}^{\mp }{Z}_b^{\pm } $$ ${\pi }^{\mp }{Z}_b^{\pm }$ orf₀(980)Υ(nS) intermediate states is found. We measure Born cross sections for thee⁺e⁻→π⁺π⁻Υ(nS) process that, combined with results from Belle, obtain the mass and width of Υ(10753) to be (10756.6 ± 2.7 ± 0.9) MeV/c²and (29.0 ± 8.8 ± 1.2) MeV, respectively. The relative ratios of the Born cross sections at the Υ(10753) resonance peak are also reported for the first time.