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A bstract We present the results of a search for charged-lepton-flavor violating decaysτ ⁻→ ℓ⁻K_(s)⁰, where ℓ − is either an electron or a muon. We combine e + e − data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb − 1 ) with samples recorded by the Belle experiment at the KEKB collider (980 fb − 1 ) to obtain a sample of 1.3 billion e + e − → τ + τ − events. We observe 0 and 1 events and set 90% confidence level upper limits of 0 . 8 × 10 − 8 and 1 . 2 × 10 − 8 on the branching fractions of the decay modesτ ⁻→ e⁻K_(S)⁰andτ ⁻→ μ ⁻K_(S)⁰, respectively. These are the most stringent upper limits to date.

Abstract We report measurements of the absolute branching fractions$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}_{s}^{\\pm }X\\right)$$,$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}^{0}/{\\overline{D} }^{0}X\\right)$$, and$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}^{\\pm }X\\right)$$, where the latter is measured for the first time. The results are based on a 121.4 fb −1 data sample collected at the Υ(10860) resonance by the Belle detector at the KEKB asymmetric-energy e + e − collider. We reconstruct one$${B}_{s}^{0}$$meson in$${e}^{+}{e}^{-}\\to \\Upsilon\\left(10860\\right)\\to {B}_{s}^{*}{\\overline{B} }_{s}^{*}$$events and measure yields of$${D}_{s}^{+}$$, D 0, and D + mesons in the rest of the event. We obtain$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}_{s}^{\\pm }X\\right)=\\left(68.6\\pm 7.2\\pm 4.0\\right)\\%$$,$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}^{0}/{\\overline{D} }^{0}X\\right)=\\left(21.5\\pm 6.1\\pm 1.8\\right)\\%$$, and$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}^{\\pm }X\\right)=\\left(12.6\\pm 4.6\\pm 1.3\\right)\\%$$, where the first uncertainty is statistical and the second is systematic. Averaging with previous Belle measurements gives$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}_{s}^{\\pm }X\\right)=\\left(63.4\\pm 4.5\\pm 2.2\\right)\\%$$and$$\\mathcal{B}\\left({B}_{s}^{0}\\to {D}^{0}/{\\overline{D} }^{0}X\\right)=\\left(23.9\\pm 4.1\\pm 1.8\\right)\\%$$. For the$${B}_{s}^{0}$$production fraction at the Υ(10860), we find$${f}_{s}=\\left({21.4}_{-1.7}^{+1.5}\\right)\\%$$.

Abstract Using data samples of 983.0 fb −1 and 427.9 fb −1 accumulated with the Belle and Belle II detectors operating at the KEKB and SuperKEKB asymmetric-energy e + e − colliders, singly Cabibbo-suppressed decays Ξ c + → p K S 0$$ {\\Xi}_c^{+}\\to p{K}_S^0 $$, Ξ c + → Λ π +$$ {\\Xi}_c^{+}\\to \\Lambda {\\pi}^{+} $$, and Ξ c + → Σ 0 π +$$ {\\Xi}_c^{+}\\to {\\Sigma}^0{\\pi}^{+} $$are observed for the first time. The ratios of branching fractions of Ξ c + → p K S 0$$ {\\Xi}_c^{+}\\to p{K}_S^0 $$, Ξ c + → Λ π +$$ {\\Xi}_c^{+}\\to \\Lambda {\\pi}^{+} $$, and Ξ c + → Σ 0 π +$$ {\\Xi}_c^{+}\\to {\\Sigma}^0{\\pi}^{+} $$relative to that of Ξ c + → Ξ − π + π +$$ {\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+} $$are measured to be B Ξ c + → p K S 0 B Ξ c + → Ξ − π + π + = 2.47 ± 0.16 ± 0.07 % , B Ξ c + → Λ π + B Ξ c + → Ξ − π + π + = 1.56 ± 0.14 ± 0.09 % , B Ξ c + → Σ 0 π + B Ξ c + → Ξ − π + π + = 4.13 ± 0.26 ± 0.22 % .$$ {\\displaystyle \\begin{array}{c}\\frac{\\mathcal{B}\\left({\\Xi}_c^{+}\\to p{K}_S^0\\right)}{\\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+}\\right)}=\\left(2.47\\pm 0.16\\pm 0.07\\right)\\%,\\\ {}\\frac{\\mathcal{B}\\left({\\Xi}_c^{+}\\to \\Lambda {\\pi}^{+}\\right)}{\\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+}\\right)}=\\left(1.56\\pm 0.14\\pm 0.09\\right)\\%,\\\ {}\\frac{\\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Sigma}^0{\\pi}^{+}\\right)}{\\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+}\\right)}=\\left(4.13\\pm 0.26\\pm 0.22\\right)\\%.\\end{array}} $$Multiplying these values by the branching fraction of the normalization channel, B Ξ c + → Ξ − π + π + = 2.9 ± 1.3 %$$ \\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+}\\right)=\\left(2.9\\pm 1.3\\right)\\% $$, the absolute branching fractions are determined to be B Ξ c + → p K S 0 = 7.16 ± 0.46 ± 0.20 ± 3.21 × 10 − 4 , B Ξ c + → Λ π + = 4.52 ± 0.41 ± 0.26 ± 2.03 × 10 − 4 , B Ξ c + → Σ 0 π + = 1.20 ± 0.08 ± 0.07 ± 0.54 × 10 − 3 .$$ {\\displaystyle \\begin{array}{c}\\mathcal{B}\\left({\\Xi}_c^{+}\\to p{K}_S^0\\right)=\\left(7.16\\pm 0.46\\pm 0.20\\pm 3.21\\right)\\times {10}^{-4},\\\ {}\\mathcal{B}\\left({\\Xi}_c^{+}\\to \\Lambda {\\pi}^{+}\\right)=\\left(4.52\\pm 0.41\\pm 0.26\\pm 2.03\\right)\\times {10}^{-4},\\\ {}\\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Sigma}^0{\\pi}^{+}\\right)=\\left(1.20\\pm 0.08\\pm 0.07\\pm 0.54\\right)\\times {10}^{-3}.\\end{array}} $$The first and second uncertainties above are statistical and systematic, respectively, while the third ones arise from the uncertainty in B Ξ c + → Ξ − π + π +$$ \\mathcal{B}\\left({\\Xi}_c^{+}\\to {\\Xi}^{-}{\\pi}^{+}{\\pi}^{+}\\right) $$.

Abstract We report a determination of the CKM angle ϕ 3, also known as γ, from a combination of measurements using samples of up to 711 fb −1 from the Belle experiment and up to 362 fb −1 from the Belle II experiment. We combine results from analyses of B + → DK +, B + → Dπ +, and B + → D * K + decays, where D is an admixture of D 0 and D ¯ 0 D̅⁰ mesons, in a likelihood fit to obtain ϕ 3 = (75.2 ± 7.6) ° . We also briefly discuss the interpretation of this result.

Abstract We report measurements of the e + e − → B B ¯ BB̅ , B B ¯ ∗ BB̅^(∗) , and B ∗ B ¯ ∗ B^(∗)B̅^(∗) cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct one B meson 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 ∗ B ¯ ∗ B^(∗)B̅^(∗) threshold, the e + e − → B ∗ B ¯ ∗ B^(∗)B̅^(∗) cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

Abstract We report results from a study of B ± → DK ± decays followed by D decaying to the CP-even final state K + K − and CP-odd final state K S 0 π 0 K_(S)⁰π⁰ , where D is an admixture of D 0 and D ¯ 0 D̅⁰ states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle ϕ 3. The results are based on a combined analysis of the final data set of 772 × 106 B B ¯ BB̅ pairs collected by the Belle experiment and a data set of 198 × 106 B B ¯ BB̅ pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be A 𝓐 CP+ = (+12.5 ± 5.8 ± 1.4)% and A 𝓐 CP− = (−16.7 ± 5.7 ± 0.6)%, and the ratios of branching fractions to be R 𝓡 CP+ = 1.164 ± 0.081 ± 0.036 and R 𝓡 CP− = 1.151 ± 0.074 ± 0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries A 𝓐 CP+ and 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 ° < ϕ 3 < 16.5 ° ) or (84.5 ° < ϕ 3 < 95.5 ° ) or (163.3 ° < ϕ 3 < 171.5 ° ) and 0.321 < r B < 0.465.

Abstract We measure CP asymmetries and branching-fraction ratios for B ± → DK ± and Dπ ± decays with D → K S 0$$ {K}_{\\textrm{S}}^0 $$K ± π ∓, where D is a superposition of D 0 and D ¯$$ \\overline{D} $$0. We use the full data set of the Belle experiment, containing 772 × 106 B B ¯$$ B\\overline{B} $$pairs, and data from the Belle II experiment, containing 387 × 106 B B ¯$$ B\\overline{B} $$pairs, both collected in electron-positron collisions at the Υ(4S) resonance. Our results provide model-independent information on the unitarity triangle angle ϕ 3.

Abstract We present a measurement of the Cabibbo-Kobayashi-Maskawa unitarity triangle angle ϕ 3 (also known as γ) using a model-independent Dalitz plot analysis of B + → D ( K S 0 K_(S)⁰ h + h − )h +, where D is either a D 0 or D ¯ D̅ 0 meson and h is either a π or K. This is the first measurement that simultaneously uses Belle and Belle II data, combining samples corresponding to integrated luminosities of 711 fb −1 and 128 fb −1, respectively. All data were accumulated from energy-asymmetric e + e − collisions at a centre-of-mass energy corresponding to the mass of the Υ(4S) resonance. We measure ϕ 3 = (78.4 ± 11.4 ± 0.5 ± 1.0)°, where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is from the uncertainties on external measurements of the D-decay strong-phase parameters.

Artificially-grown diamond crystals have unique properties that make them suitable as solid-state particle detectors and dosimeters in high-radiation environments. We have been using sensors based on single-crystal diamond grown by chemical vapour deposition for dosimetry and beam-loss monitoring at the SuperKEKB collider. Here we describe the assembly and the suite of test and calibration procedures adopted to characterise the diamond-based detectors of this monitoring system. We report the results obtained on 28 detectors and assess the stability and uniformity of response of these devices.