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We report on new flavor tagging algorithms developed to determine the quark-flavor content of bottom ( ) mesons at Belle II. The algorithms provide essential inputs for measurements of quark-flavor mixing and charge-parity violation. We validate and evaluate the performance of the algorithms using hadronic decays with flavor-specific final states reconstructed in a data set corresponding to an integrated luminosity of 62.8 fb-1, collected at the resonance with the Belle II detector at the SuperKEKB collider. We measure the total effective tagging efficiency to be εeff=(30.0±1.2(stat)±0.4(syst))%for a category-based algorithm and εeff=(28.8±1.2(stat)±0.4(syst))%for a deep-learning-based algorithm.

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 ¯ $$ \\overline{D} $$ 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.

We present the results of a search for the charged-lepton-flavor violating decays B$^{0}$ → K$^{*0}$τ$^{±}$ℓ$^{∓}$, where ℓ$^{∓}$ is either an electron or a muon. The results are based on 365 fb$^{−1}$ and 711 fb$^{−1}$ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic B-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed B meson. We find no evidence for B$^{0}$ → K$^{*0}$τ$^{±}$ℓ$^{∓}$ decays and set upper limits on the branching fractions in the range of (2.9–6.4)×10$^{−5}$ at 90% confidence level. 19 pages, 4 figures

We present a study of $$ {\\Xi}_c^0\\to {\\Xi}^0{\\pi}^0 $$ Ξ c 0 → Ξ 0 π 0 , $$ {\\Xi}_c^0\\to {\\Xi}^0\\eta $$ Ξ c 0 → Ξ 0 η , and $$ {\\Xi}_c^0\\to {\\Xi}^0{\\eta}^{\\prime } $$ Ξ c 0 → Ξ 0 η ′ decays using the Belle and Belle II data samples, which have integrated luminosities of 980 fb − 1 and 426 fb − 1 , respectively. We measure the following relative branching fractions $$ {\\displaystyle \\begin{array}{c}\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^0{\\pi}^0\\right)/\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^{-}{\\pi}^{+}\\right)=0.48\\pm 0.02\\left(\\textrm{stat}\\right)\\pm 0.03\\left(\\textrm{syst}\\right),\\\ {}\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^0\\eta \\right)/\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^{-}{\\pi}^{+}\\right)=0.11\\pm 0.01\\left(\\textrm{stat}\\right)\\pm 0.01\\left(\\textrm{syst}\\right),\\\ {}\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^0{\\eta}^{\\prime}\\right)/\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^{-}{\\pi}^{+}\\right)=0.08\\pm 0.02\\left(\\textrm{stat}\\right)\\pm 0.01\\left(\\textrm{syst}\\right)\\end{array}} $$ B Ξ c 0 → Ξ 0 π 0 / B Ξ c 0 → Ξ − π + = 0.48 ± 0.02 stat ± 0.03 syst , B Ξ c 0 → Ξ 0 η / B Ξ c 0 → Ξ − π + = 0.11 ± 0.01 stat ± 0.01 syst , B Ξ c 0 → Ξ 0 η ′ / B Ξ c 0 → Ξ − π + = 0.08 ± 0.02 stat ± 0.01 syst for the first time, where the uncertainties are statistical (stat) and systematic (syst). By multiplying by the branching fraction of the normalization mode, $$ \\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^{-}{\\pi}^{+}\\right) $$ B Ξ c 0 → Ξ − π + , we obtain the following absolute branching fraction results $$ {\\displaystyle \\begin{array}{c}\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^0{\\pi}^0\\right)=\\left(6.9\\pm 0.3\\left(\\textrm{stat}\\right)\\pm 0.5\\left(\\textrm{syst}\\right)\\pm 1.3\\left(\\operatorname{norm}\\right)\\right)\\times {10}^{-3},\\\ {}\\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^0\\eta \\right)=\\left(1.6\\pm 0.2\\left(\\textrm{stat}\\right)\\pm 0.2\\left(\\textrm{syst}\\right)\\pm 0.3\\left(\\operatorname{norm}\\right)\\right)\\times {10}^{-3},\\\ {}\\mathcal{B}\\left({\\varXi}_c^0\\to {\\Xi}^0{\\eta}^{\\prime}\\right)=\\left(1.2\\pm 0.3\\left(\\textrm{stat}\\right)\\pm 0.1\\left(\\textrm{syst}\\right)\\pm 0.2\\left(\\operatorname{norm}\\right)\\right)\\times {10}^{-3},\\end{array}} $$ B Ξ c 0 → Ξ 0 π 0 = 6.9 ± 0.3 stat ± 0.5 syst ± 1.3 norm × 10 − 3 , B Ξ c 0 → Ξ 0 η = 1.6 ± 0.2 stat ± 0.2 syst ± 0.3 norm × 10 − 3 , B Ξ c 0 → Ξ 0 η ′ = 1.2 ± 0.3 stat ± 0.1 syst ± 0.2 norm × 10 − 3 , where the third uncertainties are from $$ \\mathcal{B}\\left({\\Xi}_c^0\\to {\\Xi}^{-}{\\pi}^{+}\\right) $$ B Ξ c 0 → Ξ − π + . The asymmetry parameter for $$ {\\Xi}_c^0\\to {\\Xi}^0{\\pi}^0 $$ Ξ c 0 → Ξ 0 π 0 is measured to be $$ \\alpha \\left({\\Xi}_c^0\\to {\\Xi}^0{\\pi}^0\\right)=-0.90\\pm 0.15\\left(\\textrm{stat}\\right)\\pm 0.23\\left(\\textrm{syst}\\right) $$ α Ξ c 0 → Ξ 0 π 0 = − 0.90 ± 0.15 stat ± 0.23 syst .

We perform the first search for CP violation in $$ {D}_{(s)}^{+}\\to {K}_S^0{K}^{-}{\\pi}^{+}{\\pi}^{+} $$ D s + → K S 0 K − π + π + decays. We use a combined data set from the Belle and Belle II experiments, which study e + e − collisions at center-of-mass energies at or near the Υ(4 S ) resonance. We use 980 fb − 1 of data from Belle and 428 fb − 1 of data from Belle II. We measure six CP -violating asymmetries that are based on triple products and quadruple products of the momenta of final-state particles, and also the particles’ helicity angles. We obtain a precision at the level of 0.5% for $$ {D}^{+}\\to {K}_S^0{K}^{-}{\\pi}^{+}{\\pi}^{+} $$ D + → K S 0 K − π + π + decays, and better than 0.3% for $$ {D}_s^{+}\\to {K}_S^0{K}^{-}{\\pi}^{+}{\\pi}^{+} $$ D s + → K S 0 K − π + π + decays. No evidence of CP violation is found. Our results for the triple-product asymmetries are the most precise to date for singly-Cabibbo-suppressed D + decays. Our results for the other asymmetries are the first such measurements performed for charm decays.

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

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

We report measurements of the $e^+e^- \\to B\\bar{B}$, $B\\bar{B}{}^*$, and $B^*\\bar{B}{}^*$ 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 $B^*\\bar{B}{}^*$ threshold, the $e^+e^- \\to B^*\\bar{B}{}^*$ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report measurements of the e + e − → $$ B\\overline{B} $$ B B ¯ , $$ B{\\overline{B}}^{\\ast } $$ B B ¯ ∗ , and $$ {B}^{\\ast }{\\overline{B}}^{\\ast } $$ B ∗ B ¯ ∗ 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 $$ {B}^{\\ast }{\\overline{B}}^{\\ast } $$ B ∗ B ¯ ∗ threshold, the e + e − → $$ {B}^{\\ast }{\\overline{B}}^{\\ast } $$ B ∗ B ¯ ∗ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

We report the first measurement of the inclusive e$^{+}$e$^{−}$→$ b\\overline{b} $→$ {D}_s^{\\pm } $X and e$^{+}$e$^{−}$→$ b\\overline{b} $→ D$^{0}$/$ {\\overline{D}}^0 $X cross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determine σ(e$^{+}$e$^{−}$→$ {B}_s^0{\\overline{B}}_s^0 $X) and σ(e$^{+}$e$^{−}$→$ B\\overline{B} $X) in the same energy range. We measure the fraction of $ {B}_s^0 $ events at Υ(10860) to be f$_{s}$ = ($ {22.0}_{-2.1}^{+2.0} $)%. We determine also the ratio of the $ {B}_s^0 $ inclusive branching fractions $ \\mathcal{B} $($ {B}_s^0 $→ D$^{0}$/$ {\\overline{D}}^0 $X)/$ \\mathcal{B} $($ {B}_s^0 $→$ {D}_s^{\\pm } $X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e$^{+}$e$^{−}$ collider.[graphic not available: see fulltext]