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Search for two-body $B$ meson decays to $Λ^{0}$ and $Ω_{c}^{(*) 0}$

https://doi.org/10.1103/PhysRevD.110.L031102

Savinov, V; Adachi, I; Ahn, J K; Aihara, H; Asner, D M; Atmacan, H; Ayad, R; Banerjee, Sw; Bennett, J; Bessner, M; et al (August 2024, Physical Review D)

We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral $B$ mesons to $Λ^{0}$ and $Ω_{c}^{(*) 0}$ using $711 {fb}^{- 1}$ of data collected at the $ϒ (4 S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+} e^{-}$ collider. We observe no evidence of signal from any such decays and set 95% confidence-level upper limits on the products of $B^{0}$ and ${\bar{B}}^{0}$ branching fractions for these two-body decays with $B (Ω_{c}^{0} \to π^{+} Ω^{-})$ in the range between $9.5 \times 10^{- 8}$ and $31.2 \times 10^{- 8}$ . Published by the American Physical Society2024
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Measurement of Angular Coefficients of $\bar{B} \to D^{*} ℓ {\bar{ν}}_{ℓ}$ : Implications for $| V_{c b} |$ and Tests of Lepton Flavor Universality

https://doi.org/10.1103/PhysRevLett.133.131801

Prim, M T; Bernlochner, F; Metzner, F; Aihara, H; Asner, D M; Aushev, T; Ayad, R; Babu, V; Banerjee, Sw; Behera, P; et al (September 2024, Physical Review Letters)

We measure the complete set of angular coefficients $J_{i}$ for exclusive $\bar{B} \to D^{*} ℓ {\bar{ν}}_{ℓ}$ decays ( $ℓ = e$ , $μ$ ). Our analysis uses the full $711 {fb}^{- 1}$ Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the $\bar{B} \to D^{*}$ transition and the Cabibbo-Kobayashi-Maskawa matrix element $| V_{cb} |$ . Using recent lattice QCD calculations for the hadronic form factors, we find $| V_{cb} | = (40.7 \pm 0.7) \times 10^{- 3}$ using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter $w$ and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations. Published by the American Physical Society2024
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Measurement of the branching fraction and $C P$ -violating asymmetry of the decay $B^{0} \to π^{0} π^{0}$ using 387 million $ϒ (4 S)$ decays in Belle II data

https://doi.org/10.1103/PhysRevD.111.L071102

Adachi, I; Aggarwal, L; Ahmed, H; Aihara, H; Alhakami, M; Aloisio, A; Althubiti, N; Anh_Ky, N; Asner, D M; Atmacan, H; et al (April 2025, Physical Review D)

We measure the branching fraction and $C P$ -violating flavor-dependent rate asymmetry of $B^{0} \to π^{0} π^{0}$ decays reconstructed using the Belle II detector in an electron-positron collision sample containing $387 \times 10^{6} ϒ (4 S)$ mesons. Using an optimized event selection, we find $125 \pm 20$ signal decays in a fit to background-discriminating and flavor-sensitive distributions. The resulting branching fraction is $(1.25 \pm 0.23) \times 10^{- 6}$ and the $C P$ -violating asymmetry is $0.03 \pm 0.30$ . Published by the American Physical Society2025
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Search for the baryon number and lepton number violating decays $τ^{-} \to Λ π^{-}$ and $τ^{-} \to \bar{Λ} π^{-}$ at Belle II

https://doi.org/10.1103/PhysRevD.110.112003

Adachi, I; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Althubiti, N; Anh_Ky, N; Asner, D M; Atmacan, H; et al (December 2024, Physical Review D)

We present a search for the baryon number $B$ and lepton number $L$ violating decays $τ^{-} \to Λ π^{-}$ and $τ^{-} \to \bar{Λ} π^{-}$ produced from the $e^{+} e^{-} \to τ^{+} τ^{-}$ process, using a $364 {fb}^{- 1}$ data sample collected by the Belle II experiment at the SuperKEKB collider. No evidence of signal is found in either decay mode, which have $| Δ (B - L) |$ equal to 2 and 0, respectively. Upper limits at 90% credibility level on the branching fractions of $τ^{-} \to Λ π^{-}$ and $τ^{-} \to \bar{Λ} π^{-}$ are determined to be $4.7 \times 10^{- 8}$ and $4.3 \times 10^{- 8}$ , respectively. Published by the American Physical Society2024
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Measurement of the energy dependence of the e+e− → $$ B\overline{B} $$, $$ B{\overline{B}}^{\ast } $$, and $$ {B}^{\ast }{\overline{B}}^{\ast } $$ cross sections at Belle II

https://doi.org/10.1007/JHEP10(2024)114

Adachi, I; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Althubiti, N; Anh_Ky, N; Asner, D M; Atmacan, H; et al (October 2024, Journal of High Energy Physics)

A<sc>bstract</sc> We report measurements of thee⁺e⁻→$$ B\overline{B} $$ $B \bar{B}$ ,$$ B{\overline{B}}^{\ast } $$ $B {\bar{B}}^{*}$ , and$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{*} {\bar{B}}^{*}$ 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$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{*} {\bar{B}}^{*}$ threshold, thee⁺e⁻→$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{*} {\bar{B}}^{*}$ cross section increases rapidly. This may indicate the presence of a pole close to the threshold.
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Search for the decay $B^{0} \to γ γ$ using Belle and Belle II data

https://doi.org/10.1103/PhysRevD.110.L031106

Adachi, I; Aggarwal, L; Aihara, H; Akopov, N; Aloisio, A; Al_Said, S; Althubiti, N; Anh_Ky, N; Asner, D M; Atmacan, H; et al (August 2024, Physical Review D)

We report the result of a search for the rare decay $B^{0} \to γ γ$ using a combined dataset of $753 \times 10^{6} B \bar{B}$ pairs collected by the Belle experiment and $387 \times 10^{6} B \bar{B}$ pairs collected by the Belle II experiment from decays of the $ϒ (4 S)$ resonance produced in $e^{+} e^{-}$ collisions. A simultaneous fit to the Belle and Belle II data sets yields ${11.0}_{- 5.5}^{+ 6.5}$ signal events, corresponding to a $2.5 σ$ significance. We determine the branching fraction $B (B^{0} \to γ γ) = ({3.7}_{- 1.8}^{+ 2.2} (stat) \pm 0.5 (syst)) \times 10^{- 8}$ and set a 90% credibility level upper limit of $B (B^{0} \to γ γ) < 6.4 \times 10^{- 8}$ . Published by the American Physical Society2024
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New graph-neural-network flavor tagger for Belle II and measurement of sin $2 ϕ_{1}$ in $B^{0} \to J / ψ K_{S}^{0}$ decays

https://doi.org/10.1103/PhysRevD.110.012001

Adachi, I; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Anh_Ky, N; Asner, D M; Atmacan, H; Aushev, T; et al (July 2024, Physical Review D)

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral $B$ mesons produced in $ϒ (4 S)$ 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 $362 {fb}^{- 1}$ sample of electron-positron collisions collected at the $ϒ (4 S)$ 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 $B^{0} \to J / ψ K_{S}^{0}$ decays to measure the mixing-induced and direct $C P$ 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
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