Пахлова Галина Владимировна

New graph-neural-network flavor tagger for Belle II and measurement of sin 2⁢𝜙1 in 𝐵0→𝐽/𝜓⁢𝐾0S decays

2024 в печати · ARTICLE · en

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−1sample 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 ±0.43 ±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 𝐵0 →𝐽/𝜓⁢𝐾0S decays to measure the mixing-induced and direct 𝐶⁢𝑃 violation parameters, 𝑆 =(0.724 ±0.035 ±0.009) and 𝐶 =(−0.035 ±0.026±0.029).

Search for a 𝜇+⁢𝜇− resonance in four-muon final states at Belle II

2024 в печати · ARTICLE · en

Search for lepton-flavor-violating τ− → μ−μ+μ− decays at Belle II

2024 в печати · ARTICLE · en

We present the result of a search for the charged-lepton-flavor violating decay τ− → μ−μ+μ− using a 424 fb−1 sample of data recorded by the Belle II experiment at the SuperKEKB e+e− collider. The selection of e+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 × 10−8 on the branching fraction of the τ− → μ−μ+μ− decay, which is the most stringent bound to date.

Measurement of the branching fractions of B→ D(*)KKs* and B→ D(*)Ds decays at Belle II

2024 в печати · ARTICLE · en

Measurements of the branching fractions of Ξ0c→ Ξ0π0, Ξ0c→ Ξ0η, and Ξ0c→ Ξ0η′ and asymmetry parameter of Ξ0c→ Ξ0π0

2024 в печати · ARTICLE · en

A test of lepton flavor universality with a measurement of R(D*) using hadronic tagging at the Belle II experiment

2024 в печати · ARTICLE · en

The ratio of branching fractions 𝑅⁡(𝐷*) =ℬ⁡(¯𝐵 →𝐷*⁢𝜏−⁢¯𝜈𝜏)/ℬ⁡(¯𝐵 →𝐷*⁢ℓ−⁢¯𝜈ℓ), where ℓ is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of 189 fb−1 at the SuperKEKB asymmetric-energy 𝑒+⁢𝑒− collider. Data is collected at the ϒ⁡(4⁢S) resonance, and one 𝐵meson in the ϒ⁡(4⁢S) →𝐵⁢¯𝐵 decay is fully reconstructed in hadronic decay modes. The accompanying signal 𝐵 meson is reconstructed as ¯𝐵 →𝐷*⁢𝜏−⁢¯𝜈𝜏 using leptonic 𝜏 decays. The normalization decay, ¯𝐵→𝐷*⁢ℓ−⁢¯𝜈ℓ, 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⁡(𝐷*) =0.262+0.041−0.039⁢(stat)+0.035−0.032⁢(syst). This result is consistent with the current world average and with Standard Model predictions.

Determination of the CKM angle ϕ3 from a combination of Belle and Belle II results

2024 в печати · ARTICLE · en

We report a determination of the CKM angle ϕ3, 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 D0 and mesons, in a likelihood fit to obtain ϕ3 = (75.2 ± 7.6)°. We also briefly discuss the interpretation of this result.

Study of Υ(10753) decays to π+π−Υ(nS) final states at Belle II

2024 в печати · ARTICLE · en

We present an analysis of the process e+e− → π+π−Υ(nS) (where n = 1, 2, or 3) reconstructed in 19.6 fb−1 of 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 the π+π−Υ(nS) system and the Born cross sections of the e+e− → π+π−Υ(nS) process, we report the first observation of Υ(10753) decays to the π+π−Υ(1S) and π+π−Υ(2S) final states, and find no evidence for decays to π+π−Υ(3S). Possible intermediate states in the π+π−Υ(1S, 2S) transitions are also investigated, and no evidence for decays proceeding via the or f0(980)Υ(nS) intermediate states is found. We measure Born cross sections for the e+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/c2 and (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.

Measurement of the energy dependence of the e+e− →BB ,BB*, and B*B* cross sections at Belle II

2024 в печати · ARTICLE · en

We report measurements of the e+e− → , , and 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 threshold, the e+e− → cross section increases rapidly. This may indicate the presence of a pole close to the threshold.

Measurements of the branching fractions B(B0-bar → D*+ π−) and B(B0-bar → D*+ K−) and tests of QCD factorization

2023 · ARTICLE · en

Using (771.6±10.6)×106 B¯B meson pairs recorded by the Belle experiment at the KEKB e+e−collider, we report the branching fractions B(¯B0→D*+π−)=(2.62±0.02±0.09)×10−3 and B(¯B0→D*+K−)=(2.22±0.06±0.08)×10−4; the quoted uncertainties are statistical and systematic, respectively. A measurement of the ratio of these branching fractions is also presented, RK/π=B(¯B→D*+K−)/B(¯B→D*+π−)=(8.41±0.24±0.13)×10−2, where systematic uncertainties due to the D*+ meson reconstruction cancel out. Furthermore, we report a new QCD factorization test based on the measured ratios for ¯B→D*+h− and ¯B→D*+ℓ−ν decays at squared momentum transfer values equivalent to the mass of the h=π or K hadron. The parameters |a1(h)|are measured to be |a1(π)|=0.884±0.004±0.003±0.016 and |a1(K)|=0.913±0.019±0.008±0.013, where the last uncertainties account for all external inputs. These values are approximately 15% lower than those expected from theoretical predictions. Subsequently, flavor SU(3) symmetry is tested by measuring the ratios for pions and kaons, |a1(K)|2/|a1(π)|2=1.066±0.042±0.018±0.023, as well as for different particle species. The ratio is consistent with unity and therefore no evidence for SU(3) symmetry breaking effects is found at the 5% precision level.