From April to July 2018, a data sample at the peak energy of the resonance was collected with the Belle II detector at the SuperKEKB electron-positron collider. This is the first data sample of the Belle II experiment. Using Bhabha and digamma events, we measure the integrated luminosity of the data sample to be ( , where the first uncertainty is statistical and the second is systematic. This work provides a basis for future luminosity measurements at Belle II.

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We present the results of the re-discovery of the decay B 0 → π − + ν in 34.6 fb −1 of Belle II data using hadronic B-tagging via the Full Event Interpretation algorithm. We observe 21 signal events on a background of 155 in a fit to the distribution of the square of the missing mass, M 2 miss , with a significance of 5.69σ, and determine a total branching fraction of (1.58 ± 0.43 stat ± 0.07 sys ) ×10 −4 .

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of τ lepton decays into a light charged lepton (ℓ = e± or μ±) and a vector meson (V0 = ρ0, ϕ, ω, K*0, or $$ \overline{K} $$ K ¯ *0) using 980 fb−1 of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the τ → ℓV0 branching fractions in the range of (1.7–4.3) × 10−8. These limits are improved by 30% on average from the previous results.