Erlangen 2026 – scientific programme
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T: Fachverband Teilchenphysik
T 72: Flavour Physics IV
T 72.7: Talk
Thursday, March 19, 2026, 17:45–18:00, KH 01.011
Measurement of the Energy Dependence of Strong Isospin Violation in Υ(4S) Decays — •Lena Nowatzki, Thomas Kuhr, and Thomas Lück — Ludwig-Maximilians-Universität München (LMU), München, Germany
Precise measurements of B meson branching fractions are essential both for testing Standard Model predictions and for many measurements that rely on accurate modeling of data composition in flavor-physics analyses. At Belle II, B mesons are produced via the process e+e− → Υ(4S) → BB. The total number of such events can be determined with high precision, yet using this quantity for B meson branching fraction measurements requires knowledge of the production fractions of neutral and charged B meson pairs in Υ(4S) decays, respectively f00 and f±. Although strong isospin symmetry predicts equal production rates, recent theoretical studies indicate a possible energy dependence of the ratio f±/f00, motivating an experimental investigation.
In this analysis, we aim to measure the energy dependence of the Υ(4S) decay fractions using Belle II data. We employ two different approaches. The first relies on direct B meson energy measurements using the channels B+ → J/ψ K+ and B0 → J/ψ KS0. The second includes a multivariate classifier based on event-level observables to distinguish between neutral and charged BB pair production. This talk will present the current status and an outlook on future developments of this analysis.
Keywords: B Mesons; Branching Fractions; Neural Network; Flavor Physics; Belle II