Two precision tests of lepton universality are presented. For this purpose, events with one fully reconstructed π΅ meson and a charged lepton candidate are selected from 189 fb^-1 of electron-positron collision data at the πΆ(4π) resonance collected with the Belle II detector.
The first measurement of the ratio of light-lepton branching fractions of inclusive semileptonic π΅-meson decays, π (ππ/π) = B(π΅ β πππ)/B(π΅ β πππ) is extracted in a binned maximum-likelihood fit to the lepton-momentum spectra above 1.3 GeV. The value of π (ππ/π) = 1.007 Β± 0.009 (stat.) Β± 0.019 (syst.) is obtained, which aligns with the Standard-Model expectation. This is the most precise branching fraction-based test of lepton universality in semileptonic π΅-meson decays to date.
For the first successful measurement of inclusive π΅ β πππ decays, measured in a ratio relative to π΅ β πlπ decays (l = π, π), π (ππ/l) = B(π΅ β πππ)/B(π΅ β πlπ), significant discrepancies between experimental data and simulation are revealed. They are mostly attributed to the modeling of π·-meson decays within the hadronic system π. A new data-driven method to identify, quantify, and correct these modeling inaccuracies is presented, enabling the extraction of π (ππ/l) = 0.228 Β± 0.016 (stat.) Β± 0.036 (syst.). This value is in agreement with the Standard-Model predictions and represent a complementary test of the longstanding flavor anomaly observed in the exclusive ratios π (π·(β)).
Henrik Junkerkalefeld