PhD Colloquium: Tests of lepton universality in the rates of inclusive semileptonic B-meson decays at Belle II

by Henrik Junkerkalefeld (Arbeitsgruppe Jochen Dingfelder)

Friday 28 Jun 2024, 15:00 → 16:00 Europe/Berlin

Kreuzbergweg 24/0.052 (FTD) - Präsentationsraum (FTD)

Two precision tests of lepton universality are presented. For this purpose, events with one fully reconstructed 𝐵 meson and a charged lepton candidate are selected from 189 fb^-1 of electron-positron collision data at the 𝛶(4𝑆) resonance collected with the Belle II detector.

The first measurement of the ratio of light-lepton branching fractions of inclusive semileptonic 𝐵-meson decays, 𝑅(𝑋𝑒/𝜇) = B(𝐵 → 𝑋𝑒𝜈)/B(𝐵 → 𝑋𝜇𝜈) is extracted in a binned maximum-likelihood fit to the lepton-momentum spectra above 1.3 GeV.  The value of 𝑅(𝑋𝑒/𝜇)  = 1.007 ± 0.009 (stat.) ± 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 𝐵-meson decays to date.

For the first successful measurement of inclusive 𝐵 → 𝑋𝜏𝜈 decays, measured in a ratio relative to 𝐵 → 𝑋l𝜈 decays (l = 𝑒, 𝜇), 𝑅(𝑋𝜏/l) = B(𝐵 → 𝑋𝜏𝜈)/B(𝐵 → 𝑋l𝜈), significant discrepancies between experimental data and simulation are revealed. They are mostly attributed to the modeling of 𝐷-meson decays within the hadronic system 𝑋. A new data-driven method to identify, quantify, and correct these modeling inaccuracies is presented, enabling the extraction of 𝑅(𝑋𝜏/l) = 0.228 ± 0.016 (stat.) ± 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 𝑅(𝐷(∗)).