Test of lepton flavor universality with a measurement of R (D∗) using hadronic B tagging at the Belle II experiment

The ratio of branching fractions RðD Þ ¼ Bð ¯B → D τ− ¯ν τÞ=Bð ¯B → D l− ¯νlÞ, 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 eþe− collider. Data is collected at the ϒð4SÞ resonance, and one B meson in the ϒð4SÞ → B ¯B decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as ¯B → D τ− ¯ν τ 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 RðD Þ ¼ 0.262þ0.041 −0.039 ðstatÞ þ0.035 −0.032 ðsystÞ. This result is consistent with the current world average and with Standard Model predictions.