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 R(D∗)=B(B→D∗τ-ντ)/B(B→D∗-ν), where 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 e+e-collider. Data is collected at the I(4S) resonance, and one B meson in the I(4S)→BB decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as B→D∗τ-ντ using leptonic τ decays. The normalization decay, B→D∗-ν, 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 R(D∗)=0.262-0.039+0.041(stat)-0.032+0.035(syst). This result is consistent with the current world average and with Standard Model predictions.
