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

Abstract

The ratio of branching fractions R(D*)=B(B¯→D*τ−ν¯τ)/B(B¯→D*ℓ−ν¯ℓ), 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 e+e− collider. Data is collected at the ϒ(4S) resonance, and one B meson in the ϒ(4S)→BB¯ 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−ν¯ℓ, 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. Published by the American Physical Society 2024