Measurement of R ( D ) and R ( D ∗ ) with a Semileptonic Tagging Method

Abstract

The experimental results on the ratios of branching fractions R ( D ) = B ( ¯ B → D τ − ¯ ν τ ) / B ( ¯ B → D ℓ − ¯ ν ℓ ) and R ( D ∗ ) = B ( ¯ B → D ∗ τ − ¯ ν τ ) / B ( ¯ B → D ∗ ℓ − ¯ ν ℓ ) , where ℓ denotes an electron or a muon, show a long-standing discrepancy with the standard model predictions, and might hint at a violation of lepton flavor universality. We report a new simultaneous measurement of R ( D ) and R ( D ∗ ) , based on a data sample containing 772 × 10 6

B ¯ B events recorded at the Υ ( 4 S ) resonance with the Belle detector at the KEKB e + e − collider. In this analysis the tag-side B meson is reconstructed in a semileptonic decay mode and the signal-side τ is reconstructed in a purely leptonic decay. The measured values are R ( D ) = 0.307 ± 0.037 ± 0.016 and R ( D ∗ ) = 0.283 ± 0.018 ± 0.014 , where the first uncertainties are statistical and the second are systematic. These results are in agreement with the standard model predictions within 0.2, 1.1, and 0.8 standard deviations for R ( D ) , R ( D ∗ ) , and their combination, respectively. This work constitutes the most precise measurements of R ( D ) and R ( D ∗ ) performed to date as well as the first result for R ( D ) based on a semileptonic tagging method.