The experimental results on the ratios of branching fractions R(D) = B((B) over bar -> D tau(-)(nu) over bar (tau))/B((B) over bar -> Dl(-)(nu) over bar (e)) and R(D*) = B((B) over bar -> D*tau(-)(nu) over bar (tau))/B((B) over bar -> D*tau(-)(nu) over bar (e)), 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 x 10(6) B (B) over bar events recorded at the T(4S) 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 tau 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.

We thank the KEKB group for excellent operation of the accelerator; the KEK cryogenics group for efficient solenoid operations; and the KEK computer group, the NII, and PNNL/EMSL for valuable computing and SINET5 network support. We acknowledge support from MEXT, JSPS and Nagoya's Tau-Lepton Physics Research Center of Nagoya University (TLPRC); ARC (Australia); FWF (Austria); NSFC and CAS Center for Excellence in Particle Physics(CCEPP) (China); MSMT (Czechia); Carl Zeiss Foundation (CZF), DFG, Excellence Cluster Universe (EXC153), and VolkswagenStiftung (VS) (Germany); DST (India); INFN (Italy); MOE, MSIP, NRF, Radiation Science Research Institute (RSRI), Foreign Large-size Research Facility Application Supporting project (FLRFAS) project, GSDC of KISTI and KREONET/GLORIAD (Korea); MNiSW and NCN (Poland), Agreement No. 14.W03.31.0026 (Russia); ARRS (Slovenia); IKERBASQUE (Spain); SNSF (Switzerland); MOE and MOST (Taiwan); and DOE and NSF (U.S.A.). We acknowledge the support provided by the Albert Shimmins Fund for the writing of this Letter.