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

G. Caria et al. (Belle Collaboration)
Phys. Rev. Lett. 124, 161803 – Published 24 April 2020

Abstract

The experimental results on the ratios of branching fractions R(D)=B(B¯Dτν¯τ)/B(B¯Dν¯) and R(D*)=B(B¯D*τν¯τ)/B(B¯D*ν¯), where 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 772×106BB¯ events recorded at the ϒ(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 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.

  • Figure
  • Received 13 October 2019
  • Accepted 10 March 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.161803

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

Published by the American Physical Society

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Vol. 124, Iss. 16 — 24 April 2020

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