# Test of Lepton Flavor Universality by the measurement of the $B^0 \to D^{*-} \tau^+ \nu_{\tau}$ branching fraction using three-prong $\tau$ decays

Abstract : The ratio of branching fractions R(D*-)≡B(B0→D*-τ+ντ)/B(B0→D*-μ+νμ) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3  fb-1. The τ lepton is reconstructed with three charged pions in the final state. A novel method is used that exploits the different vertex topologies of signal and backgrounds to isolate samples of semitauonic decays of b hadrons with high purity. Using the B0→D*-π+π-π+ decay as the normalization channel, the ratio B(B0→D*-τ+ντ)/B(B0→D*-π+π-π+) is measured to be 1.97±0.13±0.18, where the first uncertainty is statistical and the second systematic. An average of branching fraction measurements for the normalization channel is used to derive B(B0→D*-τ+ντ)=(1.42±0.094±0.129±0.054)%, where the third uncertainty is due to the limited knowledge of B(B0→D*-π+π-π+). A test of lepton flavor universality is performed using the well-measured branching fraction B(B0→D*-μ+νμ) to compute R(D*-)=0.291±0.019±0.026±0.013, where the third uncertainty originates from the uncertainties on B(B0→D*-π+π-π+) and B(B0→D*-μ+νμ). This measurement is in agreement with the Standard Model prediction and with previous measurements.
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Journal articles

https://hal.archives-ouvertes.fr/hal-01797474
Contributor : Inspire Hep <>
Submitted on : Tuesday, May 22, 2018 - 3:58:34 PM
Last modification on : Wednesday, June 19, 2019 - 11:32:41 PM

### Citation

R. Aaij, Bernardo Adeva, Marco Adinolfi, Ziad Ajaltouni, Simon Akar, et al.. Test of Lepton Flavor Universality by the measurement of the $B^0 \to D^{*-} \tau^+ \nu_{\tau}$ branching fraction using three-prong $\tau$ decays. Phys.Rev.D, 2018, 97 (7), pp.072013. ⟨10.1103/PhysRevD.97.072013⟩. ⟨hal-01797474⟩

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