Abstract
The ratio of branching fractions R(D^{*-})≡B(B^{0}→D^{*-}τ^{+}ν_{τ})/B(B^{0}→D^{*-}μ^{+}ν_{μ}) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8TeV, corresponding to an integrated luminosity of 3 fb^{-1}. For the first time, R(D^{*-}) is determined using the τ-lepton decays with three charged pions in the final state. The B^{0}→D^{*-}τ^{+}ν_{τ} yield is normalized to that of the B^{0}→D^{*-}π^{+}π^{-}π^{+} mode, providing a measurement of B(B^{0}→D^{*-}τ^{+}ν_{τ})/B(B^{0}→D^{*-}π^{+}π^{-}π^{+})=1.97±0.13±0.18, where the first uncertainty is statistical and the second systematic. The value of B(B^{0}→D^{*-}τ^{+}ν_{τ})=(1.42±0.094±0.129±0.054)% is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the B^{0}→D^{*-}μ^{+}ν_{μ} decay, a value of R(D^{*-})=0.291±0.019±0.026±0.013 is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and B^{0}→D^{*-}μ^{+}ν_{μ} modes. This measurement is in agreement with the standard model prediction and with previous results.
Highlights
The ratio of branching fractions RðDÃ−Þ ≡ BðB0 → DÃ−τþντÞ=BðB0 → DÃ−μþνμÞ 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
In the standard model (SM) of particle physics, flavorchanging processes such as semileptonic decays of b hadrons are mediated by a W boson with universal coupling to leptons
Differences between the expected branching fraction of semileptonic decays into the three lepton families originate from the different masses of the charged leptons
Summary
Where 3π ≡ πþπ−πþ and Nsig (Nnorm) and εsig (εnorm) are the yield and selection efficiency for the signal (normalization) channel, respectively. The BDT response in the simulation is validated using three control samples: a B → DÃ−Dþs ðXÞ data sample, which is obtained by using partial reconstruction under the background hypothesis; a B → DÃ−D0ðXÞ data sample, with the subsequent D0 → K−3π decay, which is obtained by removing the charged-particle isolation criterion and requiring a particle satisfying kaon identification criteria with an origin at the 3π vertex; and a B → DÃ−DþðXÞ data sample, with Dþ → K−πþπþ, which is obtained replacing the negative pion with a candidate identified as a kaon. The first (second) includes Dþs decays into ηπ or ηρ (η0π or η0ρ), where at least one pion
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