Abstract
A measurement is reported of the ratio of branching fractions R(J/ψ)=B(B_{c}^{+}→J/ψτ^{+}ν_{τ})/B(B_{c}^{+}→J/ψμ^{+}ν_{μ}), where the τ^{+} lepton is identified in the decay mode τ^{+}→μ^{+}ν_{μ}ν[over ¯]_{τ}. This analysis uses a sample of proton-proton collision data corresponding to 3.0 fb^{-1} of integrated luminosity recorded with the LHCb experiment at center-of-mass energies of 7 and 8TeV. A signal is found for the decay B_{c}^{+}→J/ψτ^{+}ν_{τ} at a significance of 3 standard deviations corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be R(J/ψ)=0.71±0.17(stat)±0.18(syst). This result lies within 2 standard deviations above the range of central values currently predicted by the standard model.
Highlights
Semileptonic b-hadron decays provide powerful probes for testing the standard model (SM) and for searching for the effects of physics beyond the SM. Because of their relatively simple theoretical description via tree-level processes in the SM, these decay modes serve as an ideal setting for examining the universality of the couplings of the three charged leptons in electroweak interactions
This Letter presents the first study of the semitauonic decay Bþc → J/ψτþντ and a measurement of the ratio of branching fractions
For which the central values of the current SM predictions are in the range of 0.25–0.28, where the spread arises from the choice of modeling approach for form factors [17,18,19,20]
Summary
The combinatorial background in the selected J/ψμþ sample is predominantly due to J/ψ mesons from Bu;d;s → J/ψX decays paired with muon candidates from the rest of the event This background source is modeled using a set of three template histograms taken from simulation for the three B-meson species, with their relative fractions constrained in accordance with the production cross sections and their respective branching fractions. A separate background comes from pairing unrelated muons to form J/ψ candidates The template for this combinatorial J/ψ component is determined using events where the J/ψ invariant mass lies above the nominal selection threshold, with its normalization fixed using a fit to the μþμ− invariant-mass distribution.
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