Observation of the Decay Λb0 → Λc+ τ-(ν)over-barτ

The first observation of the semileptonic b-baryon decay Λb0 → Λc+ τ− anti-ν_τ, with a significance of 6.1 σ, is reported using a data sample corresponding to 3 fb^−1 of integrated luminosity, collected by the LHCb experiment at center-of-mass energies of 7 and 8 TeV at the LHC. The τ− lepton is reconstructed in the hadronic decay to three charged pions. The ratio K = B(Λb0 → Λc+ τ− anti-ν_τ) / B(Λb0 → Λc+ π− π+ π−) is measured to be 2.46 ± 0.27 ± 0.40, where the first uncertainty is statistical and the second systematic. The branching fraction B(Λb0 → Λc+ τ− anti-ν_τ) = (1.50 ± 0.16 ± 0.25 ± 0.23)% is obtained, where the third uncertainty is from the external branching fraction of the normalization channel Λb0 → Λc+ π− π+ π−. The ratio of semileptonic branching fractions R(Λc+) ≡ B(Λb0 → Λc+ τ− anti-ν_τ) / B(Λb0 → Λc+ μ− anti-ν_μ) is derived to be 0.242 ± 0.026 ± 0.040 ± 0.059, where the external branching fraction uncertainty from the channel Λb0 → Λc+ μ− anti-ν_μ contributes to the last term. This result is in agreement with the standard model prediction.