Observation of the Decay Λ 0 b → Λ + c τ − ¯ ν τ

The first observation of the semileptonic b-baryon decay Λ0b→Λ+cτ−¯ντ, with a significance of 6.1σ, 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(Λ0b→Λ+cτ−¯ντ)/B(Λ0b→Λ+cπ−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(Λ0b→Λ+cτ−¯ντ)=(1.50±0.16±0.25±0.23)% is obtained, where the third uncertainty is from the external branching fraction of the normalization channel Λ0b→Λ+cπ−π+π−. The ratio of semileptonic branching fractions R(Λ+c)≡B(Λ0b→Λ+cτ−¯ντ)/B(Λ0b→Λ+cμ−¯νμ) is derived to be 0.242±0.026±0.040±0.059, where the external branching fraction uncertainty from the channel Λ0b→Λ+cμ−¯νμ contributes to the last term. This result is in agreement with the standard model prediction.