Observation of the Decay Λ0𝑏→Λ+𝑐⁢𝜏−⁢¯𝜈𝜏

The first observation of the semileptonic 𝑏-baryon decay Λ0𝑏→Λ+𝑐⁢𝜏−⁢¯𝜈𝜏, 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⁡(Λ0𝑏→Λ+𝑐⁢𝜏−⁢¯𝜈𝜏)/ℬ⁡(Λ0
𝑏→Λ+𝑐⁢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⁡(Λ0𝑏→Λ+𝑐⁢𝜏−⁢
¯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+𝑐⁢𝜋−⁢𝜋+⁢𝜋−. The ratio of semileptonic branching fractions ℛ⁡(Λ+𝑐)≡ℬ⁡(Λ0𝑏→Λ+𝑐⁢𝜏−¯𝜈𝜏)/ℬ⁡(Λ0
𝑏→Λ+𝑐⁢𝜇−⁢¯𝜈𝜇) is derived to be 0.242±0.026±0.040±0.059, where the external branching fraction uncertainty from the channel Λ0𝑏→Λ+𝑐⁢𝜇−⁢¯𝜈𝜇 contributes to the last term. This result is in agreement with the standard model prediction.