First observation of the decay Λ0𝑏→𝜂𝑐⁡(1⁢𝑆)⁢𝑝⁢𝐾−

The decay Λ0𝑏→𝜂𝑐⁡(1⁢𝑆)⁢𝑝⁢𝐾− is observed for the first time using a data sample of proton-proton collisions, corresponding to an integrated luminosity of 5.5  fb−1, collected with the LHCb experiment at a center-of-mass energy of 13 TeV. The branching fraction of the decay is measured, using the Λ0𝑏→𝐽/𝜓⁢𝑝⁢𝐾− decay as a normalization mode, to be ℬ⁡(Λ0𝑏→𝜂𝑐⁡(1⁢𝑆)⁢𝑝⁢𝐾−)=(1.06±0.16±0.0⁢6+0.22
−0.19)×10−4, where the quoted uncertainties are statistical, systematic and due to external inputs, respectively. A study of the 𝜂𝑐⁡(1⁢𝑆)⁢𝑝 mass spectrum is performed to search for the 𝑃𝑐⁢(4312)+ pentaquark state. No evidence is observed and an upper limit of ℬ⁡(Λ0𝑏→𝑃𝑐⁢(4312)+⁢𝐾−)×ℬ⁡(𝑃𝑐⁢(4312)+→𝜂𝑐⁡(1⁢𝑆)⁢𝑝)ℬ⁡(Λ0𝑏→𝜂𝑐⁡(1⁢𝑆)⁢𝑝⁢𝐾−)<0.24 is obtained at the 95% confidence level.