Theoretical description of the decays $Λ_b \to Λ^{(\ast)}(\frac12^\pm,\frac32^\pm) + J/ψ$
We calculate the invariant and helicity amplitudes for the transitions $Λ_b~\to~Λ^{(\ast)}(J^P)~+~J/ψ$ where the $Λ^{(\ast)}(J^P)$ are $Λ(sud)$-type ground and excited states with $J^P$ quantum numbers $J^P=\frac12^{\pm},\frac32^{\pm}$. The calculations are performed in the framework of a covariant confined quark model previously developed by us. We find that the values of the helicity amplitudes for the $Λ^\ast(1520,\,\frac32^-)$ and the $Λ^\ast(1890,\,\frac32^+)$ are suppressed compared with those for the ground state $Λ(1116,\,\frac12^+)$ and the excited state $Λ^\ast(1405,\,\frac12^-)$. This analysis is important for the identification of the hidden charm pentaquark states $P_c^+(4380)$ and $P_c^+(4450)$ which were discovered in the decay chain $Λ_b^0~\to~P_c^+(~\to~p~J/ψ)~+~K^- $ because the cascade decay chain $Λ_b~\to~Λ^\ast(\frac32^\pm)(~\to~pK^-)~+~J/ψ$ involves the same final state.