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We calculate the invariant and helicity amplitudes for the transitions$\Lambda_b~\to~\Lambda^{(\ast)}(J^P)~+~J/\psi$ where the$\Lambda^{(\ast)}(J^P)$ are $\Lambda(sud)$-type ground and excited states with$J^P$ quantum numbers $J^P=\frac12^{\pm},\frac32^{\pm}$. The calculations areperformed in the framework of a covariant confined quark model previouslydeveloped by us. We find that the values of the helicity amplitudes for the$\Lambda^\ast(1520,\,\frac32^-)$ and the $\Lambda^\ast(1890,\,\frac32^+)$ aresuppressed compared with those for the ground state $\Lambda(1116,\,\frac12^+)$and the excited state $\Lambda^\ast(1405,\,\frac12^-)$. This analysis isimportant for the identification of the hidden charm pentaquark states$P_c^+(4380)$ and $P_c^+(4450)$ which were discovered in the decay chain$\Lambda_b^0~\to~P_c^+(~\to~p~J/\psi)~+~K^- $ because the cascade decay chain$\Lambda_b~\to~\Lambda^\ast(\frac32^\pm)(~\to~pK^-)~+~J/\psi$ involves the samefinal state.

Theoretical description of the decays $\Lambda_b \to \Lambda^{(\ast)}(\frac12^\pm,\frac32^\pm) + J/\psi$