  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 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
$\Lambda^\ast(1520,\,\frac32^-)$ and the $\Lambda^\ast(1890,\,\frac32^+)$ are
suppressed compared with those for the ground state $\Lambda(1116,\,\frac12^+)$
and the excited state $\Lambda^\ast(1405,\,\frac12^-)$. 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
$\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 same
final state.
