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We study here the $\Lambda_b^0 \to J/\psi p \pi^-$ reaction in analogy to the$\Lambda^0_b \to J/\psi p K^-$ one, and we note that in both decays there is asharp structure (dip or peak) in the $J/\psi p$ mass distribution around $4450$MeV, which is associated in the $\Lambda^0_b \to J/\psi p K^-$ experiment to anexotic pentaquark baryonic state, although in $\Lambda_b^0 \to J/\psi p \pi^-$it shows up with relatively low statistics. We analyze the $\Lambda^0_b \toJ/\psi p \pi^-$ interaction along the same lines as the $\Lambda^0_b \to J/\psip K^-$ one, with the main difference stemming from the reduced Cabibbo strengthin the former and the consideration of the $\pi^-p$ final state interactioninstead of the $K^-p$ one. We find that with a minimal input, introducing the$\pi^-p$ and $J/\psi p$ interaction in $S$-wave with realistic interactions,and the empirical $P$-wave and $D$-wave contributions, one can accomplish aqualitative description of the $\pi^-p$ and $J/\psi p$ mass distributions. Moreimportantly, the peak structure followed by a dip of the experimental $J/\psip$ mass distribution is reproduced with the same input as used to describe thedata of $\Lambda^0_b\rightarrow J/\psi p K^-$ reaction. The repercussion forthe triangular singularity mechanism, invoked in some works to explain thepentaquark peak, is discussed.

A hidden-charm pentaquark state in $\Lambda^0_b \to J/\psi p \pi^-$ decay