A hidden-charm pentaquark state in $Λ^0_b \to J/ψp π^-$ decay

Abstract

We study here the $Λ_b^0 \to J/ψp π^-$ reaction in analogy to the $Λ^0_b \to J/ψp K^-$ one, and we note that in both decays there is a sharp structure (dip or peak) in the $J/ψp$ mass distribution around $4450$ MeV, which is associated in the $Λ^0_b \to J/ψp K^-$ experiment to an exotic pentaquark baryonic state, although in $Λ_b^0 \to J/ψp π^-$ it shows up with relatively low statistics. We analyze the $Λ^0_b \to J/ψp π^-$ interaction along the same lines as the $Λ^0_b \to J/ψp K^-$ one, with the main difference stemming from the reduced Cabibbo strength in the former and the consideration of the $π^-p$ final state interaction instead of the $K^-p$ one. We find that with a minimal input, introducing the $π^-p$ and $J/ψp$ interaction in $S$-wave with realistic interactions, and the empirical $P$-wave and $D$-wave contributions, one can accomplish a qualitative description of the $π^-p$ and $J/ψp$ mass distributions. More importantly, the peak structure followed by a dip of the experimental $J/ψp$ mass distribution is reproduced with the same input as used to describe the data of $Λ^0_b\rightarrow J/ψp K^-$ reaction. The repercussion for the triangular singularity mechanism, invoked in some works to explain the pentaquark peak, is discussed.