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

Abstract

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

9 pages, 5 figures; to be published in PRD

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