Pentaquark states in diquark-triquark model

Abstract

The diquark-triquark model is adopted to explain the charmonium-pentaquark states, i.e. the $P_c(4380)$ and $P_c(4450)$ which recently have been observed by the LHCb collaboration. We for the first time investigate the properties of the color attractive configuration of a triquark and define a nonlocal light cone distribution amplitude for pentaquark states, where both diquark and triquark are not pointlike, but have a nonzero size. An effective diquark-triquark Hamiltonian based on spin-orbital interaction is established. According to the Hamiltonian, we find that the minimum mass splitting between $\frac{5}{2}^+$ and $\frac{3}{2}^-$ is around $100$~MeV, which may naturally solve the challenging problem of the small mass splitting between $P_c(4450)$ and $P_c(4380)$. The peculiarities of $P_c(4380)$ having a broad decay width while $P_c(4450)$ having a narrow decay width are understandable. Based on diquark-triquark model we predict more pentaquark states, which are hopefully measurable in future experiment.