Compact hidden charm pentaquark states and QCD isomers

Abstract

We make an exhaustive investigation on the pentaquark states $qqqc\overline{c}$ ($q=u$, $d$ and $s$) and discuss the effect of color structures in a multiquark color flux-tube model. We exhibit a novel picture of the structure and properties of the states ${P}_{c}$ and ${P}_{cs}$ observed by the LHCb Collaboration. We can describe the states as the compact pentaquark states in the model. The spin-parity of the group of ${P}_{c}(4312{)}^{+}$ and ${P}_{c}(4337{)}^{+}$ is ${\frac{1}{2}}^{\ensuremath{-}}$ while that of the group of ${P}_{c}(4380{)}^{+}$, ${P}_{c}(4440{)}^{+}$ and ${P}_{c}(4457{)}^{+}$ is ${\frac{3}{2}}^{\ensuremath{-}}$. Their structures are pentagon, diquark, pentagon, diquark, and octet, respectively. The members in each group can be analogically called QCD isomers because of their the same spin-parity and quark content but different color structures. The singlet ${P}_{cs}(4459{)}^{0}$ has pentagon structure and spin-parity of ${\frac{1}{2}}^{\ensuremath{-}}$. In addition, we also predict the ${P}_{cs}$, ${P}_{css}$ and ${P}_{csss}$ families in the model. The five-body confinement potential based on the color flux-tube picture, which is a collective degree of freedom and induces QCD isomer phenomenon, plays an important role in the formation of the compact states.