Heavy pentaquark states and a novel color structure
  Encouraged by the observation of the pentaquark states $P^+_c(4380)$ and $P^+_c(4450)$, we propose a novel color flux-tube structure, pentagonal state, for pentaquark states within the framework of color flux-tube mode involving a five-body confinement potential. Numerical results on the heavy pentaquark states indicate that the states with three color flux-tube structures, diquark, octet and pentagonal structures, have the close masses, which can therefore be called QCD isomers analogous to isomers in Chemistry. The pentagonal structure has lowest energy. The state $P^+_c(4380)$ can be described as the compact pentaquark state $uudc\bar{c}$ with the pentagonal structure and $J^P=\frac{3}{2}^-$ in the color flux-tube model. The state $P^+_c(4450)$ can not be accommodated into the color flux-tube model. The heavy pentaquark states $uudc\bar{b}$, $uudb\bar{c}$ and $uudb\bar{b}$ are predicted in the color flux-tube model. The five-body confinement potential basing on the color flux-tube picture as a collective degree of freedom is a dynamical mechanism in the formation of the compact heavy pentaquark states. 
Chengrong Deng, Jialun Ping, Hongxia Huang, and Fan Wang
hep-ph