The hidden charm pentaquarks are the hidden color-octet $uud$ baryons?

Abstract

The $I(J^P)={1\over 2}({1\over 2}^-)$, ${1\over 2}({3\over 2}^-)$, and ${1\over 2}({5\over 2}^-)$ $uudc\overline{c}$ pentaquarks are investigated by the quark cluster model. This model, which reproduces the mass spectra of the color-singlet $S$-wave $q^3$ baryons and $q\overline{q}$ mesons, also enables us to evaluate the quark interaction in the color-octet $uud$ configurations. It is shown that the color-octet isospin-${1\over 2}$ spin-${3\over 2}$ $uud$ configuration gains attraction. The $uudc\overline{c}$ states with this configuration have structures around the $Σ_c{}^{(*)}\overline{D}{}^{(*)}$ thresholds: one bound state, two resonances, and one large cusp are found. We argue that the negative parity pentaquark found by the LHCb experiments may be given by these structures.