The ccbar pentaquarks by a quark model

Recent LHCb experiments have shown us that there are two resonances in the J/ψp channel in the Λ_b decay, whose spin and parity are most probably (3/2⁻ 5/2⁺). In this work, we investigate the I(J^P)=1/2(1/2^-), 1/2(3/2^-), and 1/2(5/2^-) uudcc-bar pentaquark states by employing the quark cluster model. It is found that the color-octet isospin -1/2 spin -3/2 und configuration gives an attraction to such five-quark systems. This configuration together with the color-octer cc-bar pair gives structures around the Σ_c(*)D-bar(*) thresholds: one bound state, two resonances, and one large cusp are found in the uudcc-bar negative parity channels. We argue that these resonances and cusp may correspond to, or combine to form, the negative parity pentaquark peak observed by LHCb. (author)