Narrow Nucleon-\u3cem\u3eψ\u3c/em\u3e(2\u3cem\u3eS\u3c/em\u3e) Bound State and LHCb Pentaquarks

We interpret the newly discovered pentaquark Pc(4450) as a bound state of charmonium ψ(2S) and the nucleon. The binding potential is due to the charmonium-nucleon interaction that in the heavy quark approximation is proportional to the product of the charmonium chromoelectric polarizability and the nucleon energy-momentum distribution. We use the large Nc expansion to estimate the quarkonium polarizability and calculate the nucleon properties in the framework of the mean-field picture of light baryons. Two almost degenerate states JP=(1/2)− and JP=(3/2)− are predicted at the position of the Pc(4450) pentaquark. We find that the nucleon-ψ(2S) bound state has a naturally narrow width in the range of tens of MeV. The unitary multiplet partners of the Pc(4450) pentaquark and the generalization to bb¯-nucleon pentaquark bound states are discussed