Narrow nucleon-ψ(2S) bound state and LHCb pentaquarks

We interpret the newly discovered pentaquark P-c(4450) as a bound state of charmonium psi(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 N-c 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 J(P) = (1/2)(-) and J(P) = (3/2)(-) are predicted at the position of the P-c(4450) pentaquark. We find that the nucleon-psi(2S) bound state has a naturally narrow width in the range of tens of MeV. The unitary multiplet partners of the P-c(4450) pentaquark and the generalization to b (b) over bar -nucleon pentaquark bound states are discussed.