Positive parity pentaquark towers in large N c QCD

We construct the complete set of positive parity pentaquarks, which correspond in the quark model to s̄qNc+1 states with one unit of orbital angular momentum L = 1. In the large Nc limit they fall into the K = 1/2 and K = 3/2 irreps (towers) of the contracted SU(4)c symmetry. We derive predictions for the mass spectrum and the axial couplings of these states at leading order in 1/Nc. The strong decay width of the lowest-lying positive parity exotic state is of order O(1/Nc), such that this state is narrow in the large Nc limit. Replacing the antiquark with a heavy antiquark Q̄q Nc+1, the two towers become degenerate, split only by O(1/mQ) hyperfine interactions. We obtain predictions for the strong decay widths of heavy pentaquarks to ordinary baryons and heavy H (∗) Q̄ mesons at leading order in 1/Nc and 1/mQ.