Interpretation of the narrow J/ψp peaks in Λb → J/ψ p K- decay in the compact diquark model

Recently, the LHCb Collaboration have updated their analysis of the resonant J/psi p mass spectrum in the decay Lambda(0)(b) -> J/psi p K-. In the combined Run 1 and Run 2 LHCb data, three peaks are observed, with the former P-c(4450)(+) state split into two narrow states, P-c(4440)(+) and P-c(4457)(+), having the masses M = (4440.3 +/- 1.3(-4.7)(+4.1)) MeV and M = (4457.3 +/- 0.6(-1.7)(+4.1)) MeV, and decay widths Gamma = (20.6 +/- 4.9(-10.1)(+8.7)) MeV and Gamma = (6.4 +/- 2.0(-1.9)(+5.7)) MeV, respectively. In addition, a third narrow peak, P-c(4312)(+), having the mass M = (4311.9 +/- 0.7(-0.6)(+6.8)) MeV and decay width Gamma = (9.8 +/- 2.7(-4.5)(+3.7)) MeV is also observed. The LHCb analysis is not sensitive to broad J/psi p contributions like the former P-c(4380)(+), implying that there could be more states present in the data. Also the spin-parity, J(P), assignments of the states are not yet determined. We interpret these resonances in the compact diquark model as hidden-charm diquark-diquark-antiquark baryons, having the following spin and angular momentum quantum numbers: P-c(4312)(+)={(c) over bar [cu](s=1) [ud](s=0); L-P = 0, J(P) = 3/2(-)}, the S-wave state, and the other two as P-wave states, with P-c(4440)(+)={(c) over bar [cu](s=1) [ud](s=0); L-P = 1, J(P) = 3/2(+)} and P-c(4457)(+)={(c) over bar [cu](s=1) [ud](s=0); L-P = 1, J(P) = 5/2(+)}. The subscripts denote the spins of the diquarks and L-P = 0, 1 is the orbital angular momentum quantum number of the pentaquark. These assignments are in accord with the heavy-quark-symmetry selection rules for Lambda(b)-baryon decays, in which the spin S = 0 of the light diquark [ud](s=0) is conserved. The masses of observed states can be accommodated in this framework and the two heaviest states have the positive parities as opposed to the molecular-like interpretations. In addition, we predict several more states in the J/psi p mass spectrum, and urge the LHCb Collaboration to search for them in their data. (C) 2019 The Author(s). Published by Elsevier B.V