Interpretation of the narrow J/ψ p peaks in Λ  → J/ψ p K− decay in the compact diquark model

Recently, the LHCb Collaboration have updated their analysis of the resonant J/ψp mass spectrum in the decay Λb0→J/ψpK−. In the combined Run 1 and Run 2 LHCb data, three peaks are observed, with the former Pc(4450)+ state split into two narrow states, Pc(4440)+ and Pc(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 Γ=(20.6±4.9−10.1+8.7) MeV and Γ=(6.4±2.0−1.9+5.7) MeV, respectively. In addition, a third narrow peak, Pc(4312)+, having the mass M=(4311.9±0.7−0.6+6.8) MeV and decay width Γ=(9.8±2.7−4.5+3.7) MeV is also observed. The LHCb analysis is not sensitive to broad J/ψp contributions like the former Pc(4380)+, implying that there could be more states present in the data. Also the spin-parity, JP, 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: Pc(4312)+={c¯[cu]s=1[ud]s=0;LP=0,JP=3/2−}, the S-wave state, and the other two as P-wave states, with Pc(4440)+={c¯[cu]s=1[ud]s=0;LP=1,JP=3/2+} and Pc(4457)+={c¯[cu]s=1[ud]s=0;LP=1,JP=5/2+}. The subscripts denote the spins of the diquarks and LP=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 Λb-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/ψp mass spectrum, and urge the LHCb Collaboration to search for them in their data.