Abstract
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.
Highlights
In 2015, the LHCb Collaboration reported the first observation of two hidden-charm pentaquark states Pc (4380)+ and Pc(4450)+ in the decay 0 b → J /ψ p K−[1], having the masses M = (4380 ±8 ± 29) MeV and M = (4449.8 ± 1.7 ± 2.5) MeV, and decay widths= (205 ± 18 ± 86) MeV and = (39 ± 5 ± 19) MeV, with the preferred spin-parity assignments J P = 3/2− and J P = 5/2+, respectively, but the reversed spin-parity assignments were tenable
Imposing the spin conservation in the heavyquark symmetry limit, which was already advocated in the analysis of the older LHCb data [22], we argue that only that part of the pentaquark spectrum is reachable in b -decays, in which the pentaquarks have a “good” light diquark, i.e., having the spin
In the case of the orbitally excited pentaquarks, the orbital angular momentum L of the pentaquark is the sum of two terms, Lt, arising from the triquark system consisting of the heavy diquark and the charmed antiquark, and Lld, which determines the relative motion of the light diquark around the doubly-heavy triquark
Summary
In 2015, the LHCb Collaboration reported the first observation of two hidden-charm pentaquark states Pc (4380)+ and Pc(4450)+. Imposing the spin conservation in the heavyquark symmetry limit, which was already advocated in the analysis of the older LHCb data [22], we argue that only that part of the pentaquark spectrum is reachable in b -decays, in which the pentaquarks have a “good” light diquark, i.e., having the spin. There are yet more states present in the J /ψ p mass spectrum in the compact diquark model, whose masses and J P quantum numbers are given here, and we urge the LHCb Collaboration to search for them in their data.
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