Novel pentaquark picture for singly heavy baryons from chiral symmetry

We propose a new type of structure for singly heavy baryons of $Qqq\overline{q}q$ in addition to the conventional one of $Qqq$. Based on chiral symmetry of the light quarks, we show that the $Qqq\overline{q}q$ baryon offers a novel picture for heavy-quark spin-singlet and flavor-antisymmetric baryons. By making use of the effective Lagrangian approach, we find that ${\mathrm{\Lambda }}_c(2765)$ and ${\mathrm{\Xi }}_c(2967)$ are mostly $Qqq\overline{q}q$, while ${\mathrm{\Lambda }}_c(2286)$ and ${\mathrm{\Xi }}_c(2470)$ are mostly $Qqq$. The masses of negative-parity baryons are predicted. We also derive a sum rule and the extended Goldberger-Treiman relation that the masses of the baryons satisfy. Furthermore, a mass degeneracy of parity partners of the baryons with the restoration of chiral symmetry is discussed. These are the unique features that the conventional picture of radial excitation in the quark model does not accommodate. Our findings provide useful information not only for future experiments but also for future lattice simulations on diquarks.

Figure 1

A sketch of the conventional picture (left) and the novel picture proposed in this paper (right) for describing singly heavy baryons.

Figure 2

Mass spectrum of ${\mathrm{\Lambda }}_c$’s and ${\mathrm{\Xi }}_c$’s with $J^P={\frac{1}{2}}^{+},{\frac{1}{2}}^{-}$ obtained in our model. The asterisk ($*$) stands for the inputs. The ratios indicated under the bars correspond to the components of three-quark and pentaquark states in each baryon: $Qqq:Qqq\overline{q}q$, in which the upper and lower ratios correspond to the parameter sets (I) and (II), respectively.

Figure 3

The left panel shows the ${\sigma }_s$ dependence of $M(B_{+,i=3}^L)$, $M(B_{-,i=3}^L)$, $M(B_{+,i=3}^H)$, and $M(B_{-,i=3}^H)$, while the right panel shows the ${\sigma }_q$ dependence of $M(B_{+,i=1,2}^L)$, $M(B_{-,i=1,2}^L)$, $M(B_{+,i=1,2}^H)$, and $M(B_{-,i=1,2}^H)$. The results are identical for the parameter sets (I) and (II) in Table 1.