Lowest-lying ${\frac{1}{2}}^{-}$ and ${\frac{3}{2}}^{-}$ ${\Lambda }_Q$ resonances: from the strange to the bottom sectors

Abstract

We present a detailed study of the lowest-lying ${\frac{1}{2}}^{-}$ and ${\frac{3}{2}}^{-}$ ${\Lambda }_Q$ resonances both in the heavy quark (bottom and charm) and the strange sectors. We have paid special attention to the interplay between the constituent quark-model and chiral baryon-meson degrees of freedom, which are coupled using a unitarized scheme consistent with leading-order heavy quark symmetries. We show that the ${\Lambda }_b(5912)$ [$J^P=1/2^{-}$], ${\Lambda }_b(5920)$ [$J^P=3/2^{-}$] and the ${\Lambda }_c(2625)$ [$J^P=3/2^{-}$], and the $\Lambda (1520)$ [$J^P=3/2^{-}$] admitting larger breaking corrections, are heavy-quark spin-flavor siblings. They can be seen as dressed quark-model states with ${\Sigma }_Q^{(\ast )}\pi$ molecular components of the order of 30\%. The ${J^P=\frac{1}{2}}^{-}$ ${\Lambda }_c(2595)$ has, however, a higher molecular probability of at least $50$\%, and even values greater than 70\% can be easily accommodated. This is because it is located almost on top of the threshold of the ${\Sigma }_c\pi$ pair, which largely influences its properties. Although the light degrees of freedom in this resonance would be coupled to spin-parity $1^{-}$ as in the ${\Lambda }_b(5912)$, ${\Lambda }_b(5920)$ and ${\Lambda }_c(2625)$, the ${\Lambda }_c(2595)$ should not be considered as a heavy-quark spin-flavor partner of the former ones. We also show that the $\Lambda (1405)$ chiral two-pole pattern does not have analogs in the ${\frac{1}{2}}^{-}$ charmed and bottomed sectors, because the $N{D}^{(\ast )}$ and $N\overline{B}{}^{(\ast )}$ channels do not play for heavy quarks the decisive role that the $N\overline{K}$ does in the strange sector, and the notable influence of the bare quark-model states for the charm and bottom resonances. Finally, we predict the existence of two ${\Lambda }_b(6070)$ and two ${\Lambda }_c(2765)$ heavy-quark spin and flavor sibling odd parity states.

Comments: 41 pages, 13 figures, and 7 tables