The LHCb Collaboration has recently observed two narrow baryon resonances with beauty. Their masses and decay modes look consistent with the quark model orbitally excited states ${\Lambda }_b(5912)$ and ${\Lambda }_b^{*}(5920)$, with quantum numbers $J^P=1/2^{-}$ and $3/2^{-}$, respectively. We predict the existence of these states within a unitarized meson-baryon coupled-channel dynamical model, which implements heavy-quark spin symmetry. Masses, quantum numbers and couplings of these resonances to the different meson-baryon channels are obtained. We find that the resonances ${\Lambda }_b^0(5912)$ and ${\Lambda }_b^0(5920)$ are heavy-quark spin symmetry partners, which naturally explains their approximate mass degeneracy. Corresponding bottom-strange baryon resonances are predicted at ${\Xi }_b(6035.4)$ ($J^P=\frac{1}{2}\genfrac{}{}{0ex}{}{{\mathrm{}}^{-}}{}$) and ${\Xi }_b(6043.3)$ ($J^P=\frac{3}{2}\genfrac{}{}{0ex}{}{{\mathrm{}}^{-}}{}$). The two ${\Lambda }_b$ and two ${\Xi }_b$ resonances complete a multiplet of the combined symmetry SU(3)-flavor times heavy-quark spin.

• Received 15 January 2013

DOI:https://doi.org/10.1103/PhysRevD.87.034032