The observation of the $P_{ cs}(4459)$ by the LHCb collaboration adds a new member to the set of known hidden-charm pentaquarks, which includes the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$. The $P_{ cs}(4459)$ is expected to have the light-quark content of a $\Lambda$ baryon ($I=0$, $S=-1$), but its spin is unknown. Its closeness to the $\bar{ D}^* \Xi_c$ threshold -- $4478\,{ \rm MeV}$ in the isospin-symmetric limit -- suggests the molecular hypothesis as a plausible explanation for the $P_{ cs}(4459)$. While in the absence of coupled-channel dynamics heavy-quark spin symmetry predicts the two spin-states of the $\bar{ D}^* \Xi_c$ to be degenerate, power counting arguments indicate that the coupling with the nearby $\bar{ D} \Xi_c'$ and $\bar{ D} \Xi_c^*$ channels might be a leading order effect. This generates a hyperfine splitting in which the $J=\tfrac{ 3}{ 2}$ $\bar{ D}^* \Xi_c$ pentaquark will be lighter than the $J=\tfrac{ 1}{ 2}$ configuration, which we estimate to be of the order of $5-15\,{ \rm MeV}$. We also point out an accidental symmetry between the $P_{ cs}(4459)$ and $P_c(4440/4457)$ potentials. Finally, we argue that the spectroscopy and the $J/\psi \Lambda$ decays of the $P_{ cs}(4459)$ might suggest a marginal preference for $J = \tfrac{ 3}{ 2}$ over $J = \tfrac{ 1}{ 2}$.
Categories: hep-ph hep-ex hep-lat nucl-th