Very narrow excited $Ω_c$ baryons

Abstract

Recently LHCb reported the discovery of five extremely narrow excited $Ω_c$ baryons decaying into $Ξ_c^+ K^-$. We interpret these baryons as bound states of a $c$-quark and a $P$-wave $ss$-diquark. For such a system there are exactly five possible combinations of spin and orbital angular momentum. The narrowness of the states could be a signal that it is hard to pull apart the two $s$-quarks in a diquark. We predict two of spin 1/2, two of spin 3/2, and one of spin 5/2, all with negative parity. Of the five states two can decay in $S$-wave and three can decay in $D$-wave. Some of the $D$-wave states might be narrower than the $S$-wave states. We discuss relations among the five masses expected in the quark model and the likely spin assignments and compare with the data. A similar pattern is expected for negative-parity excited $Ω_b$ states. An alternative interpretation is noted in which the heaviest two states are $2S$ excitations with $J^P = 1/2^+$ and $3/2^+$, while the lightest three are those with $J^P = 3/2^-,3/2^-,5/2^-$ expected to decay via $D$-waves. In this case we expect $J^P = 1/2^-$ $Ω_c$ states around 2904 and 2978 MeV.