Flavor-singlet charm pentaquark

A new type of charm pentaquark $P_{cs}$ with quark content $c\bar{c}uds$ in light-flavor singlet state is studied in the quark model. This state is analogous to the $P_{c}$ with $c\bar{c}uud$ in light-flavor octet, which was observed in LHC in 2015. Considering various combinations of color, spin and light flavor as internal quantum numbers in $P_{cs}$, we investigate the mass ordering of the $P_{cs}$'s by adopting both the one-gluon exchange interaction and the instanton-induced interaction in the quark model. The most stable configuration of $P_{cs}$ is identified to be total spin $1/2$ in which the $c\bar{c}$ is combined to be color octet and spin $1$, while the $uds$ cluster is in a color octet state. The other color octet configurations, the total spin $1/2$ state with the $c\bar{c}$ spin 0 and the state with total spin $3/2$ and $c\bar{c}$ spin 1, are found as excited states. We also discuss possible decay modes of these charm pentaquarks.