The nature of the $P_c$ states from compositeness criteria

Abstract: Based on a coupled-channel approach, we investigate the structures of four $P_c$ states through compositeness criteria. Towards a more precise description of the states, we have obtained refined fit results of the LHCb data on the $J/\psi p$ invariant mass distribution of the $\Lambda_b^0\to J/\psi p K^-$ decay. Allowing for the fact that each of the four $P_c$ states couples strongly to a nearby $S$-wave channel, three criteria on the compositeness/elementariness are adopted in this study: the pole-counting rule, the spectral density function, and the Gamow wave function. Compositeness information is extracted from the scattering amplitudes and the pole parameters (pole positions and residues), without any preconceived assumptions on the nature of the $P_c$ states. Consistently within the framework of all the three methods, it has been found that the $P_c(4312)\,1/2^-$ is mainly composed by $\bar{D}\Sigma_c$, $P_c(4380)\,3/2^-$ by $\bar{D}\Sigma_c^*$, while the $P_c(4440)\,1/2^-$ and $P_c(4457)\,3/2^-$ states are both turn out as composite states of $\bar{D}^*\Sigma_c$. The upper limits of the values of their elementariness are estimated to be rather small. This paper provides an additional confirmation of the molecular interpretation for the $P_c$ states in the literature.