Observation of a narrow pentaquark state, $P_c(4312)^+$, and of two-peak structure of the $P_c(4450)^+$

A narrow pentaquark state, $P_c(4312)^+$, decaying to $J/psi p$ is discovered with a statistical significance of $7.3sigma$ in a data sample of ${Lambda_b^0to J/psi p K^-}$ decays which is an order of magnitude larger than that previously analyzed by the LHCb collaboration. The $P_c(4450)^+$ pentaquark structure formerly reported by LHCb is confirmed and observed to consist of two narrow overlapping peaks, $P_c(4440)^+$ and $P_c(4457)^+$, where the statistical significance of this two-peak interpretation is $5.4sigma$. Proximity of the $Sigma_c^+bar{D}^{0}$ and $Sigma_c^+bar{D}^{*0}$ thresholds to the observed narrow peaks suggests that they play an important role in the dynamics of these states.