  A narrow pentaquark state, $P_c(4312)^+$, decaying to $J/\psi p$ is
discovered with a statistical significance of $7.3\sigma$ in a data sample of
${\Lambda_b^0\to J/\psi p K^-}$ 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 $5.4\sigma$.
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.
