Observation of a Narrow Pentaquark State, $P_c(4312{)}^{+}$, and of the 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 $7.3\sigma$ in a data sample of ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ 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 $5.4\sigma$. The proximity of the ${\mathrm{\Sigma }}_c^{+}{\overline{D}}^0$ and ${\mathrm{\Sigma }}_c^{+}{\overline{D}}^{*0}$ thresholds to the observed narrow peaks suggests that they play an important role in the dynamics of these states.

Figure 1

Distribution of (left) $m_{J/\psi p}$ and (right) $m_{Kp}$ for ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ candidates. The prominent peak in $m_{Kp}$ is due to the $\mathrm{\Lambda }(1520)$ resonance.

Figure 2

Dalitz plot of ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ candidates. The data contain 6.4% of non-${\mathrm{\Lambda }}_b^0$ backgrounds, which are distributed smoothly over the phase space. The vertical bands correspond to the ${\mathrm{\Lambda }}^{*}$ resonances. The horizontal bands correspond to the $P_c(4312{)}^{+}$, $P_c(4440{)}^{+}$, and $P_c(4457{)}^{+}$ structures at $m_{J/\psi p}^2=18.6$, 19.7, and $19.9{\mathrm{GeV}}^2$, respectively.

Figure 3

Distribution of $m_{J/\psi p}$ from ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ candidates after suppression of the dominant ${\mathrm{\Lambda }}^{*}\to p{K}^{-}$ contributions with the $m_{Kp}>1.9\mathrm{GeV}$ requirement. The inset shows a zoom into the region of the narrow $P_c^{+}$ peaks.

Figure 4

Weight function $w(\cos {\theta }_{Pc})$ applied to candidates, determined as the inverse of the density of ${\mathrm{\Lambda }}_b^0$ candidates in the narrow $P_c^{+}$ peak region. The red line is a spline function used to interpolate between bin centers.

Figure 5

Fits to the $m_{J/\psi p}$ distributions of the (top row) inclusive, (middle row) $m_{Kp}>1.9\mathrm{GeV}$, and (bottom row) $\cos {\theta }_{Pc}$-weighted samples with three incoherently summed BW amplitudes representing the narrow $P_c^{+}$ signals on top of a (left column) high-order polynomial function or (right column) lower-order polynomial plus a broad $P_c^{+}$ state represented by a fourth BW amplitude.

Figure 6

Fit to the $\cos {\theta }_{Pc}$-weighted $m_{J/\psi p}$ distribution with three BW amplitudes and a sixth-order polynomial background. This fit is used to determine the central values of the masses and widths of the $P_c^{+}$ states. The mass thresholds for the ${\mathrm{\Sigma }}_c^{+}{\overline{D}}^0$ and ${\mathrm{\Sigma }}_c^{+}{\overline{D}}^{*0}$ final states are superimposed.