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

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Abstract

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.

Figures and captions

Distribution of (left) $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}$ and (right) $m_{Kp}$ for $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates. The prominent peak in $m_{Kp}$ is due to the $\Lambda (1520)$ resonance.

mjpsip[..].pdf [28 KiB]
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mjpsip-spectrum-all.pdf
mkp-sp[..].pdf [22 KiB]
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mkp-spectrum.pdf

Dalitz plot of $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates. The data contain $6.4\%$ of non-$\Lambda ^0_ b $ backgrounds, which are distributed smoothly over the phase-space. The vertical bands correspond to the $\Lambda ^*$ resonances. The horizontal bands correspond to the $P_c(4312)^+$, $P_c(4440)^+$, and $P_c(4457)^+$ structures at $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}^2 = 18.6$, $19.7$, and $19.9\text{ Ge V} ^2$, respectively.

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Distribution of $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}$ from $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates after suppression of the dominant $\Lambda ^*\rightarrow p K ^- $ contributions with the $m_{Kp}>1.9$ $\text{ Ge V}$ requirement. The inset shows a zoom into the region of the narrow $P_c^+$ peaks.

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mjpsip-spectrum-19.pdf

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

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weight.pdf

Fits to the $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}$ distributions of the (top row) inclusive, (middle row) $m_{Kp}>1.9$ $\text{ Ge V}$ , 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.

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mjpsip2x3.pdf

Fit to the $\cos\theta_{Pc}$-weighted $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}$ 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 $\Sigma ^+_ c \overline{ D } {}^0 $ and $\Sigma ^+_ c \overline{ D } {}^{*0} $ final states are superimposed.

pentaq[..].pdf [52 KiB]
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pentaquarks_nominal_fit_and_thresholds.pdf

Invariant mass spectrum of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates. The $\Lambda ^0_ b $ signal region is between the vertical red lines. A linear interpolation of the background, determined from the sideband regions (bounded by the shorter vertical blue lines), to the signal region is shown by the dashed blue line.

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Lb_mass.pdf

Fit to the $\cos\theta_{Pc}$-weighted $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p}$ distribution with four BW amplitudes and a linear background. The broad $P_c^+$ state is added coherently to the $P_c(4312)^+$ amplitude. In this fit model, the magnitude of the $P_c(4312)^+$ peak in the data is dominated by its interference with the broad $P_c^+$ state. Each $P_c^+$ contribution is displayed as the BW amplitude squared (the interference contributions are not shown individually).

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bw4p1_14_cosw_nom_did_0_npoly_1.pdf

Triangle diagram for the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ decay. The figure defines the symbols used in the formulae in the text.

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triangle.png

Fit of three triangle-diagram amplitudes and a quadratic background to the $\cos\theta_{Pc}$-weighted distribution. The widths of the excited particles exchanged in the triangles is (top) an unrealistic value of 1 $\text{ Me V}$ or (bottom) a more plausible value of 50 $\text{ Me V}$ . Individual triangle diagram contributions are also shown. The dashed vertical lines are the $\Lambda ^+_ c \overline{ D } {}^{*0} $, $\chi_{c0}p$ and $\Lambda ^+_ c (2595)\overline{ D } {}^0 $ thresholds.

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triangleFits.pdf

Fit of two BW amplitudes, one triangle-diagram amplitude, and a sixth-order polynomial background to the $\cos\theta_{Pc}$-weighted distribution. The width of the excited $D_s^*$ state exchanged in the triangle loop is set to $\Gamma(D_{s1}(2860)^-)=159$ $\text{ Me V}$ \cite{Tanabashi:2018oca,LHCb-PAPER-2014-035}. The predicted width for this $D_s^*$ state, interpreted as the $1^3D_1$ $s\bar{c}$ excitation in the quark model, is 197 $\text{ Me V}$ \cite{Godfrey:2015dva}.

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bw2t1p6_cosw_159MeV_did_0_npoly_6.pdf

Animated gif made out of all figures.

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Tables and captions

Summary of $P_c^+$ properties. The central values are based on the fit displayed in Fig. ???.

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Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [873 KiB]
supplementary.pdf

Please see supplementary.pdf for description of figures.

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Created on 20 August 2019.Citation count from INSPIRE on 23 August 2019.