Observation of $J/ψp$ resonances consistent with pentaquark states in ${Λ_b^0\to J/ψK^-p}$ decays

TL;DR

The LHCb analysis performs a full amplitude analysis of Λb0→J/ψK−p decays to search for exotic pentaquark-like states. Employing a sophisticated helicity formalism with two interfering decay chains, the study finds two resonant structures in the J/ψp channel, Pc+(4380) and Pc+(4450), with opposite parity assignments (J^P ~ 3/2^− and 5/2^+ respectively) and significances well above 9σ individually and 15σ combined. The results require both Pc+ states to describe the J/ψp mass–angular distributions, and the analysis carefully accounts for Λ* backgrounds via reduced and extended Λ* models, mass- and width-dependent line shapes, and efficiency effects. The findings provide strong evidence for charmonium-pentaquark states, expanding the spectrum of exotic hadrons and informing models of multiquark dynamics in QCD.

Abstract

Observations of exotic structures in the $J/ψp$ channel, that we refer to as pentaquark-charmonium states, in $Λ_b^0\to J/ψK^- p$ decays are presented. The data sample corresponds to an integrated luminosity of 3/fb acquired with the LHCb detector from 7 and 8 TeV pp collisions. An amplitude analysis is performed on the three-body final-state that reproduces the two-body mass and angular distributions. To obtain a satisfactory fit of the structures seen in the $J/ψp$ mass spectrum, it is necessary to include two Breit-Wigner amplitudes that each describe a resonant state. The significance of each of these resonances is more than 9 standard deviations. One has a mass of $4380\pm 8\pm 29$ MeV and a width of $205\pm 18\pm 86$ MeV, while the second is narrower, with a mass of $4449.8\pm 1.7\pm 2.5$ MeV and a width of $39\pm 5\pm 19$ MeV. The preferred $J^P$ assignments are of opposite parity, with one state having spin 3/2 and the other 5/2.

Figures (19)

• Figure 1: Feynman diagrams for (a) ${{\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace}\xspace\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace }}\xspace {\Lambda\xspace}\xspace^*$ and (b) ${{\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace}\xspace\rightarrow\xspace P_c^+ K^-$ decay.
• Figure 2: Invariant mass of (a) $K^-p$ and (b) ${{\mathrm{J}\xspace / \uppsi\xspace }}\xspace p$ combinations from ${{\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace}\xspace\rightarrow\xspace{{\mathrm{J}\xspace / \uppsi\xspace }}\xspace K^-p$ decays. The solid (red) curve is the expectation from phase space. The background has been subtracted.
• Figure 3: Fit projections for (a) $m_{Kp}$ and (b) $m_{{{\mathrm{J}\xspace / \uppsi\xspace }}\xspace p}$ for the reduced ${\Lambda\xspace}\xspace^*$ model with two $P_c^+$ states (see Table \ref{['tab:Lstar']}). The data are shown as solid (black) squares, while the solid (red) points show the results of the fit. The solid (red) histogram shows the background distribution. The (blue) open squares with the shaded histogram represent the $P_c(4450)^+$ state, and the shaded histogram topped with (purple) filled squares represents the $P_c(4380)^+$ state. Each ${\Lambda\xspace}\xspace^*$ component is also shown. The error bars on the points showing the fit results are due to simulation statistics.
• Figure 4: Invariant mass spectrum of ${{\mathrm{J}\xspace / \uppsi\xspace }}\xspace K^-p$ combinations, with the total fit, signal and background components shown as solid (blue), solid (red) and dashed lines, respectively.
• Figure 5: Invariant mass squared of $K^-p$ versus ${{\mathrm{J}\xspace / \uppsi\xspace }}\xspace p$ for candidates within $\pm15$ MeV of the ${\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace$ mass.