Evidence of a $J/ψ\varLambda$ structure and observation of excited $\varXi^-$ states in the $\varXi_b^-\to J/ψ\varLambda K^-$ decay

Abstract: First evidence of a structure in the $J/\psi \varLambda$ invariant mass distribution is obtained from an amplitude analysis of $\varXi_b^- \to J/\psi \varLambda K^-$ decays. The observed structure is consistent with being due to a charmonium pentaquark with strangeness. Its mass and width are determined to be $4458.8\pm2.9\,^{+4.7}_{-1.1}$MeV and $17.3\pm6.5\,^{+8.0}_{-5.7}$MeV, where the quoted uncertainties are statistical and systematic, respectively. The structure is also consistent with being due to two resonances. In addition, the narrow excited $\varXi^-$ states, $\varXi(1690)^-$ and $\varXi(1820)^-$, are seen for the first time in a $\varXi_b^-$ decay, and their masses and widths are measured with improved precision. The analysis is performed using $pp$ collision data corresponding to a total integrated luminosity of 9fb$^{-1}$, collected with the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13$TeV.

• The paper demonstrates first evidence for a charmonium pentaquark (P_cs^0) with strangeness, measured at 3.1σ significance with precise mass and width values.
• It observes excited Ξ(1690)⁻ and Ξ(1820)⁻ states, achieving improved mass and width measurements through detailed amplitude analysis.
• A sophisticated signal selection method, using bootstrapped decision trees and the sPlot technique, enabled robust background subtraction and resonance identification.

Analysis and Implications of Charmonium Pentaquark and Excited Baryon States in $\jpsi\Lambda$ Mass Spectrum

This paper outlines a comprehensive amplitude analysis conducted by the LHCb Collaboration, focusing on the decay $\Xi_{b}^{-} \rightarrow J/\psi \Lambda K^{-}$. The study reveals significant insights into the structure of the $\jpsi\Lambda$ invariant mass distribution, offering the first evidence for a charmonium pentaquark with strangeness. It further observes excited $\Xi^{-}$ states in the decay process, contributing to our understanding of the baryon spectrum.

Key Findings and Results

1. Discovery of Pentaquark States: The analysis reveals a structure consistent with a charmonium pentaquark with strangeness, designated as $P_{cs}^0$, with a significance of 3.1σ after accounting for systematic uncertainties and the look-elsewhere effect. The mass and width of the identified state are measured at $4458.8\pm2.9\,^{+4.7}_{-1.1}$ MeV and $17.3\pm6.5\,^{+8.0}_{-5.7}$ MeV, respectively.
2. Observation of Excited $\Xi^-$ States: The study marks the first observation of the $\Xi(1690)^-$ and $\Xi(1820)^-$ states in a $\Xi_{b}^{-}$ decay. The masses and widths of these states were determined with improved precision compared to previous experiments. Specifically:
   • $\Xi(1690)^-$: Measured mass $1692.0\pm1.3\,^{+1.2}_{-0.4}$ MeV and width $25.9\pm9.5\,^{+14.0}_{-13.5}$ MeV.
   • $\Xi(1820)^-$: Measured mass $1822.7\pm1.5\,^{+1.0}_{-0.6}$ MeV and width $36.0\pm4.4\,^{+7.8}_{-8.2}$ MeV.
3. Decay Data and Methodology: The study utilizes proton-proton collision data with a total integrated luminosity of 9 fb$^{-1}$ across center-of-mass energies of 7, 8, and 13 TeV. An advanced signal selection and background subtraction methodology was employed using techniques such as bootstrapped Decision Trees and the sPlot method, alongside a kinematic Dalitz plot analysis to ensure accurate representation of underlying resonances and nonresonant contributions.

Implications and Future Directions

• Theoretical Implications: The results have significant implications for understanding the quark-level structure of exotic hadronic states. The confirmed existence of a $P_{cs}^0$ charmonium pentaquark aligns with theoretical predictions that extended the concept of the pentaquark family to include states with strange quarks. These findings provide crucial data which can refine theoretical models of QCD that predict tetraquark and pentaquark states.
• Future Prospects in High-Energy Experiments: The results highlight the potential for discovering further exotic states in heavy quark baryon decays. Continued investigation into similar decays could uncover more about the structure and interactions of other hypothesized exotic hadrons. Future studies could aim to explore other decay channels and energy levels to test the robustness and generalizability of current theoretical models.
• Experimental Techniques: Advances in experimental techniques outlined in this paper, particularly the robust methods for handling background noise in particle collision data, set a precedent for future studies in high-energy physics, enhancing the precision and accuracy of discovery in heavily data-driven environments.

In conclusion, the paper by the LHCb Collaboration offers pivotal insights into the spectrum of strangeness quark-rich pentaquark states. This reveals a complex interplay of quarks and gluons that challenge existing models of particle physics, prompting further theoretical and experimental inquiries. The successful identification of these states underscores the richness of QCD and its yet unexplored frontier in particle physics.