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

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.

• The paper reports the discovery of the narrow Pc(4312)+ state with a 7.3σ significance using a tenfold larger data sample.
• It resolves the previously observed Pc(4450)+ into two distinct peaks, Pc(4440)+ and Pc(4457)+, with a combined significance of 5.4σ.
• Results imply that these pentaquark states may be molecular baryon-meson bound structures, prompting refinements in exotic hadron models.

Observation of Narrow Pentaquark States by LHCb

The study conducted by the LHCb collaboration provides significant insights into exotic baryon spectroscopy, particularly through the discovery and characterization of pentaquark states. By analyzing a comprehensive dataset from the LHCb experiment, the study identifies a narrow pentaquark state, $P_c(4312)^+$, and refines the existing understanding of the $P_c(4450)^+$ structure by revealing two separate states within it: $P_c(4440)^+$ and $P_c(4457)^+$.

Key Findings

The paper reports several key results:

1. Discovery of $P_c(4312)^+$: The $P_c(4312)^+$ pentaquark state, decaying into $J/\psi p$, was observed with a statistical significance of 7.3σ. This discovery was facilitated by a tenfold increase in the data sample size compared to previous analyses.
2. Resolution of $P_c(4450)^+$: The previously reported $P_c(4450)^+$ structure is now understood to comprise two narrow overlapping peaks at $M = 4440.3 \pm 1.3$ MeV and $M = 4457.3 \pm 0.6$ MeV, with statistical significance for this two-peak structure at 5.4σ. The resolution of these two peaks provides deeper insights into the nature of pentaquark states and their dynamics.
3. Spectroscopy and Dynamics: The proximity of the observed states to the $\Sigma_c \bar{D}^{(*)}$ thresholds suggests molecular-like dynamics might play a crucial role in the formation of these narrow peaks. This observation supports the hypothesis that these entities are not simply kinematic effects or reflections but potentially bound states or resonant structures with verified exotic hadronic character.

Implications and Future Directions

The observations made in this study have several significant implications:

• Hypotheses in Exotic Hadron Physics: The precise measurements constrain theoretical models, supporting interpretations which suggest these structures could be molecularly bound states of a baryon-meson combination. Specifically, their placement relative to $\Sigma_c \bar{D}^{(*)}$ thresholds corroborates this theory, pointing towards a meson-baryon molecular interpretation rather than compact pentaquark states.
• Amendments to the Quark Model: These findings necessitate refinements to the quark model, particularly concerning the role of quark components such as $c\bar{c}$ in the hybridization or interaction leading to pentaquark structures.
• Further Experimental Approaches: The significant increase in data and subsequent discoveries underscore the need for future experimental investigations into the $J/\psi p$ decay modes and other potential channels. Such studies may further uncover wide $P_c^+$ states or provide alternative explanations such as effects from triangle diagrams.

Overall, this research provides a crucial step towards a comprehensive understanding of the structure and dynamics of exotic baryonic states and paves the way for further inquiry into the quantum chromodynamics governing such systems. The LHCb's advancements in data analysis techniques and increased data resolution capabilities continue to shape our understanding of hadronic physics, advocating for ongoing exploration in this domain.