The hidden-charm pentaquark and tetraquark states (1601.02092v3)

Abstract: In the past decade many charmonium-like states were observed experimentally. Especially those charged charmonium-like $Z_c$ states and bottomonium-like $Z_b$ states can not be accommodated within the naive quark model. These charged $Z_c$ states are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. Recently, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. We list the puzzles and theoretical challenges of these models when confronted with the experimental data. We also discuss possible future measurements which may distinguish the theoretical schemes on the underlying structures of the hidden-charm multiquark states.

• The paper presents a comprehensive review of experimental discoveries and theoretical interpretations of hidden-charm pentaquark and tetraquark states.
• It employs methodologies such as meson exchange models, coupled-channel dynamics, and QCD sum rules to analyze mass spectra and bound state configurations.
• The findings challenge conventional quark models and guide future experiments in probing non-traditional hadronic matter and refining QCD predictions.

The Hidden-Charm Pentaquark and Tetraquark States: A Comprehensive Overview

The paper of hidden-charm pentaquark and tetraquark states has gained significant interest due to their intriguing implications in particle physics, specifically concerning the limitations of the quark model. These states, which cannot be easily accommodated within the conventional quark model, demand alternative theoretical frameworks for their explanation. This paper provides an extensive review of both experimental findings and theoretical interpretations related to these exotic hadron states.

Experimental Context

In recent experimental advances, collaborations, such as LHCb and Belle, have reported resonances that suggest the existence of hidden-charm pentaquark and tetraquark states. Notably, the discovery of the $P_c(4380)$ and $P_c(4450)$ by LHCb exemplifies states that challenge conventional hadron classifications. These pentaquarks were observed in the $J/\psi p$ invariant mass spectrum and are significant because their quark content, $c\bar cuud$, goes beyond traditional baryon structure, indicating intricate substructures involving five quark components.

Theoretical Interpretations

1. Molecular States:

The molecular interpretation considers these states as bound systems of a charmed baryon and anti-charmed meson, linked analogously to the deuteron. The interaction mechanisms are predominantly described by meson exchange forces, particularly highlighting the one-pion exchange (OPE) contributions. Potential molecular candidates for the $P_c(4380)$ resonance include the $\Sigma_c \bar{D}^*$ with isospin $I=1/2$ and spin-parity $J^P=3/2^-$. Meanwhile, the $P_c(4450)$ could be interpreted as a $\Sigma_c^*\bar{D}^*$ molecular state. These dynamics suggest complex interplay of binding energies and structural orientations in multi-quark systems.

2. Dynamically Generated States:

In contrast, the dynamical generation approach utilizes coupled-channel unitary frameworks within chiral effective field theories. These models predict the formation of narrow baryonic resonances through meson-baryon scattering processes. They provide precise but varied predictions regarding mass and decay properties, serving as a crucial tool for understanding multi-quark resonances without imposing significant phenomenological inputs beyond QCD constraints.

3. QCD Sum Rules:

QCD sum rules offer insights at the quark-gluon level, where spectral density functions are analyzed to estimate masses and coupling constants of potential multiquark states. This approach reinforces the possibility of both molecular and tightly bound pentaquark states by showing relative consistency with experimental masses and providing mass predictions for these exotic baryons.

4. Tightly Bound Pentaquarks:

This viewpoint treats the pentaquark as a tightly bound configuration within a diquark framework. The interactions focus on the color-antitriplet diquark dynamics and quark-quark correlations, offering another angle by which these resonances might stabilize under strong forces distinct from conventional combinations.

Implications and Future Directions

The hidden-charm pentaquark discovery, coupled with theoretical anticipations, invigorates the search for new hadronic matter and challenges current paradigms of QCD. It poses a formidable challenge to lattice QCD simulations to provide higher precision in predicting such states. Experimentally, further confirmation of these states' properties, including spin-parity and decay modes, is essential.

These investigations have broader implications for understanding the strong force and the emergence of matter beyond traditional baryons and mesons. Future studies in experimental arenas such as LHC, Belle II, and upcoming facilities like J-PARC are poised to further demystify these states, providing deeper insight into non-perturbative QCD and the underlying architecture of matter.