Pentaquark and Tetraquark states (1903.11976v2)

Abstract: The past seventeen years have witnessed tremendous progress on the experimental and theoretical explorations of the multiquark states. The hidden-charm and hidden-bottom multiquark systems were reviewed extensively in [Phys. Rept. 639 (2016) 1-121]. In this article, we shall update the experimental and theoretical efforts on the hidden heavy flavor multiquark systems in the past three years. Especially the LHCb collaboration not only confirmed the existence of the hidden-charm pentaquarks but also provided strong evidence of the molecular picture. Besides the well-known $XYZ$ and $P_c$ states, we shall discuss more interesting tetraquark and pentaquark systems either with one, two, three or even four heavy quarks. Some very intriguing states include the fully heavy exotic tetraquark states $QQ\bar Q\bar Q$ and doubly heavy tetraquark states $QQ\bar q \bar q$, where $Q$ is a heavy quark. The $QQ\bar Q\bar Q$ states may be produced at LHC while the $QQ\bar q \bar q$ system may be searched for at BelleII and LHCb. Moreover, we shall pay special attention to various theoretical schemes. We shall emphasize the model-independent predictions of various models which are truly/closely related to Quantum Chromodynamics (QCD). There have also accumulated many lattice QCD simulations through multiple channel scattering on the lattice in recent years, which provide deep insights into the underlying structure/dynamics of the $XYZ$ states. In terms of the recent $P_c$ states, the lattice simulations of the charmed baryon and anti-charmed meson scattering are badly needed. We shall also discuss some important states which may be searched for at BESIII, BelleII and LHCb in the coming years.

• The paper comprehensively reviews experimental findings and theoretical models, highlighting chromomagnetic interaction frameworks for describing multiquark mass splittings.
• Experimental breakthroughs from collaborations like LHCb and BelleII are analyzed for their role in confirming hidden-charm pentaquarks and molecular tetraquark configurations.
• Advanced computational techniques, including lattice QCD and meson exchange models, are utilized to predict binding energies and stability of multiquark states.

An Overview of Multiquark States: Pentaquark and Tetraquark Systems

The multiquark states, particularly tetraquarks and pentaquarks, have garnered considerable interest within the physics community due to their potential to expand our understanding of hadronic matter beyond the conventional mesons and baryons. This paper, authored by a collaboration of researchers from multiple Chinese physics institutions, reviews recent theoretical and experimental advances related to hidden-charm and other varied quark flavor multiquark systems.

Advances in Experimental Observations

In the past few years, experimental efforts, primarily by the LHCb collaboration, have significantly enriched our understanding of multiquark states, including the confirmation of hidden-charm pentaquarks and tangible evidence supporting the molecular interpretation of these states. Experiments have identified intriguing tetraquark and pentaquark systems with configurations involving multiple heavy quarks, such as fully heavy tetraquarks ($QQ\bar{Q}\bar{Q}$) and doubly heavy tetraquarks ($QQ\bar{q}\bar{q}$). The exploration at facilities like the LHC and BelleII is ongoing, highlighting the crucial role of experimental physics in uncovering the nuances of multiquark systems.

Theoretical Frameworks: From Chromomagnetic Interaction to QCD Symmetries

The paper explores various theoretical models employed to explain and predict the masses and properties of multiquark states. Significant emphasis is placed on chromomagnetic interaction models, which are pivotal in understanding the mass splittings in hadrons owing to spin-related interactions between quarks. These models, finely tuned to offer insights into tetraquark and pentaquark configurations, show considerable promise in explicating observed experimental data.

Moreover, the paper discusses how the heavy quark symmetry, including heavy quark spin symmetry and heavy diquark symmetry, provides a compelling framework for understanding multiquark systems. Such symmetry considerations are particularly pertinent when extrapolating the properties of $QQ\bar{q}\bar{q}$ tetraquarks and their related heavy baryon systems.

Chromomagnetic Interaction Models: Systematic Application

The paper offers a comprehensive review of various chromomagnetic interaction (CMI) models. These models, adapted for multiquark systems, facilitate detailed predictions about potential tetraquark and pentaquark masses. Particularly noteworthy are model-independent results related to the QCD-motivated approaches within these models. The chromomagnetic interaction, as arising from gluon exchanges, champions the explanation of multiquark mass dynamics, contributing significantly to the theoretical groundwork reinforcing experimental findings.

Meson Exchange and Computational Methods

Besides the CMI models, the paper elaborates on meson exchange and lattice QCD calculations, pivotal in assessing the stability and interactions within these multiquark states. Meson exchange models, akin to those operational in nuclear physics, offer insights into the long-range forces at play within molecular picture analyses of these states.

Additionally, lattice QCD simulations are instrumental in providing first-principle computations related to the binding energies and scattering properties of multiquark systems. These simulations augment the understanding afforded by experimental and CMI models by offering computational validation and predictions of observed phenomena.

Practical and Theoretical Implications

The theoretical frameworks and experimental findings discussed have substantial implications on how multiquark states are perceived in the context of particle physics. Practically, identifying new multiquark states will inform the exploration of QCD under different conditions. Theoretically, this research underscores potential directions for model refinement, particularly through improved symmetry considerations and computational methodologies, such as the implementation of heavier computational schemes in lattice QCD.

Future Prospects

The speculated mass spectra of multiquark states and model predictions chart exciting future research courses in both experimental and theoretical domains. With accelerators like the LHC and projects such as BelleII poised to advance experimental capabilities, the detection and subsequent studies of heavier multiquark states could unravel deeper insights into the strong force and the invariant properties prescribed by QCD symmetries.

Overall, this review positions itself at a critical juncture, bridging the gap between foundational theoretical constructs and the burgeoning experimental realities of multiquark states, paving a pathway for future discoveries in hadron spectroscopy and beyond.