The $XYZ$ states: experimental and theoretical status and perspectives (1907.07583v2)

Abstract: The quark model was formulated in 1964 to classify mesons as bound states made of a quark-antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as $XYZ$ states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the $XYZ$ states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic $XYZ$ hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges.

• The paper presents key experimental findings and theoretical analyses that challenge the conventional quark model.
• It utilizes methods like potential models, coupled channel effects, lattice QCD, and EFTs to decode the properties of XYZ states.
• The study underscores the need for precise experiments to further clarify the multiquark dynamics in particle physics.

Overview and Implications of the $XYZ$ States: Experimental and Theoretical Insights

The landscape of particle physics has witnessed significant developments with the exploration of exotic hadrons, notably the enigmatic $XYZ$ states. These states, primarily observed in the spectrum of charmonium and bottomonium, challenge the conventional quark-antiquark model of mesons and the quark triplet configuration of baryons. This paper, authored by Brambilla et al., provides an extensive review of both experimental data and theoretical frameworks pertaining to these states, highlighting their potential implications for Quantum Chromodynamics (QCD).

Experimental Observations and the Spectrum of $XYZ$ States

The $XYZ$ states are a category of charmonium-like and bottomonium-like states that exhibit properties inconsistent with traditional quarkonium structures. These states have been extensively studied in collider experiments like Belle, BaBar, LHCb, BESIII, and more. Among these, the $X(3872)$ stands out due to its proximity to the $D^0 \bar{D}^{*0}$ threshold and its decay into isospin-violating channels like $\rho J/\psi$. The $Z_c(3900)$ and $Z_c(4020)$ are notable as charged states, indicating a tetraquark configuration or molecular states beyond simple quark-antiquark pairs.

The experimental data, particularly from electron-positron annihilation and B-meson decays, reveal these exotic states with varying degrees of statistical significance. The paper of processes such as $e^+e^- \to \pi^+ \pi^- J/\psi$ and $e^+e^- \to \omega \chi_{c0}$ has been pivotal in identifying these states. The results indicate the presence of several resonant structures, such as the $Y(4260)$, which further complicate the understanding of hadron spectroscopy.

Theoretical Perspectives: Quark Models and QCD

The theoretical landscape explored in the paper involves several models attempting to accommodate these exotic states within the framework of QCD. Traditional quark models suggest that mesons are primarily quark-antiquark pairs, yet the $XYZ$ states challenge this with evidence pointing towards configurations involving more than three quarks, including tetraquarks and pentaquarks. Theoretical approaches discussed include:

1. Potential Models: These use potential energy functions, typically a combination of a Coulombic term and a linear confining term, to describe the interactions between quarks. While successful for low-lying quarkonium states, these models struggle with explaining states around and above open-flavor thresholds without additional dynamics such as quark-pair creation mechanisms.
2. Coupled Channel Effects: These address the influence of hadronic loops and open-flavor thresholds on meson properties. The incorporation of these dynamics often leads to mass shifts and changes in decay widths, offering a potential explanation for the unexpected masses and decay pathways of some $XYZ$ states.
3. Lattice QCD: As a non-perturbative method, lattice QCD calculations aim to provide first-principles insights into hadron structures. These computations help in understanding the static potentials and mass spectra of heavy quark systems, though they are computationally intensive and have limitations in handling dynamically coupled channels.
4. Effective Field Theories (EFTs): These theories simplify the complex dynamics by focusing on relevant degrees of freedom at different energy scales. Heavy quark effective theory (HQET) and non-relativistic QCD (NRQCD) are particularly useful in studying mesons with one or more heavy quark contents.

Implications and Future Directions

The investigation into the $XYZ$ states offers profound implications for our understanding of QCD, especially the aspects of confinement and the dynamics of multiquark interactions. The existence of these states necessitates a re-examination of the quark model, possibly extending it to include configurations like hybrids and glueballs.

Future theoretical and experimental work is poised to focus on the precise measurement of these states' properties, which will enable more stringent tests of theoretical models. Experiments with increased luminosity and energy, such as those planned at the upgraded Belle II, are expected to unearth more data on these elusive states, providing clarity on their nature.

In conclusion, the research on $XYZ$ states stands at the frontier of particle physics, challenging long-held assumptions and driving advancements in both experimental techniques and theoretical models. The pursuit of understanding these states not only enhances our grasp of the strong force but also enriches the entire field of quantum field theories.