An updated review of the new hadron states (2204.02649v2)

Abstract: The past decades witnessed the golden era of hadron physics. Many excited open heavy flavor mesons and baryons have been observed since 2017. We shall provide an updated review of the recent experimental and theoretical progresses in this active field. Besides the conventional heavy hadrons, we shall also review the recently observed open heavy flavor tetraquark states $X(2900)$ and $T_{cc}^+(3875)$ as well as the hidden heavy flavor multiquark states $X(6900)$, $P_{cs}(4459)^0$, $Z_{cs}(3985)^-$, $Z_{cs}(4000)^+$, and $Z_{cs}(4220)^+$. We will also cover the recent progresses on the glueballs and light hybrid mesons, which are the direct manifestations of the non-Abelian $SU(3)$ gauge interaction of the Quantum Chromodynamics in the low-energy region.

• The paper synthesizes experimental discoveries and QCD theoretical frameworks to decode exotic hadron configurations.
• It details key observations like X₀(2900), X₁(2900), and T_cc⁺(3875), emphasizing the need for extending the conventional quark model.
• The review outlines future research directions for high-energy experiments to probe multiquark dynamics and refine Standard Model predictions.

An Updated Review of the New Hadron States

The resurgence of hadron physics over the past decades has led to heightened theoretical and experimental activity aimed at deciphering the intricate dynamics of subatomic particles, specifically in the domain of hadron spectroscopy. The paper by Chen, Chen, Liu, Liu, and Zhu systematically chronicles the landscape of hadron physics, particularly emphasizing the recent advancements and experimental discoveries in the field. This essay offers an insightful overview, focusing on several key aspects, including the latest observations of hadrons with exotic configurations and the theoretical frameworks used to comprehend them.

Conventional and Exotic Hadron Configurations

The conventional quark model, introduced by Gell-Mann and Zweig, has been remarkably successful in describing hadron structures, positing mesons as quark-antiquark pairs and baryons as tri-quark conglomerates. However, recent experimental insights have necessitated extensions to this model, incorporating configurations such as tetraquarks and pentaquarks, which exhibit more exotic quark arrangements. The observation of states like the $X(3872)$ and others has invigorated the exploration of multiquark entities, offering substantial challenges and opportunities in theoretical and experimental particle physics.

New Observations of Hadronic States

The paper highlights several recently discovered states that defy conventional categorization:

• Open-charm Tetraquark States: The $X_0(2900)$ and $X_1(2900)$, observed by the LHCb collaboration, have been identified as containing four distinct quark flavors ($u$, $d$, $\bar{s}$, and $\bar{c}$). The identification of these states presents compelling evidence for configurations beyond the classic quark-antiquark pairing in mesons.
• Doubly Charmed Tetraquark State ($T_{cc}^+(3875)$): This state, observed distinctly near the $D^*D$ threshold, has been interpreted as a $cc\bar{u}\bar{d}$ entity with $J^P = 1^+$. Its observation exemplifies the culmination of years of anticipation rooted in theoretical proposals that forecasted tightly bound tetraquark configurations involving two heavy quarks.

Theoretical Frameworks: Quantum Chromodynamics (QCD) and Symmetries

The intricate QCD interactions govern how quarks and gluons assemble into hadrons. The authors employ heavy quark effective theory and symmetries, such as flavor SU(3) and heavy quark spin symmetries, to model and predict these exotic states.

1. Hadronic Molecules vs. Compact Tetraquarks: The debate between hadronic molecule configurations versus compact tetraquark states remains active. Hadronic molecules suggest weakly bound states due to residual strong interactions, akin to Van der Waals forces in molecular physics. On the other hand, compact tetraquark models entail strongly bound diquark-antidiquark pairs.
2. Lattice QCD and Sum Rules: These computational and semi-analytic methods provide non-perturbative calculations crucial for modeling hadrons that cannot be directly probed through simpler analytical approaches. They predict mass spectra and decay constants for proposed particles, grounding theoretical predictions against which experimental data can be juxtaposed.

Implications and Future Prospects

The discovery of states such as $X(3872)$ and $T_{cc}^+(3875)$ emphasizes the complexity of QCD and presents new frontiers for exploration, both in altering QCD parameterizations and in the potential discovery of whole families of exotic states. These discoveries have profound implications for understanding the strong force and the binding mechanics of quarks, with further potential repercussions in understanding the early universe's quark-gluon plasma phase.

Theorizations about yet-unobserved states contribute to a roadmap for upcoming experiments, guiding searches at high-energy facilities such as the LHC, Belle II, and potential future accelerators.

Conclusion

This extensive review by Chen et al. is a comprehensive resource that reflects both the present achievements and the ongoing challenges within the field of hadron physics. The synthesis of experimental and theoretical insights creates a valuable narrative for the scientific community, fostering advances in our understanding of the fundamental strong interactions that bind the atomic substrata of matter. As the field progresses, continued collaboration between theorists and experimentalists will be vital in unveiling the full tapestry of hadronic states, thereby refining the Standard Model and its extensions.