Exotics: Heavy Pentaquarks and Tetraquarks (1706.00610v2)

Abstract: For many decades after the invention of the quark model in 1964 there was no evidence that hadrons are formed from anything other than the simplest pairings of quarks and antiquarks, mesons being formed of a quark-antiquark pair and baryons from three quarks. In the last decade, however, in an explosion of data from both $e^+e^-$ and hadron colliders, there are many recently observed states that do not fit into this picture. These new particles are called generically "exotics". They can be either mesons or baryons. Remarkably, they all decay into at least one meson formed of either a $c\overline{c}$ or $b\overline{b}$ pair. In this review, after the introduction, we explore each of these new discoveries in detail first from an experimental point of view, then subsequently give a theoretical discussion. These exotics can be explained if the new mesons contain two-quarks and two-antiquarks (tetraquarks), while the baryons contain four-quarks plus an antiquark (pentquarks). The theoretical explanations for these states take three divergent tracks: tightly bound objects, just as in the case of normal hadrons, but with more constituents, or loosely bound "molecules" similar to the deuteron, but formed from two mesons, or a meson or baryon, or more wistfully, they are not multiquark states but appear due to kinematic effects caused by different rescatterings of virtual particles; most of these models have all been post-dictions. Both the tightly and loosely bound models predict the masses and related quantum numbers of new, as yet undiscovered states. Thus, future experimental discoveries are needed along with theoretical advances to elucidate the structure of these new exotic states.

• The paper presents experimental evidence for heavy pentaquark and tetraquark states, notably Pc(4380) and X(3872).
• It evaluates competing theories including compact quark clusters, meson molecules, and hybrid configurations.
• The analysis underscores significant QCD implications and calls for advanced simulations and precision collider experiments.

Overview of the Paper: "Exotics: Heavy Pentaquarks and Tetraquarks"

The paper by Ahmed Ali, Jens Sören Lange, and Sheldon Stone is a comprehensive review centered on the discovery and classification of exotic hadrons known as pentaquarks and tetraquarks. Historically, the quark model, introduced in 1964, posited that hadrons were composed exclusively of the simplest combinations: quark-antiquark pairs for mesons and three-quark states for baryons. However, in the last decade, various hadron colliders have furnished evidence of hadronic states that defy this paradigm, leading to the classification of these states as "exotics."

Key Discoveries and Theoretical Insights:

The exotics exhibit decay patterns suggesting they may consist of more than the conventional quark pairings. Tetraquarks are described as states containing two quarks and two antiquarks, while pentaquarks are postulated to have four quarks and one antiquark. These new findings have been substantiated through experimental observations with $e^+e^-$ and hadron colliders, revealing states that do not conform to traditional quark pairing models.

Pentaquark States:

Recent evidence from different experiments, such as LHCb, have confirmed the existence of pentaquarks, notably the $P_c^+(4380)$ and $P_c^+(4450)$, which are differentiated by their mass spectra. These states suggest the presence of a structure involving a composite quark framework, compatible with pentaquark configurations.

Tetraquark States:

The tetraquarks are further divided based on decay modes and quantum numbers, suggesting states like $X(3872)$ and $Y(4260)$ might be tetraquarks. The paper highlights the role of bound states versus molecular structures in these interpretations. Here, theoretical models propose either tightly bound quark-based objects similar to existing hadron structures or looser "molecular" configurations resembling deuteron assembly in nuclear physics.

Theoretical Models:

The theoretical landscape for these exotic states is diverse:

1. Compact Tetraquarks and Pentaquarks: These models suggest the existence of highly bound quark clusters forming the exotic states. Theoretical computations suggest these objects, while composed of more quarks, follow compact structures and resonate at specific mass levels, creating the observed resonant states.
2. Molecule Models: Alternatively, some models propose that these exotic states are not bound quark states but weakly bound meson molecules. This hypothesis is prompted by resonant masses lying close to the thresholds of their constituent mesons.
3. Hybrids and Kinematic Effects: Other models attribute the observed states to either hybrid configurations involving gluonic components ($Q\bar{Q}g$ states) or as artifacts of rescattering processes, producing apparent resonances through kinematic effects rather than true bound states.
4. Lattice QCD and Effective Field Theories: The paper also suggests potential advancements in lattice QCD might validate or refute some of the current theoretical models. Effective field theories based on QCD are also potential tools for delving deeper into these structures.

Implications and Future Directions:

The confirmation of exotic hadron states challenges the prevailing understanding of QCD at energy scales involved in heavy quark processes. These findings invite significant implications, specifically in the domains of nuclear physics, particle collision experiments, and potentially beyond the Standard Model theories. The future developments in this area are contingent on both experimental advancements with higher precision and theoretical progress with sophisticated QCD modeling and simulations. The paper emphasizes the need for novel approaches to accurately predict and explain the emergence of such states, hinting at new horizons in the paper of quantum chromodynamics.

This review paper provides an insightful synthesis of experimental findings and theoretical conjectures, positioning heavy pentaquarks and tetraquarks at the forefront of contemporary particle physics research and elucidating the vast and uncharted landscape of quark dynamics. It invites ongoing investigation into the quantum structure of matter, advocating for broad collaboration across experimental and theoretical disciplines to further unravel the idiosyncrasies of these exotic states.