Exotics: Heavy Pentaquarks and Tetraquarks

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.

Review article to appear in Prog. Part. Nucl. Phys. 109 pages, 55 figures, v2 contains corrections due to reviewers and others comments

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Cited by in corpus (92)

• Pentaquark and Tetraquark states
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• A survey of heavy-heavy hadronic molecules
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• Anatomy of the newly observed hidden-charm pentaquark states: $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$
• Explaining the Many Threshold Structures in the Heavy-Quark Hadron Spectrum
• Isospin breaking decays as a diagnosis of the hadronic molecular structure of the $P_c(4457)$
• A Theory of X and Z Multiquark Resonances
• Masses of doubly heavy tetraquarks $T_{Q{Q}^{\prime }}$ in a relativized quark model
• Implications of Heavy Quark-Diquark Symmetry for Excited Doubly Heavy Baryons and Tetraquarks
• Stability of tetrons
• Novel Method for Precisely Measuring the $X(3872)$ Mass
• Spectroscopy, lifetime and decay modes of the $T_{bb}^{-}$ tetraquark
• The Dynamical Diquark Model: First Numerical Results
• Magnetic dipole moments of the hidden-charm pentaquark states: $P_c(4440)$, $P_c(4457)$ and $P_{cs}(4459)$
• Study of the $DK$ interaction with femtoscopic correlation functions
• Spectrum of the Hidden-Bottom and the Hidden-Charm/Strange Exotics in the Dynamical Diquark Model
• Prediction of exotic doubly charmed baryons within chiral effective field theory
• Heavy pentaquark spectroscopy in the diquark model
• Observation of the decay $X(3872)\to {\pi }^0{\chi }_{c1}(1P)$
• Production of $X(3872)$ at High Multiplicity
• $S$-wave fully charmed tetraquark resonant states
• How to reveal the nature of three or more pentaquark states?
• Spectroscopy of Exotic Hadrons Formed from Dynamical Diquarks
• Very heavy flavored dibaryons
• Production of exotic tetraquarks $QQ\bar{q}\bar{q}$ in heavy-ion collisions at the LHC
• Benchmark calculations of fully heavy compact and molecular tetraquark states
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• Tetraquark-adequate formulation of QCD sum rules
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• Heavy flavor pentaquarks with four heavy quarks
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• Radiative decays and magnetic moments of the predicted $B_c$-like molecules
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• The structure of the X(3872) as explained by a Diffusion Monte Carlo calculation
• The interplay between compact and molecular structures in tetraquarks
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• Looking for the exotic $X_0(2866)$ and its $J^P=1^{+}$ partner in the ${\bar{B}}^0\to D^{(\ast )+}{K}^{-}{K}^{(\ast )0}$ reactions
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• Radiative and pionic transitions $Z_c(4020{)}^0\to X(3872)\gamma$ and $Z_c(4020{)}^{\pm }\to X(3872){\pi }^{\pm }$
• Charmed baryon$-$nucleon interaction
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• Study of conventional and non-conventional scalar and vector mesons
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• Parametrically narrow tetraquark states containing one heavy quark and one heavy antiquark
• Spectroscopy of the tetraquark $c\bar{c}$-$c\bar{c}$ in a non-relativistic approach using a phenomenological QCD model