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The $XYZ$ states: experimental and theoretical status and perspectives
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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.
Forward citations
Cited by 28 Pith papers
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An improved moment QCD sum rule
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Singly heavy tetraquarks
A hybrid quark model with gluon and meson exchange predicts that LHCb's T-c̄s̄0(2870) and T-cs̄0(2900) are compact tetraquarks, and that Ds0(2317), Ds1(2460), Tbs(5568), and Tcs(2327) are not.
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Effective Range Expansion with the Left-Hand Cut: Higher Order Improvements
Extended the generalized effective-range expansion, including the left-hand cut from one-particle exchange, to O(k^6) with a new relativistic version, validated against exact Yukawa-potential solutions of the Lippmann...
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A Paradigm for the Coupled-Channel Origin of Resonances: the Exotic $T_{c\bar{s}}$ in $D_{s1}(2460/2536)\to D_s\pi\pi$
An off-diagonal DK–D_sπ coupled-channel interaction generates the T_cbar{s} pole on the second Riemann sheet, and predicts a single-peak D_s1(2536) decay.
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Prediction of $QQqq\bar{s}$ molecular pentaquarks within the extended local hidden gauge approach
A model calculation predicts fourteen double-heavy molecular pentaquark states, but their existence and binding energies depend critically on an unconstrained regularization parameter.
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Probing the structure of the $D_{s 0}^*(2317)$ and $X(3872)$ states through correlation functions
Femtoscopic correlation functions for D0K+ and D0Dbar*0 pairs are predicted to be sensitive to the molecular versus bare-state composition of D_s0*(2317) and X(3872).
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Eigenstates in coupled-channel scattering amplitude and their effects on spectrum
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Magnetic dipole moments as probes of doubly-bottom molecular pentaquarks
For the molecular pentaquark configurations BΣ_b, BΣ_b*, and B*Σ_b, the predicted magnetic dipole moments are 2.40, −2.84, and 5.17 nuclear magnetons respectively, with a sign and magnitude pattern sensitive to spin s...
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Analytic decomposition of two-body electroweak processes with left-hand cuts
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Possibility of the antibottom-strange molecular pentaquarks near $ B\Sigma$ and $ B^*\Sigma$ thresholds
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Symmetry Analysis of Compact Tetraquark States and Implications for the Level Ordering of the Fully Charmed Candidates $X(6600)$, $X(6900)$, and $X(7100)$
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Structural dissection of hadronic molecules: The $D^{(*)}\bar{K}^{(*)}$ family under QCD light-cone sum rules
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The dynamically generated $h_1$ state by the $K^*\bar{K}^*$ interaction and its $K_1(1270)\bar{K}$ and $b_1(1235)\pi$ decays
The dynamically generated h1(1790) K*Kbar* molecule is predicted to decay into K1(1270)Kbar and b1(1235)pi with partial widths of about 0.5 to several MeV, plus stable width ratios R1≈0.3 and R2≈0.53.
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Electromagnetic form factors: A window into the $D\Lambda_c$, $D^*\Lambda_c$, and $D\Lambda_c^*$ molecular structure
Using light-cone QCD sum rules, the paper predicts negative magnetic dipole moments of roughly -1.27, -2.78, and -3.80 nuclear magnetons for the DΛc, D*Λc, and DΛc* molecular pentaquark candidates, plus small quadrupo...
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ZS-Puffin: Design, Modeling and Implementation of an Unmanned Aerial-Aquatic Vehicle with Amphibious Wings
An unmanned aerial-aquatic prototype uses a single-pitch-axis amphibious wing for lift in air and flapping propulsion underwater, driven by an artificial central pattern generator.
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Correlation functions for $n\,\bar{D}_{s1}(2460)$ and $n\,\bar{D}_{s1}(2536)$
The neutron-D_{s1}(2460) and neutron-D_{s1}(2536) systems are predicted to have bound states, with correlation functions sensitive to the molecular structure of the D_{s1} mesons.
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$\Upsilon(5S)$ in the unquenched quark model
A coupled-channel quark-model calculation shifts the bare Upsilon(5S) mass down by 31.4 MeV to 10896 MeV and argues that Upsilon(10860) and Upsilon(10753) are distinct states.
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$\lambda$, $\rho$, and $\sigma$ Regge trajectories for the quadruply heavy pentaquark $bb\bar{u}cc$ in the diquark-triquark picture
Regge relations for bbūcc in the diquark-triquark picture give four trajectory series with M∼x^{2/3} or √x behavior plus spin-averaged mass estimates for λ, ρ1, ρ2 and σ excitations.
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Decay constants of the two-pole $D_0^*(2300)$
Molecular two-pole D0*(2300) decay constants are 65 and 81 MeV—much smaller than compact c¯q estimates—and imply Cabibbo-favored b-hadron branching fractions of order 10^{-5}.
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A Unified Study of Hidden-Charm and Hidden-Bottom Mesons
A common screened-potential and QCD sum-rule framework yields consistent spectra, E1/M1 widths, Regge slopes, decay constants and finite-spectrum thermal indicators for c¯c and b¯b mesons.
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Investigating the two-pion exchange of the double charm $DD^*$ chiral interactions and $T_{cc}$
In this chiral EFT calculation the I=0 DD* two-pion-exchange potential is repulsive, and its near-cancellation with attractive contact and one-pion terms provides the weak binding of Tcc.
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Cross section of the process $e^+ + e^- \to \Xi^0 + \bar{\Xi}^0$ in the vicinity of charmonium $\psi(3770)$ and in the charmonium (-like) state including the $D$-meson loop and three-gluon contributions
A fitted Born term combined with a D-meson loop and a three-gluon mechanism reproduces the BESIII e+e- to Xi0 anti-Xi0 cross section, with a near-180-degree phase at psi(3770).
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Scalar Triple-Heavy Tetraquark States With Quark Content $cc\bar{c}\bar{s}$
QCD sum rule analysis predicts scalar cc̄c̄s tetraquark states with masses near 4.94 to 5.08 GeV, including an ηc-Ds molecule and two diquark-antidiquark compact states.
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The Pattern of Exotic Hidden-Heavy Hadrons Revealed
Exotic hidden-heavy hadrons are proposed as near-threshold bound states and resonances in Born-Oppenheimer potentials that are repulsive at short range and cross the heavy-hadron-pair threshold.
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Unquenched Charmonium and Beyond
This Lanzhou-group review argues that coupled-channel (unquenched) effects, not exotic constituents, are the common thread explaining charmonium anomalies from the rho-pi puzzle to the Y-problem states.
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Heavy quarkonia and new hadrons with two heavy quarks
A literature review that compiles the current experimental landscape of charmonium, bottomonium, and XYZ exotic states; it contains no new measurements, derivations, or predictions.
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Charm physics
A review chapter summarizing the theoretical framework and experimental status of charm hadron lifetimes, D0 mixing, CP violation, and rare decays.
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