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The XYZ states: experimental and theoretical status and perspectives
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The XYZ states: experimental and theoretical status and perspectives
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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 47 Pith papers
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Electromagnetic form factors and structure of the $T_{bb}$ tetraquark from lattice QCD
Lattice QCD on one ensemble yields electromagnetic form factors for T_bb, indicating a compact heavy diquark plus light antidiquark bound state with charge radius smaller than the BB* threshold.
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First evidence of $X(3872)\to\pi^0\chi_{c0}(1P)$ and search for $X(3915)\to\pi^0\chi_{c1}(1P)$
First evidence reported for X(3872)→π⁰χ_c0 decay at 3.4σ, with branching fraction ratio 2.3 relative to π⁺π⁻J/ψ, upper limits on other modes, and no signal for X(3915)→π⁰χ_c1.
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Analysis of the $D_0^*(2300)$ resonance from lattice QCD under chiral symmetry
Chiral symmetry corrections in lattice QCD fits shift the D0*(2300) resonance pole closer to the Dπ threshold and reduce its width, while coupled channels produce a two-pole structure.
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Compositeness of near-threshold eigenstates with Coulomb plus short-range interactions
The compositeness of near-threshold eigenstates with Coulomb plus short-range forces is determined solely by the Coulomb scattering length, effective range, and Bohr radius, with the Coulomb interaction qualitatively ...
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Open-flavor threshold effects on quarkonium spectrum in the BOEFT
BOEFT quantifies threshold-induced shifts in quarkonium masses below threshold by solving coupled Schrödinger equations using lattice potentials and one parameter fixed to the χ_c1(3872) mass.
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Dispersive Analysis of $D$- and $B$-Meson Form Factors with Chiral and Heavy-Quark Constraints
The isovector D/D*/B/B* electromagnetic form factors are reconstructed dispersively with ππ rescattering, yielding ρ(770) coupling constants from pole residues.
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Model-independent mass determination of near-threshold states from short-range production
A model-independent minimum in short-range production rates of dimer-spectator systems allows precise mass extraction for near-threshold states via a fixed relation to the observed dip position.
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An improved moment QCD sum rule
An improved moment sum rule incorporating duality and dependence conditions uniquely fixes the continuum threshold and ground-state mass without ad-hoc OPE/pole criteria, matching prior Laplace results for a light tetraquark.
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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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Mechanisms of high energy polarized photoproduction of $\pi^{-}\Delta^{++}$
Regge amplitude fit to high-energy polarized pi- Delta++ photoproduction confirms pion dominance at small t and extracts pi N Delta coupling consistent with Delta decay plus first values for rho, b1, and a2 N Delta couplings.
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$\boldsymbol{B_c}$ Meson Spectroscopy from Bayesian MCMC: Probing Confinement and State Mixing
Bayesian MCMC sampling of Cornell and log-modified Cornell potentials reproduces known B_c states and supplies mass predictions for higher excitations with propagated uncertainties.
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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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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
An on-shell decomposition of 2+J o2 electroweak amplitudes isolates OPE poles, logs, and triangle singularities, leaving only smooth short-distance functions.
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Possibility of the antibottom-strange molecular pentaquarks near $ B\Sigma$ and $ B^*\Sigma$ thresholds
Coupled-channel OBE dynamics with S–D mixing produce three near-threshold poles dominated by BΣ/B*Σ that should show as narrow enhancements in open Bs0N, BΛ and B*Λ channels.
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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 $X(7100)$
Symmetry analysis of compact tetraquarks shows low-energy states favor J^P=2+ and places X(6600), X(6900), X(7100) among the lower levels of the fully charmed spectrum.
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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 $X(7100)$
Counting symmetry-allowed states up to orbital angular momentum L=3 predicts low-lying compact tetraquarks prefer J^P=2^+, matching the observed 2^{++} fully charmed X states.
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Radiative decays of dynamically generated pentaquarks in the chiral unitary approach: the $P_c(4457)\to P_c(4312)\,\gamma$ transition
The M1 radiative decay width Pc(4457)(3/2-) → Pc(4312)(1/2-)γ is computed as 6.7 keV (range 2-9 keV) via 19 triangle loops with photon coupling to molecular components.
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Fully-heavy multiquarks in neural-network quantum states
Neural-network quantum states are used to compute spectra of fully-heavy multiquarks in a non-relativistic quark model, claiming to overcome dimensionality issues with superior accuracy over prior approximations.
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$\boldsymbol{\chi_{c1}}(3872)$ and its Partners in the Diabatic Born-Oppenheimer Approximation for QCD
χ_c1(3872) is treated as a bound state in adjoint-meson Born-Oppenheimer potentials within a diabatic model, with calculated spin splittings and decay widths for its multiplet and bottom analogs after tuning to threshold.
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Simultaneous Dalitz-plot decomposition of the $e^+ e^- \to J/\psi \, \pi \, \pi \, (K \bar{K})$ processes in the 4.13-4.36 GeV region using dispersive final-state interactions
Joint Dalitz decomposition of two e+e- processes with dispersive pi pi / KKbar final-state interactions shows a non-resonant production term is required and extracts Breit-Wigner parameters for Zc(3900), Y(4220), and Y(4320).
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Vector charmonium(-like) states in the energy range of 4.1-4.6 GeV
A coupled-channel framework is developed and fitted to BESIII data on vector charmonium-like states in the 4.1-4.6 GeV range, concluding that coupled-channel effects with dynamically generated poles explain the line shapes.
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Isospin-breaking effects on the threshold cusp structures in $\Lambda N$-$\Sigma N$ scattering
Isospin breaking splits threshold cusps in ΛN-ΣN scattering into constrained structures whose relative sharpness or type can change, as shown via K-matrix classification and N²LO chiral EFT calculations.
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Multimodal Fragmentation of All-Heavy Pentaquarks: Uncertainty-Aware Predictions for Hadron Colliders
A multimodal, uncertainty-quantified set of leading-power fragmentation functions for all-charm pentaquarks is constructed and applied to NLL/NLO+ pentaquark-plus-jet production at future hadron colliders.
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Determination of the $Z_c(3900)$ and the $Z_{cs}(3985)$ states from joint analysis of experimental and lattice data
Joint analysis of experimental and lattice data confirms Z_c(3900) and Z_cs(3985) as SU(3) flavor partners with pole masses (3879.6 ± 4.8) MeV and (3976.9 ± 5.1) MeV, half-widths (32.2 ± 4.7) MeV and (28.8 ± 5.9) MeV,...
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Coupled-channel study of the three-body $DDK$ and $D^{*}D^{*}K$
The DDK system supports a deeply bound compact state across wide parameters and possibly a shallow three-body halo state near the D-DK threshold, with negligible D*D*K coupling and no resonances.
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Study of the molecular Properties of the $P_c$ and $P_{cs}$ States
Coupled-channel calculations show Pc and Pcs states as molecular bound states with RMS radii 0.5-2 fm when heavy-quark spin symmetry is respected across all channels.
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Structural dissection of hadronic molecules: The $D^{(*)}\bar{K}^{(*)}$ family under QCD light-cone sum rules
QCD light-cone sum rules predict magnetic moments from −2.0 to +3.1 nuclear magnetons and quadrupole moments near 10⁻³ fm² for JP=1+ D(*)K̅(*) molecular tetraquarks, dominated by light-quark contributions.
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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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Short-range production of three bottom mesons
Leading-order predictions for three-body point production rates of B and B* meson systems are derived in short-range NREFT from two-body input alone.
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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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$\lambda$, $\rho$, 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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Probing the structure of $\chi_{c1}(3872)$: Heavy quark symmetries at work
Heavy-quark large-mass limit applied to E1 transitions of χ_c1(2P) yields decay rates that match LHCb measurements for χ_c1(3872) under the 2P assignment.
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Does $\psi(4660)$ exist?
Fits using coupled channels and bare states show the psi(4660) as a conventional charmonium state above the Lambda_c pair threshold in both Belle and BESIII datasets.
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Hunting for $B\bar B$ molecular state $X_{b0}$ via radiative transition of $\Upsilon(10753)$
The decay Υ(10753) → γ X_b0 is predicted to have partial width 0.2-1.5 keV and branching fraction 10^{-6} to 10^{-5} for binding energies 0-10 MeV, dominated by B1(') meson loops.
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QCD sum rule analysis of local meson-meson currents for the $K(1690)$ state
QCD sum rules with local meson-meson currents for the K(1690) consistently predict masses around 2 GeV or above, disfavoring a molecular interpretation in favor of a compact multiquark state.
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Comprehensive Mass Predictions: From Triply Heavy Baryons to Pentaquarks
Machine learning models trained on known hadron data and an extended Gürsey-Radicati mass formula predict masses for triply heavy baryons and numerous pentaquark states, agreeing with available data and forecasting un...
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Three-body molecular states composed of $D^{(*)}$ and two nucleons
The DNN system forms a robust compact bound state in the I=1/2 (1^-) channel across cutoffs, while D*NN exhibits spin-dependent bound states in 0^-, 1^-, and 2^- channels with no resonances found.
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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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Hadronic Molecules and $\chi_{cJ}(2P)$ Coupled States
A coupled-channel model mixing χ_cJ(2P) cores with D(*)D(*) hadrons describes X(3872) as mostly molecular while X(3860) and Z(3930) have larger compact components and predicts a 0++ state.
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Multimodal Fragmentation of All-Heavy Pentaquarks: Uncertainty-Aware Predictions for Hadron Colliders
Develops uncertainty-aware fragmentation functions PQ5Q1.1 for all-charm pentaquarks using multimodal perturbative and nonperturbative modeling for collider predictions.
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Prospects for observing the missing $2D$ and $1F$ charmonium states around 4 GeV
The paper computes spectroscopic properties, OZI-allowed strong decays, and radiative transitions for the 2D and 1F charmonium states around 4 GeV to guide experimental searches.
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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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Colloquium: Hadron Production in Open-charm Meson Pair at $e^+e^-$ Collider
A review summarizing experimental contributions from BABAR, Belle, BESIII, and CLEO-c to the study of hadron production in open-charm meson pairs and insights into nonstandard hadrons.
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