Scalar glueball in a holographic model of QCD

Two-gluon and trigluon glueballs from dynamical holography QCD

We study the scalar, vector and tensor two-gluon and trigluon glueball spectra in the framework of 5-dimension dynamical holographic QCD model, where the metric structure is deformed self-consistently by the dilaton field. For comparison, the glueball spectra are also calculated in the hard-wall and soft-wall holographic QCD models. In order to distinguish glueballs with even and odd parities, we introduce the positive and negative coupling between the dilaton field and glueballs, and for higher spin glueballs, we introduce a deformed 5-dimension mass. With this set-up, there is only one free …

Glueballs at high temperature within the Hard-Wall holographic model

In this investigation an holographic description of the deconfined phase transition of scalar and tensor glueballs is presented within the so called hard-wall model. The spectra of these bound states of gluons have been calculated from the linearized Einstein equations for a graviton propagating from a thermal $AdS_5$ space to an AdS Black-Hole. In this framework, the deconfined phase is reached via a two steps mechanism. We propose that the transition between the AdS thermal sector to the BH is described via a first order phase transition, with discontinuous masses at the critical temperature…

Glueball decay patterns in top-down holographic QCD

We discuss our results on scalar glueball decay in the top-down holographic Witten-Sakai-Sugimoto model for low-energy QCD and compare with available experimental data, which appear to disfavor the glueball candidate $f_0(1500)$ but seem to be perfectly consistent with interpreting $f_0(1710)$ as a nearly unmixed glueball. The holographic model moreover makes definite predictions for future experiments.

Tensor Glueball in a Top-Down Holographic Approach to QCD

Properties of the tensor glueball are discussed in the Witten-Sakai-Sugimoto Model, a top-down holographic approach to the non-perturbative region of Quantum Chromodynamics (QCD).

Pseudoscalar Glueball Mass: QCD vs. Lattice Gauge Theory Prediction

We study whether the pseudoscalar glueball mass in full QCD can differ from the prediction of quenched lattice calculations. Using properties of the correlator of the vacuum topological susceptibility we derive an expression for the upper bound on the QCD glueball mass. We show that the QCD pseudoscalar glueball is lighter than the pure Yang-Mills theory glueball studied in quenched lattice calculations. The mass difference between those two states is of order of $1/N_c$. The value calculated for the $0^{-+}$ QCD glueball mass can not be reconciled with any physical state observed so far in th…

The Glueball; The Fundamental Particle of Non-Perturbative QCD

Theoretical ideas related to the existence of glueballs in QCD are reviewed. These include non-perturbative phenomena such as confinement, instantons, vacuum condensates and renormalons. We also discuss glueball dominance of the trace of the stress-tensor, the mass content of the nucleon and a theorem on the lightest glueball state.

Glueballs, Hybrids, Multiquarks. Experimental facts versus QCD inspired concepts

The spectroscopy of light and heavy mesons is reviewed with emphasis on glueballs, hybrids, and tetraquarks.

Glueball Decay in Holographic QCD

Using holographic QCD based on D4-branes and D8-anti-D8-branes, we have computed couplings of glueballs to light mesons. We describe glueball decay by explicitly calculating its decay widths and branching ratios. Interestingly, while glueballs remain less well understood both theoretically and experimentally, our results are found to be consistent with the experimental data for the scalar glueball candidate f_0(1500). More generally, holographic QCD predicts that decay of any glueball to 4 $π_0$ is surpressed, and that mixing of the lightest glueball with $q\bar{q}$ mesons is small.

Fermion correction to the mass of the scalar glueball in QCD sum rule

Contributions of fermions to the mass of the scalar glueball $0^{++}$ are calculated at two-loop level in the framework of QCD sum rules. It obviously changes the coefficients in the operator product expansion (OPE) and shifts the mass of glueball.

Mass of the Lowest Scalar (0++) Glueball in the QCD Sum Rules

The mass of the lowest scalar glueball is discussed by using QCD sum rules. We find that the glueball mass is sensitive to the choice of moments and slightly depends on the radiative corrections. With the help of suitable moments and stability criteria, we get the scalar glueball mass: $1710\pm110$ MeV without radiative corrections and $1580\pm150$ MeV with radiative corrections.

QCD Vacuum Topology and Glueballs

We outline a comprehensive study of spin-0 glueball properties which, in particular, keeps track of the topological gluon structure. Specifically, we implement (semi-hard) topological instanton physics as well as topological charge screening in the QCD vacuum into the operator product expansion (OPE) of the glueball correlators. A realistic instanton size distribution and the (gauge-invariant) renormalization of the instanton contributions are also implemented. Predictions for 0^{++} and 0^{-+} glueball properties are presented.

Odd spin glueball masses and the odderon Regge trajectories from the holographic hardwall model

We use the holographic hardwall model to calculate the masses of light glueball states with odd spin and P= C= -1 associated with odderons. Considering Dirichlet and Neumann boundary conditions we obtain expressions for the odderon Regge trajectories consistent with those calculated using other approaches.

Glueball spectra and Regge trajectories from a modified holographic softwall model

In this work we propose a modified holographic softwall model, analytically solvable, to calculate the masses of lightest scalar glueball and its radial excitations and of higher spin glueball states for both even and odd spins. From these results we obtain their respective Regge trajectories, associated with the pomeron for even spins and with the odderon for odd spins. These results are in agreement with those calculated using other approaches.

Scalar and higher even spin glueball masses from an anomalous modified holographic model

In this work, within an anomalous modified holographic softwall model, we calculate analytically the masses of the scalar glueball with its radial excitations and higher even glueball spin states, with $P=C=+1$, from a single mass equation. Using this approach we achieved an unified treatment for both scalar and high even spin glueballs masses. Furthermore, we also obtain the Regge trajectory associated with the pomeron compatible with other approaches.

Scalar Glueball in a Top-Down Holographic Approach to QCD

Identification of glueballs -- bound states of gauge bosons in Quantum Chromodynamics (QCD) -- is a very important open question in dynamics of the strong interaction. The search for the glueball ground state, carrying scalar quantum numbers, poses a particular challenge due to the existence of (i) several candidates for its realisation in the physical spectrum and (ii) inevitable mixing of the pure glueball state with those comprised of quarks. In this article, I discuss implications of an approach in holographic QCD where, among others, the mass and the two-pion decay of the pure scalar glue…

Scalar Glueball--Quarkonium Mixing and the Structure of the QCD Vacuum

We use Ward identities of broken scale invariance to infer the amount of scalar glueball--$\bar{q}q$ meson mixing from the ratio of quark and gluon condensates in the QCD vacuum. Assuming dominance by a single scalar state, as suggested by a phase-shift analysis, we find a mixing angle $γ\sim 36^{\circ}$, corresponding to near-maximal mixing of the glueball and $\bar{s}s$ components.

Glueballs, a fulfilled promise of QCD?

This is a contribution to the review "50 Years of Quantum Chromdynamics" edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJC. The contribution remembers the early searches and explains how to find a glueball, based on its properties. The results of a coupled-channel analysis are presented that provides evidence for the scalar glueball and first hints for the tensor glueball. Data on radiative decays of $ψ(2S)$ and $Υ(1S)$ show scalar intensity that is likely due to glueball production.

Determination of the scalar glueball mass in QCD sum rules

The $0^{++}$ glueball mass is analyzed in the QCD sum rules. We show that in order to determine the $0^{++}$ glueball mass by using the QCD sum rules method, it is necessary to clarify the following three ingredients: (1) to choose the appropriate moment with acceptable parameters which satisfy all of the criteria; (2) to take account into the radiative corrections; (3) to estimate an additional contribution to the glueball mass from the lowest lying ${\bar q}q$ resonance. We conclude that it is the key point to choose suitable moments to determine the $0^{++}$ glueball mass, the radiative cor…

Scalar glueball in a soft-wall model of AdS/QCD

The scalar glueball is investigated in a soft-wall model of AdS/QCD. Constraints of the mass of the scalar glueball are given through an analysis of a relation between the bulk mass and the anomalous dimension. The mass of the ground scalar glueball is located at $0.96^{+0.04}_{-0.07} \rm{GeV}<m_G<1.36^{+0.05}_{-0.10}\rm{GeV}$. In terms of a background dilaton field $Φ(z)=cz^2$, the two-point correlation function for the scalar gluon operator is obtained. The two-point correlation function at $Δ=4$ gives a different behavior compared with the one in QCD.

Holographic Oddballs

The spectrum of the glueball with $J^{PC}=0^{--}$ is computed using different bottom-up holographic models of QCD. The results indicate a lowest-lying state lighter than in the determination by other methods, with mass $m \simeq 2.8$ GeV. The in-medium properties of this gluonium are investigated, and stability against thermal and density effects is compared to other hadronic systems. Production and decay modes are identified, useful for searching the $J^{PC}=0^{--}$ glueball.

A Remark on the Large Difference between the Glueball Mass and T(C) in Quenched QCD

The lattice QCD studies indicate that the critical temperature $T_c \simeq 260-280$ MeV of the deconfinement phase transition in quenched QCD is considerably smaller than the lowest-lying glueball mass $m_{\rm G} \simeq 1500-1700$ MeV, i.e., $ T_c \ll m_{\rm G}$. As a consequence of this large difference, the thermal excitation of the glueball in the confinement phase is strongly suppressed by the statistical factor as $e^{-m_{\rm G}/T_c} \simeq 0.00207$ even near $T \simeq T_c$. We consider its physical implication, and argue the abnormal feature of the deconfinement phase transition in quenc…

Glueball masses and Pomeron trajectory in nonperturbative QCD

Using a nonperturbative method based on asymptotic behaviour of Wilson loops we calculate masses of glueballs and corresponding Regge-trajectories. The method contains no fitting parameters and the mass scale is fixed by the meson Regge slope. Theoretical predictions for lowest glueball states are in a perfect agreement with lattice results. The leading glueball trajectory and its relation to the Pomeron is discussed in details. Important role of mixing between glueball and q\bar q trajectories is emphasized.

Top-down Holographic Glueball Decay Rates

We present new results on the decay patterns of scalar and tensor glueballs in the top-down holographic Witten-Sakai-Sugimoto model. This model, which has only one free dimensionless parameter, gives semi-quantitative predictions for the vector meson spectrum, their decay widths, and also a gluon condensate in agreement with SVZ sum rules. The holographic predictions for scalar glueball decay rates are compared with experimental data for the widely discussed gluon candidates f0(1500) and f0(1710).

Glueball Masses in QCD_3

We discuss how to extract the spectroscopy of quantum chromodynamics (QCD) in the pure gauge sector from the Hamiltonian lattice field theory approach. The recently developed truncated eigenvalue equation method is applied to the estimation of the scalar glueball $0^{++}$ and $0^{--}$ masses in the (2+1)-dimensional case. These masses reach the constant values in a scaling region as required by the renormalizability.

Abelian Decomposition and Glueball-Quarkonium Mixing in QCD

The Abelian decomposition of QCD which decomposes the gluons to the color neutral binding gluons (the neurons) and the colored valence gluons (the chromons) gauge independently naturally generalizes the quark model to the quark and chromon model which could play the central role in hadron spectroscopy. We discuss the color reflection symmetry, the fundamental symmetry of the quark and chromon model, and explain how it describes the glueballs and the glueball-quarkonium mixing in QCD. We present the numerical analysis of glueball-quarkonium mixing in $0^{++}$, $2^{++}$, and $0^{-+}$ sectors bel…

Exotic glueball $0^{\pm -}$ states in QCD Sum Rules

The lowest dimension three-gluon currents that couple to the exotic $0^{\pm-}$ glueballs have been constructed using the helicity formalism. Based on the constructed currents, we obtain new QCD SRs that have been used to extract the masses and the decay constants of the scalar exotic $0^{\pm-}$ glueballs. We estimate the masses for the scalar state and for the pseudoscalar state to be ${m_+=9.8^{+1.3}_{-1.4}}$ GeV and $m_-=6.8^{+1.1}_{-1.2}$ GeV.

Holographic QCD predictions for production and decay of pseudoscalar glueballs

The top-down holographic Witten-Sakai-Sugimoto model for low-energy QCD, augmented by finite quark masses, has recently been found to be able to reproduce the decay pattern of the scalar glueball candidate f0(1710) on a quantitative level. In this Letter we show that this model predicts a narrow pseudoscalar glueball heavier than the scalar glueball and with a very restricted decay pattern involving eta or eta' mesons. Production should be either in pairs or in association with eta(') mesons. We discuss the prospect of discovery in high-energy hadron collider experiments through central exclus…

Investigating AdS/QCD duality through scalar glueball correlators

We investigate AdS/QCD duality for the two-point correlation function of the lowest dimension scalar glueball operator, in the case of the IR soft wall model. We point out the role of the boundary conditions for the bulk-to-boundary propagator in determining the gluon condensates. We show that a low energy QCD theorem can be obtained within the AdS approach, together with a gluon condensate close to the commonly accepted value and robust against perturbation of the background dilaton field.

Conditional entropy of glueball states

The conditional entropy of glueball states is calculated using a holographic description. Glueball states are represented by a supergravity dual picture, consisting of a 5-dimensional graviton-dilaton action of a dynamical holographic AdS/QCD model. The conditional entropy is studied as a function of the glueball spin and of the mass, providing information about the stability of the glueball states.

Glueball Physics in QCD

The Abelian decomposition of QCD which decomposes the gluons to the color neutral binding gluons and the colored valence gluons shows that QCD can be viewed as the restricted QCD (RCD) made of the binding gluons which has the valence gluons as colored source, and simplifies the QCD dynamics greatly. In particular, it tells that the gauge covariant valence gluons can be treated as the constituents of hadrons, and generalizes the quark model to the quark and valence gluon model. So it provides a comprehensive picture of glueballs and their mixing with quarkoniums, and predicts new hybrid hadrons…

On holographic relation between radial meson trajectories and deconfinement temperature

The interrelation between the deconfinement temperature of hadron medium and parameters of radial Regge trajectories within the bottom-up holographic models for QCD is scrutinized. We show that the lattice data on the deconfinement temperature can yield a powerful restriction on the spectrum of excited mesons and glueballs within the framework of holographic approach. The best phenomenological agreement and theoretical self-consistency are achieved if the scalar meson $f_0(1500)$ is considered as the lightest glueball.

A broad pseudovector glueball from holographic QCD

We study the decay of a pseudovector glueball with quantum numbers $J^{PC}=1^{+-}$, the lightest glueball with spin different from zero or two, in the top-down holographic Witten-Sakai-Sugimoto model. This glueball is dual to the string-theoretic Kalb-Ramond tensor field, which completely fixes all its couplings by gauge invariance and $D$-brane anomaly inflow. The dominant decay channels are determined by a Chern-Simons-like action for the flavor branes; couplings from the Dirac-Born-Infeld action turn out to be subdominant. While the resulting width is parametrically of order $N_c^{-2}λ^{-1}…

Holographic theories at finite $θ$-angle, CP-violation, glueball spectra and strong-coupling instabilities

A general class of holographic theories with a nontrivial $θ$-angle are analyzed. The instanton density operator is dual to a bulk axion field. We calculate the ground-state solutions with nontrivial source, $a_{UV}$, for the axion, for both steep and soft dilaton potentials in the IR, and both in $d=3$ and $d=4$. We find all cases to be qualitatively similar. We also calculate the spin$=2,0$ glueball spectra and show that the glueball masses monotonically decrease as functions of $a_{UV}$ (or $θ$-angle). The slopes of glueball masses are different, generically, in different potentials. In the…

On the AdS/QCD estimates of the scalar glueball mass

We review standard holographic derivations of the scalar glueball spectra and emphasize computational assumptions which rely strongly on the original AdS/CFT correspondence, and those which are more arbitrary. In the soft-wall model, we show explicitly that the dual of the spectral mass constraint in the Kallen-Lehmann representation of the 2-point scalar gluonic correlator is an eigenvalue constraint on the motion equation of the bulk field corresponding this correlator.

Pseudoscalar glueball and $η'$-meson in low-energy QCD expansion

An effective chiral lagrangian of order $p^2$, describing the interaction of light pseudoscalar (PS) mesons with $η'$-meson and PS-glueball, has been determined taking into consideration the renorm-group requirements imposed by QCD renormalization. It is shown that the interpolating fields for the lowest singlet quarkic and gluonic states, $η^0$ and $η^G$, may be involved into the effective theory to be renorm-invariant objects not mixing due to QCD renormalization. It is established that the potential describing the ``mass'' term of the lagrangian does not depend on $η^0$. The dependence on $…

Witten-Veneziano mechanism and pseudoscalar glueball-meson mixing in holographic QCD

We revisit the $U(1)_A$ anomaly in the holographic model of low-energy QCD by Witten, Sakai, and Sugimoto, presenting a new and direct derivation of the Witten-Veneziano mechanism for generating the mass of the $η'$ through an anomalous mixing of the Ramond-Ramond $C_1$ field with the singlet component of the pseudoscalar mesons. The latter turns out to have a kinetic mixing with the normalizable modes of the $C_1$ field representing pseudoscalar glueballs, yielding additional vertices for their production and their decay that dominate over those of the unmixed case considered previously in th…

Heavy glueballs: status and large-$N_{c}$ width's estimate

Glueballs, an old and firm prediction of various QCD approaches (lattice QCD, bag models, AdS/QCD, effective models, etc.), have not yet been experimentally confirmed. While for glueballs below $2.6$ GeV some candidates exist, the situation for heavy glueballs (above $2.6$ GeV) is cloudy. Here, after a brief review of scalar, tensor, and pseudoscalar glueballs, we present predictions for the decays of a putative pseudotensor glueball with a lattice predicted mass of $3.04$ GeV and a putative vector glueball with a lattice predicted mass of $3.81$ GeV. Moreover, we discuss in general the width …

Direct instantons, topological charge screening and QCD glueball sum rules

Nonperturbative Wilson coefficients of the operator product expansion (OPE) for the spin-0 glueball correlators are derived and analyzed. A systematic treatment of the direct instanton contributions is given, based on realistic instanton size distributions and renormalization at the operator scale. In the pseudoscalar channel, topological charge screening is identified as an additional source of (semi-) hard nonperturbative physics. The screening contributions are shown to be vital for consistency with the anomalous axial Ward identity, and previously encountered pathologies (positivity violat…

The low lying glueball spectrum

The complete low-lying positive charge conjugation glueball spectrum is obtained from QCD. The formalism relies on the construction of an efficient quasiparticle gluon basis for Hamiltonian QCD in Coulomb gauge. The resulting rapidly convergent Fock space expansion is exploited to derive quenched low-lying glueball masses with no free parameters which are in remarkable agreement with lattice gauge theory.

QCD/String holographic mapping and glueball mass spectrum

Recently Polchinski and Strassler reproduced the high energy QCD scaling at fixed angles from a gauge string duality inspired in AdS/CFT correspondence. In their approach a confining gauge theory is taken as approximately dual to an AdS space with an IR cut off. Considering such an approximation (AdS slice) we found a one to one holographic mapping between bulk and boundary scalar fields. Associating the bulk fields with dilatons and the boundary fields with glueballs of the confining gauge theory we also found the same high energy QCD scaling. Here, using this holographic mapping we give a si…

QCD Glueball Regge Trajectories and the Pomeron

We report a glueball Regge trajectory emerging from diagonalizing a confining Coulomb gauge Hamiltonian for constituent gluons. Using a BCS vacuum ansatz and gap equation, the dressed gluons acquire a mass, of order 800 $MeV$, providing the quasiparticle degrees of freedom for a TDA glueball formulation. The TDA eigenstates for two constituent gluons have orbital, $L$, excitations with a characteristic energy of 400 $MeV$ revealing a clear Regge trajectory for $\vec{J} = \vec{L} + \vec{S}$, where $S$ is the total (sum) gluon spin. Significantly, the $S = 2$ glueball spectrum coincides with the…

Deconfinement temperature in AdS/QCD from the spectrum of scalar glueballs

We scrutinize various holographic estimations of the deconfinement temperature within the bottom-up AdS/QCD models. A special emphasis is put on the recent idea of isospectral potentials in the holographic approach. It is demonstrated that different models from an isospectral family (i.e., the models leading to identical predictions for the spectrum of hadrons with fixed quantum numbers) result in different predictions for the deconfinement temperature. This difference is found to be quite small in the scalar glueball channel but very large in the vector meson channel which is often used for f…

Modelling glueballs

Glueballs are predicted in various theoretical approaches of QCD (most notably lattice QCD), but their experimental verification is still missing. In the low-energy sector some promising candidate for the scalar glueball exist, and some (less clear) candidates for the tensor and pseudoscalar glueballs were also proposed. Yet, for heavier gluonic states there is much work to be done both from the experimental and theoretical points of view. In these proceedings, we briefly review the current status of research of glueballs and discuss future developments.

The Glueball Filter in Central Production and Broken Scale Invariance

We propose a possible explanation of the kinematical dependence of the central production of the scalar glueball candidate observed recently by the WA91 and WA102 Collaborations, and discussed by Close and Kirk, in the context of the broken scale invariance of QCD. The dependences of glueball production on the transverse momenta and azimuthal angles of the final-state protons may be related to the structure of the trace anomaly in QCD.

Holographic Glueball Decay

We announce new results on glueball decay rates in the Sakai-Sugimoto model, a realization of holographic QCD from first principles that has only one coupling constant and an overall mass scale as free parameters. We extend a previous investigation by Hashimoto, Tan, and Terashima who have considered the lowest scalar glueball which arises from a somewhat exotic polarization of supergravity modes and whose mass is uncomfortably small in comparison with lattice results. On the other hand, the scalar glueball dual to the dilaton turns out to have a mass of about twice the mass of the rho meson (…

Odd-spin glueballs, AdS/QCD and information entropy

Odd-spin glueballs in the dynamical AdS/QCD model are scrutinized, in the paradigm of the configurational entropy (CE). Configurational-entropic Regge trajectories, that relate the CE underlying odd-spin glueballs to their mass spectra and spin, are then engendered. They predict the mass spectra of odd-spin glueballs, besides pointing towards the configurational stability of odd-spin glueball resonances. The exponential modified dilaton with logarithmic anomalous dimensions comprises the most suitable choice to derive the mass spectra of odd-spin glueballs, compatible to lattice QCD. It is the…

Estimate for the $0^{++}$ glueball mass in QCD

We obtain accurate result for the lightest glueball mass of QCD in 3 dimensions from lattice Hamiltonian field theory. Using the dimensional reduction argument, a good approximation for confining theories, we suggest that the $0^{++}$ glueball mass in 3+1 dimensional QCD be about $1.71$ GeV.

Holographic glueball structure

We derive and systematically analyze scalar glueball correlation functions in both the hard-wall and dilaton soft-wall approximations to holographic QCD. The dynamical content of the holographic correlators is uncovered by examining their spectral density and by relating them to the operator product expansion, a dilatational low-energy theorem and a recently suggested two-dimensional power correction associated with the short-distance behavior of the heavy-quark potential. This approach provides holographic estimates for the three lowest-dimensional gluon condensates or alternatively their Wil…

数据驱动的爱因斯坦-稀释子模型用于纯杨-米尔斯热力学与胶球谱

Data-Driven Einstein-Dilaton Model for Pure Yang-Mills Thermodynamics and Glueball Spectrum

本文提出数据驱动的全息模型，通过神经网络重建爱因斯坦-稀释子引力，统一描述纯杨-米尔斯理论的热力学与胶球谱。

Baryonium, tetra-quark state and glue-ball in large N_c QCD

From the large-N_c QCD point of view, baryonia, tetra-quark states, hybrids, and glueballs are studied. The existence of these states is argued for. They are constructed from baryons. In N_f=1 large N_c QCD, a baryonium is always identical to a glueball with N_c valence gluons. The ground state 0^{-+} glueball has a mass about 2450 MeV. f_0(1710) is identified as the lowest 0^{++} glueball. The lowest four-quark nonet should be f_0(1370), a_0(1450), K^*_0(1430) and f_0(1500). Combining with the heavy quark effective theory, spectra of heavy baryonia and heavy tetra-quark states are predicted. …

Scalar Mesons and glueballs in $Dp-Dq$ hard-wall models

We investigate light scalar mesons and glueballs in the $Dp-Dq$ hard-wall models, including $D3-Dq$, $D4-Dq$, and $D6-Dq$ systems. It is found that only in the $D4-D6$ and $D4-D8$ hard wall models, the predicted masses of the ${\bar q} q$ scalar meson $f_0$, scalar glueball are consistent with their experimental or lattice results. This indicates that $D4-D6$ and $D4-D8$ hard-wall models are favorite candidates of the realistic holographic QCD model.

Glueball correlators as holograms

We investigate the dynamical content of both hard- and soft-wall approximations to holographic QCD by deriving the corresponding glueball correlation functions and by confronting them with a variety of QCD results. We further calculate the glueball decay constants in both holographic duals, discuss emerging limitations and improvement strategies, and comment on a recent attempt to generalize the glueball correlator in the soft-wall background.

XYZ mesons and Glueballs: effects of the quark-gluon mixing

We present the new developments in the physics of XYZ mesons and glueballs and discuss the connection between properties of the $XYZ$ mesons and glueballs, and the structure of the QCD vacuum. It is shown that the mixing between quark and gluon degree of freedoms induced by the nontrivial topological structure of the QCD vacuum might affect strongly the properties of the exotic hadrons. A new interpretation of the $XYZ$ mesons as the mixing between radial excitations of light quark-antiquark systems and states with heavy quark content is suggested. We introduce also a new mechanism of quarkoni…

The Physics of Glueballs

Glueballs are particles whose valence degrees of freedom are gluons and therefore in their description the gauge field plays a dominant role. We review recent results in the physics of glueballs with the aim set on phenomenology and discuss the possibility of finding them in conventional hadronic experiments and in the Quark Gluon Plasma. In order to describe their properties we resort to a variety of theoretical treatments which include, lattice QCD, constituent models, AdS/QCD methods, and QCD sum rules. The review is supposed to be an informed guide to the literature. Therefore, we do not d…