We first construct the effective chiral Lagrangians for the $1^{-+}$ exotic mesons. With the infrared regularization scheme, we derive the one-loop infrared singular chiral corrections to the $π_1(1600)$ mass explicitly. We investigate the variation of the different chiral corrections with the pion mass under two schemes. Hopefully, the explicit non-analytical chiral structures will be helpful to the chiral extrapolation of the lattice data from the dynamical lattice QCD simulation of either the exotic light hybrid meson or tetraquark state.
Recent results on meson spectroscopy from lattice QCD are reviewed. The emphasis is on interesting states near thresholds like $Z_c^+(3900)$, $X(3872)$ and $D_{s0}^*(2317)$. Another focus is on the meson resonances in light, strange and charm sector, where the resonance masses as well as the strong decay widths are extracted from the lattice.
Exotic and higher spin (> 1) mesons are still not throughly investigated in lattice QCD. Using a set of derivative based operators we report our exploratory study of these mesons in charmonium region. We use a 12^3 X 48 anisotropic (xi = 3) clover lattice with inverse temporal lattice spacing a^-1 = 6.05 GeV. Techniques developed in this exploratory study will be utilized in our future comprehensive study of light hybrid mesons that are to be explored in the 12 GeV GlueX experiment at Jefferson Laboratory.
Hybrid mesons, made from a quark, an antiquark and gluons, can have quantum numbers inaccessible to conventional quark-antiquark states. Confirmation of such states would give information on the role of "dynamical" color in low energy QCD. We present preliminary results for hybrid meson masses using light Wilson valence quarks.
We present highly-excited charmonium, $D_s$ and $D$ meson spectra from dynamical lattice QCD calculations with light quarks corresponding to $M_π \sim 240$ MeV and compare these to previous results with $M_π \sim 400$ MeV. Utilising the distillation framework, large bases of carefully constructed interpolating operators and a variational procedure, we extract and reliably identify the continuum spin of an extensive set of excited mesons. These include states with exotic quantum numbers which, along with a number with non-exotic quantum numbers, we identify as having excited gluonic degrees of …
Using a new quark-field construction algorithm and a large variational basis of operators, we extract a highly excited isovector meson spectrum on dynamical anisotropic lattices. We show how carefully constructed operators can be used to reliably identify the continuum spin of extracted states, overcoming the reduced cubic symmetry of the lattice. Using this method we extract, with confidence, excited states, states with exotic quantum numbers (0+-, 1-+ and 2+-) and states of high spin, including for the first time in lattice QCD, spin-four states.
We review lattice QCD results for glueballs (including a discussion of mixing with scalar mesons), hybrid mesons and exotic mesons (such as $B_s B_s$ molecules).
The evidence for the existence of mesons with exotic quantum numbers and of hybrid candidates with non-exotic quantum numbers is critically reviewed, including candidates with hidden charm. Aims and methods of future searches for hybrid mesons are briefly discussed.
Since gluons in QCD are interacting fundamental constituents just as quarks are, we expect that in addition to mesons made from a quark and an antiquark, there should also be glueballs and hybrids (bound states of quarks, antiquarks and gluons). In general, these states would mix strongly with the conventional q-bar-q mesons. However, they can also have exotic quantum numbers inaccessible to q-bar-q mesons. Confirmation of such states would give information on the role of "dynamical" color in low energy QCD. In the quenched approximation we present a lattice calculation of the masses of mesons…
We discuss two production mechanisms of the $J^{PC}=1^{-+}$ exotic meson, hadroproduction, using pion beams and photoproduction. We show that the ratio of exotic to non-exotic, in particular the $a_2$, meson production cross sections is expected to be by a factor of 5 to 10 larger, in photoproduction then in hadroproduction. Furthermore we show that the low-t photoproduction of exotic meson is enhanced as compared to hadronic production. This findings support the simple quark picture in which exotic meson production is predicted to be enhanced when the beam is a virtual $Q\bar Q$ pair with a s…
We review lattice QCD results for glueballs (including a discussion of mixing with scalar mesons), hybrid mesons and exotic mesons (such as $B_s B_s$ molecules). We also discuss string breaking as a mixing between colour flux states and $B \bar{B}$ states.
Non-exotic scalar-meson resonances in S-wave meson-meson scattering are studied in the light of a unitarised Schroedinger model. The resulting poles in the scattering matrices, by analytical continuation into the complex-energy plane, are grouped into nonets of isoscalar, isodoublet, and isotriplet resonances. All singularities can be related to quark-antiquark confinement states, the light-quark nonet of which has ground states at 1.3 to 1.4 GeV and level spacings of some 300-400 MeV, except for a nonet of light scalar mesons below 1 GeV. All non-exotic S-wave resonances reported by experimen…
A surprising number of new c c-bar mesons with masses above the D D-bar threshold have been discovered at the B factories. Some of them are ordinary charmonium states, but others are definitely exotic mesons. The current theoretical status of the new c c-bar mesons is summarized.
We examine the chiral corrections to exotic meson masses calculated in lattice QCD. In particular, we ask whether the non-linear chiral behavior at small quark masses, which has been found in other hadronic systems, could lead to large corrections to the predictions of exotic meson masses based on linear extrapolations to the chiral limit. We find that our present understanding of exotic meson decay dynamics suggests that open channels may not make a significant contribution to such non-linearities whereas the virtual, closed channels may be important.
We evaluate the leading order amplitude for the deep exclusive electroproduction of an exotic hybrid meson in the Bjorken regime. We show that, contrary to naive expectation, this amplitude factorizes at the twist 2 level and thus scales like usual meson electroproduction when the virtual photon and the hybrid meson are longitidinally polarized. Exotic hybrid mesons may thus be studied in electroproduction experiments at JLAB, HERA (HERMES) or CERN (COMPASS).
Interactions of $a_2, K^*_2, f_2$ and $f_2'$ tensor-mesons with low-energy $π, K, η, η'$ pseudo-scalar mesons are constrained by chiral symmetry. We derive a chiral Lagrangian of tensor mesons in which the tensor mesons are treated as heavy non-relativistic matter fields. Using $1/N_c$ counting, we derive relations among unknown couplings of the chiral Lagrangian. Chiral perturbation theory is applied to the tensor-meson mass matrix. At one-loop there are large corrections to the individual tensor meson masses, but the singlet-octet mixing angle remains almost unchanged. We argue that all heav…
We analyze the process $\vec γ+p\to ρ^0 π^{+}n$ at low momentum transfer focusing on a possibility of production of an exotic $J^{PC}=1^{-+}$ meson state. In particular we discuss polarization observables and conclude that linear photon polarization is instrumental for separating of the exotic wave.
We describe a four-parameter model for non-exotic meson-meson scattering, which accommodates all non-exotic mesons, hence also the light scalar mesons, as resonances and bound states characterised by complex singularities of the scattering amplitude as a function of the total invariant mass. The majority of the full $S$-matrix mesonic poles stem from an underlying confinement spectrum. However, the light scalar mesons K0*(830), a0(980), f0(400-1200), and f0(980) do not, but instead originate in 3P0-barrier semi-bound states. In the case of bound states, wave functions can be determined. For cc…
This is a conference mostly devoted to diquarks. Although an exotic meson can in principle be composed of a diquark and an antidiquark, such an exotic is unlikely to be experimentally observable in the near future. The reason, according to the model, is that diquark-antidiquark exotics have masses well above the threshold for decay into two mesons, and are likely to have widths too large to make them observable. A possible exception is a $ u d \bar b \bar b$ exotic, but it will be a long time before such a state can be observed even if it is stable against strong decay.
If the exotic baryon $Θ^+(1540)$ is a correlated $udud\bar{s}$ with $J^P = {1/2}^+$, then there should exist an exotic meson, $J^P = 1^-$ $\vartheta^+ (S=+2)$ $\to K^+K^0$ $\sim 1.6$GeV with width $O(10-100)$MeV. The $π_1(1400;1600)$ may be broad members of {\bf 10} $\pm$ \10bar in such a picture. Vector mesons in the 1.4 - 1.7GeV mass range are also compared with this picture.
We propose stable exotic nuclei bound with $\bar{D}$ and $B$ mesons with respecting heavy quark symmetry. We indicate that an approximate degeneracy of $\bar{D}$($B$) and $\bar{D}^{*}$($B^{*}$) mesons plays an important role, and discuss the stability of $\bar{D}N$ and $BN$ bound states. We find the binding energies 1.4 MeV and 9.4 MeV for each state in the $J^{P}=1/2^{-}$ with I=0 channel. We discuss also possible existence of exotic nuclei $\bar{D}NN$ and $BNN$.
The relativistic four-quark equations are found in the framework of the dispersion relation technique. The dynamical mixing of the four-quark amplitudes and the glueball amplitudes is considered. The approximate solutions of these equations using the method based on the extraction of leading singularities of the amplitudes are obtained. The four-quark amplitudes of exotic mesons including the quarks of three flavors (u, d, s) are calculated. The poles of these amplitudes determine the masses of the exotic mesons.
The exotic J^{PC}= 1^{-+} resonance π_{1}(1600)is examined in the framework of the Quark Model with Constituent Gluon (QMCG). We report the possibility of interpreting that resonance as quark-antiquark-gluon hybrid meson, with a masse = 1.65+0.05-0.04 GeV and a rho-pi decay width = 0.28+0.14-0.09 GeV.
We discuss a coupled channel analysis of the $ηπ$ and $η'π$ systems produced in $π^-p$ interactions at 18 GeV/$c$. We show that known $Q\bar Q$ resonances, together with residual soft meson-meson rescattering, saturate the spectra including the exotic $J^{PC}=1^{-+}$ channel. A possibility of a narrow exotic resonance at a mass near 1.6 GeV/$c^2$ cannot, however, be ruled out.
We reconsider the discrepancy between theory and data in the momentum distribution of slow J/psi in B decays. Beside an update of the standard color singlet and color octet QCD components, we include the contribution from XY Z exotic mesons, and show that the residual discrepancy could be accommodated considering new XYZ mesons still unobserved.
In this talk I summarize the status of exotic mesons, including both theoretical expectations and experimental candidates. The current experimental candidates are ``spin-parity exotics''; since these are most often considered possible hybrid mesons, the theoretical discussion will be mainly concerned with hybrids. The exotic meson candidates discussed are the surprisingly light pi_1(1400) and pi_1(1600).
Chiral symmetry restoration high in the hadron spectra is expected but it remains to be confirmed both in lattice QCD computations and in experiments. Recently, a theorem was derived, relating chiral symmetry restoration high in the hadron spectra to the spontaneous generation of the dynamical quark mass in QCD. We refine the theorem in the case of static-light mesons. Utilizing chiral quark model computations and lattice QCD results for the spectrum of mesons composed by a static antiquark and a light quark, we explore chiral symmetry restoration in the spectrum and the quark running mass m(k…
We show that hard electroproduction is a promising way to study exotic hybrid mesons, in particular through the hybrid decay channel $H \to πη$. We discuss the $πη$ generalized distribution amplitude, calculate the production amplitude and propose a forward-backward asymmetry as a signal for the hybrid meson production.
Two-ordinary-meson scattering in large-$N_c$ QCD implies consistency criteria for intermediate-tetraquark contributions. Their fulfilment at $N_c$-leading order constrains the nature of the spectrum of genuinely exotic tetraquark states.
We investigate the hybrid exotic meson with $J^{PC}=1^{-+}$ within the framework of an AdS/QCD model. Introducing a holographic field dual to the operator for hybrid exotic meson, we obtain the eigen-value equation for its mass. Fixing all free parameters by QCD observables such as the $ρ$-meson mass, we predict the masses of the hybrid exotic meson. The results turn out to be $1476 \mathrm{MeV}$ for the ground state, and $2611 \mathrm{MeV}$ for the first excited one. Being compared with the existing experimental data for the $π_1(1400)$, which is known to be $m_{π_1} = 1351\pm30 \mathrm{MeV}$…
We consider chiral loop corrections to the effective couplings which describe the strong interactions of two heavy mesons of even or odd parities and a light pseudoscalar meson, within a framework which combines heavy quark effective theory and chiral perturbation theory. The strong couplings are extracted from the experimental results for the decay widths, after including chiral corrections. We find that the introduction of positive parity heavy meson fields into the framework gives relevant contributions to the chiral loop corrections of the effective couplings. The counterterms' contributio…
Discussing four-point Green functions of bilinear quark currents in large-$N_c$ QCD, we formulate rigorous criteria for selecting diagrams appropriate for the analysis of potential tetraquark poles. We find that both flavor-exotic and cryptoexotic (i.e., flavor-nonexotic) tetraquarks, if such poles exist, have a width of order $O(1/N_c^2)$, so they are parametrically narrower compared to the ordinary $\bar qq$ mesons, which have a width of order $O(1/N_c)$. Moreover, for flavor-exotic states, the consistency of the large-$N_c$ behavior of "direct" and "recombination" Green functions requires t…
Tetra-quark mesons with exotic quantum numbers, their production rates and decay properties are studied, because they are useful to establish existence of tetra-quark mesons.
Exotic mesons are striking predictions of quantum chromodynamics that go beyond the quark model. They can provide great insight into understanding phenomena such as asymptotic freedom, confinement, and dynamical symmetry breaking. This work analyzes hadronic decays of exotic mesons, with a focus on the lightest one, the $J^{PC}=1^{-+}$ $π_{1}$, in a fully relativistic formalism. The relativistic spin wave functions of normal and exotic mesons are constructed based on unitary representations of the Poincare group. The radial wave functions are obtained from phenomenological considerations of th…
The potential of decays of mesons containing heavy quarks [including $B$ mesons and the $Υ(1S)$] for producing final states of matter with unusual quark configurations, such as $q q \bar q \bar q$ or $q q q q \bar q$, is investigated. The usefulness of antineutron detection in such searches is stressed.
We study heavy hadron spectroscopy near heavy meson thresholds. We employ heavy pseudoscalar meson P and heavy vector meson P* as effective degrees of freedom and consider meson exchange potentials between them. All possible composite states which can be constructed from the P and P* mesons are studied up to the total angular momentum J <= 2. We consider, as exotic states, isosinglet states with exotic J^{PC} quantum numbers and isotriplet states. We solve numerically the Schrödinger equation with channel-couplings for each state. We found B(*)barB(*) molecule states for I^G(J^{PC}) = 1^+(1^{+…
We present a general class of operators resembling compact tetraquarks which have a range of colour-flavour-spin structures, transform irreducibly under the symmetries of the lattice and respect other relevant symmetries. These constructions are demonstrated in lattice QCD calculations with light quarks corresponding to $m_π=$ 391 MeV. Using the distillation framework, correlation functions involving large bases of meson-meson and tetraquark operators are computed in the isospin-1 hidden-charm and doubly-charmed sectors, and finite-volume spectra are extracted with the variational method. We f…
The CMS phase II track trigger may allow for a displaced dimuon vertex trigger with qualitatively lower $p_T$ thresholds than existing dimuon triggers. With this technique, we show that the CMS reach for exotic $B$-meson decays involving a displaced dimuon resonance, such as a light, Higgs-mixed scalar, can be competitive with that of LHCb and Belle II.
We employ a chiral Lagrangian framework with three dynamical flavors to calculate the masses of the lowest-lying vector mesons to one loop accuracy, and use the resulting formulae to extrapolate recent QCDSF lattice data on vector meson masses and mass ratios to the physical point. Our representation for the vector meson self energies also enables us to discuss loop corrections to the $ωϕ$ mixing amplitude.
Exotic mesons with hidden strange ($s\bar s$) and heavy quark pairs ($Q\bar Q$), where $Q=c,b$, are considered as diquark-antidiquark systems, $(Qs)(\bar Q\bar s)$. Taking into account that these states can recombinate into two-meson ones, we study the interplay of these states in terms of the dispersion relation D-function technique. The classification of exotic mesons is discussed, coefficients for decay modes are given, predictions for new states are presented. The nonet structure for $((Qq)(\bar Q \bar q)$, ($(Qs)(\bar Q\bar s$)), ($(Qq)(\bar Q\bar s))$- states ($q=u,d$) is suggested.
The status of non-perturbative QCD calculations for mesons with gluonic excitation is presented. Lattice results for the glueball spectrum are reviewed. For hybrid mesons, the heavy quark results are summarised and new results are presented for light quarks. Preliminary results for the spectrum of light-quark hybrid mesons indicate substantial mixing with quark model states for non-exotic $J^{PC}$. For the exotic $J^{PC}$ hybrid mesons, the $J^{PC}=1^{-+}$, $0^{+-}$ and $2^{+-}$ states are explored.
The QCD analysis of the recent experimental data (L3@LEP) of the hard exclusive $ρρ$ production in two photon collisions shows that these data can be understood as a signal for the existence of an exotic isotensor resonance with a mass around $1.5 {\rm GeV}$. We also argue that hard exclusive reactions are a powerful tool for an experimental study of exotic hybrid mesons with $J^{PC}=1^{-+}$.
We study heavy baryons with an exotic flavor quantum number formed by a heavy meson and a nucleon (Dbar-N and B-N) through a long range one pion exchange interaction. The bound state found previously in the (I,JP) =(0,1/2-) channel survives when short range interaction is included. In addition, we find a resonant state with (I,JP)=(0,3/2-) as a Feshbach resonance predominated by a heavy vector meson and a nucleon (Dbar*-N and B*-N). We find that these exotic states exist for the charm and heavier flavor region.
We present a theoretical estimate for the cross-section of exclusive $ρ^+ρ^-$ and $ρ^0ρ^0$-meson production in two photon collisions when one of the initial photons is highly virtual. We focus on the discussion of the twist 4 contributions which are related to the production of an exotic isospin 2 resonance of two $ρ$ mesons. Our analysis shows that the recent experimental data obtained by the L3 Collaboration at LEP can be understood as a signal for the existence of an exotic isotensor resonance with a mass around $1.5 {\rm GeV}$.
We present an exact calculation of $S$ and $P$ wave $QQ\bar n\bar n$ states using different standard nonrelativistic quark--quark potentials. We explore in detail the charm and bottom sectors looking for bound states that could be measured within existing facilities. Against the proliferation of four--quark states sometimes predicted in the literature, we found a small number of candidates to be stable. We analyze their properties in a trial to distinguish between compact and molecular states. Possible decay modes are discussed.
Masses for vector 1--, axialvector 1+-, 1++, and exotic-vector 1-+ mesons are calculated within a quantum field theoretic framework based on the Dyson-Schwinger equations using a model form of the quark-antiquark interaction that is separable. The model provides an excellent description of pion and K-meson observables and the flavor-octet ground-state meson spectrum. With no adjustments to model parameters, a numerical solution of the Bethe-Salpeter equation yields two exotic 1-+ mesons with masses of 1439 and 1498 MeV.
Exotic hybrid mesons H, with quantum numbers J^{PC}= 1^{- +} may be copiously produced in the hard exclusive processes gamma^*(Q^2) gamma -> H and gamma^*(Q^2) P(p) -> H P(p') because they have a leading twist distribution amplitude with a sizable coupling constant f_H, which may be estimated through QCD sum rules. The reaction rates scale in the same way as the corresponding rates for usual mesons.
We study heavy hadron spectroscopy near open bottom thresholds. We employ B and B* mesons as effective degrees of freedom near the thresholds, and consider meson exchange potentials between them. All possible composite states which can be constructed from the B and B* mesons are studied up to the total angular momentum J = 0, 1, 2. We consider, as exotic states, isosinglet states with exotic (J,PC) quantum numbers and isotriplet states. We solve numerically the Schrodinger equation with channel-couplings for each state. The masses of twin resonances Zb(10608) and Zb(10653) recently found by Be…
We demonstrate that the azimuthal dependence of central meson production in hadronic collisions, when suitably binned, provides unambiguous tests of whether the Pomeron couples like a conserved vector-current to protons. We discuss the possibility of discriminating between q-qbar and glueball production in such processes. Our predictions apply also to meson production in tagged two-photon events at electron--positron colliders and to vector-meson production in ep collisions at HERA.
Hybrid mesons with exotic quantum numbers $J^{PC}=1^{-+}$ are examined in soft-wall AdS/QCD. The predicted mass spectrum is compared to the measured values of the candidates $π_1(1400)$, $π_1(1600)$ and $π_1(2015)$. Thermal effects are analysed through the spectral function in the AdS-Black Hole model, and the differences with the Hawking-Page description are discussed.
Exotic mesons with quantum numbers of a hybrid ($\bar{q} q g$) such as $J^{PC} = 1^{-+}$are shown to exist as narrow resonant states in the large $N_c$ limit of QCD. The width of these states is proportional to $1/N_c$. Possible phenomenological implications for the world of $N_c=3$ are discussed.
I briefly review the history of exotic meson (4-quark) and baryon (5-quark) states, which is rooted in the formalism of Regge pole and duality. There are robust model-independent predictions for the exchange of 4-quark (Baryonium) Regge trajectories in several processes, which are strongly supported by experiment. On the other hand the predictions for the spectroscopy of 4-quark resonances are based on specific QCD inspired models, with some experimental support. The corresponding predictions for the recently discovered exotic baryon (Pentaquark) state are briefly discussed.
We study exotic mesons with double charm and bottom flavor, whose quark configuration is \bar{Q}\bar{Q}qq. This quark configuration has no annihilation process of quark and antiquark, and hence is a genuinely exotic states. We take a hadronic picture by considering the molecular states composed of a pair of heavy mesons, such as DD, DD* and D*D* for charm flavor, and BB, BB* and B*B* for bottom flavor. The interactions between heavy mesons are derived from the heavy quark effective theory. All molecular states are classified by I(J^P) quantum numbers, and are systematically studied up to the t…
We analyze the role of final state interactions in decay of the lighest exotic meson, $pi_1$ with $J^{PC}=1^{-+}. We use the relativistic Lippmann-Schwinger equation for two coupled $πb_{1}$ and $πρ$ channels. The first one is the predicted dominant decay mode of the $π_{1}$, whereas in the other a narrow $π_1(1600)$ exotic signal has been reported by the E852 collaboration. The FSI potential is constructed, based on the $ω$ meson exchange between the two channels. We find that this process introduces corrections to the $π_{1}$ widths of the order of only a few MeV. Therefore, we conclude that…
The coupling of a neutral hybrid {1,3,5...}^-+ exotic particle (or current) to two neutral (hybrid) meson particles with the same J^PC and J=0 is proved to be sub-leading to the usual large-N_c QCD counting. The coupling of the same exotic particle to certain two - (hybrid) meson currents with the same J^PC and J=0 is also sub-leading. The decay of a {1,3,5...}^-+ hybrid to eta pi^0, eta' pi^0, eta' eta, eta(1295) pi^0, pi(1300)^0 pi0, eta(1440) pi^0, a_0(980)^0 sigma or f_0(980) sigma is sub-leading, assuming that these final state particles are (hybrid) mesons in the limit of large N_c.
The color anti-triplet triquark $q\bar{q}\bar{q}$ is considered as a compact component in the tetraquark structure $cq\bar{q}\bar{q}$ of exotic $D_{s}$ mesons. We discuss the mass spectrum and the flavor mixing of the triquarks by using the instanton induced interaction and the one-gluon exchange potentials. As a characteristic property of the triquark, we investigate the isospin violation. It is shown that the flavor $\bar{\bf 3}$ (isosinglet) and $\bf 6$ (isotriplet) states may be strongly mixed and then are identified with $D_{s}(2632)$.
This work analyses hadronic decays of exotic mesons, with a focus on the lightest one, the $J^{PC}=1^{-+}$ $π_{1}$, in a fully relativistic formalism, and makes comparisons with non-relativistic results. We also discuss Coulomb gauge decays of normal mesons that proceed through their hybrid components. The relativistic spin wave functions of mesons and hybrids are constructed based on unitary representations of the Lorentz group. The radial wave functions are obtained from phenomenological considerations of the mass operator. Fully relativistic results (with Wigner rotations) differ significan…
The light-mass exotics with J^{PC}=1^{-+} observed at BNL and CERN may have a simple explanation as dynamically generated resonances in eta-prime pi rescattering in the final state interaction. This dynamics is mediated by the same anomalous glue which also generates the large mass of the eta-prime. OZI violating processes are also potentially important to eta-prime production in proton-proton collisions close to threshold.
In this article a systematic quantitative analysis of the isoscalar bosonic states is performed in the framework of supersymmetric light front holographic QCD. It is shown that the spectroscopy of the $η$ and $h$ mesons can be well described if one additional mass parameter -- which corresponds to the hard breaking of chiral $U(1)$ symmetry in standard QCD -- is introduced. The mass difference of the $η$ and $η'$ isoscalar mesons is then determined by the strange quark mass content of the $η'$. The theory also predicts the existence of isoscalar tetraquarks which are bound states of diquarks a…
