Hybrid Mesons Research Articles

On some models of the exotic hadron states

There are several kinds of exotic hadron states. In this paper, we are concerned with the flavor exotic meson states: the tetraquarks and meson molecules. Two distinct models of tetraquarks are studied: diquark-antidiquark bound states and uncorrelated quarks. Although diquark is not a color singlet, there is a strong theoretical evidence that such states do exist within hadrons. Both tetraquark models as well as methods used to study such states are reviewed. The meson-meson molecules are also mentioned. It is remarked that hadron spectroscopy per se may not always be sufficient to discern between physically distinct models.

Heavy quark spin in an S-wave meson-antimeson state

The newly observed exotic mesons above [Formula: see text] threshold were widely discussed in the molecular picture. To understand deeper their structures, we here discuss the spin ([Formula: see text]) of a heavy quark-antiquark pair in an S-wave meson-antimeson system by constructing explicitly the spin wave function. One finds two selection rules for [Formula: see text] (a) the total angular momentum J is larger than the maximum angular momentum of the light degree of freedom or smaller than the minimum one; (b) JC = 1+, 2-, 3+, ⋯ if the two mesons are different but belong to the same doublet. This feature may be used to constrain possible strong decay channels.

Constituent quark model study of exotic meson states

Using high precision numerical method based on Gaussian expansion method for few-body systems, we studied the spectra of possible exotic meson states composed of [Formula: see text] (Q = c, b, and q = u, d, s) in non-relativistic constituent quark model. Some newly released experimental data by the BES, Belle, BaBar Collaboration can be interpreted in our calculation. Several possible exotic states are also predicted.

Gluonic excitations in the hadronic spectrum

QCD at low energy features a gluonic field that is strongly coupled to itself and to quarks. I will present a summary of what we know about the role that excitations of the gluonic field play in determining the spectrum of meson resonances. Recent studies using lattice techniques have suggested a phenomenology of gluonic excitations within QCD that leads to hybrid mesons with both exotic and non-exotic quantum numbers. I will discuss these calculations and describe their relationship to current experimental knowledge and to forthcoming experiments at Jefferson Lab and elsewhere.

Toward the excited isoscalar meson spectrum from lattice QCD

We report on the extraction of an excited spectrum of isoscalar mesons using lattice QCD. Calculations on several lattice volumes are performed with a range of light quark masses corresponding to pion masses down to about 400 MeV. The distillation method enables us to evaluate the required disconnected contributions with high statistical precision for a large number of meson interpolating fields. We find relatively little mixing between light and strange in most JPC channels; one notable exception is the pseudoscalar sector where the approximate SU(3)F octet, singlet structure of the {\eta}, {\eta}' is reproduced. We extract exotic JPC states, identified as hybrid mesons in which an excited gluonic field is coupled to a color-octet qqbar pair, along with non-exotic hybrid mesons embedded in a qqbar-like spectrum.

Exploring theD*ρsystem within QCD sum rules

We present a study of the ${D}^{*}\ensuremath{\rho}$ system made by using the method of QCD sum rules to determine the mass of possible resonances generated in the same system. Using isospin and spin projectors, we investigate the different configurations and obtain evidences for three ${D}^{*}$ mesons with isospin $I=1/2$, spin $S=0$, 1, 2 and with masses $2500\ifmmode\pm\else\textpm\fi{}67$, $2523\ifmmode\pm\else\textpm\fi{}60$, and $2439\ifmmode\pm\else\textpm\fi{}119\text{ }\text{ }\mathrm{MeV}$, respectively. The last state can be associated with ${D}_{2}^{*}(2460)$ (spin 2) listed by the Particle Data Group, while one of the first two might be related to ${D}^{*}(2640)$, with unknown spin parity. In the case of $I=3/2$ we also find evidences of three states with spin 0, 1, and 2, respectively, with masses $2467\ifmmode\pm\else\textpm\fi{}82$, $2420\ifmmode\pm\else\textpm\fi{}128$, and $2550\ifmmode\pm\else\textpm\fi{}56\text{ }\text{ }\mathrm{MeV}$. The results for the sector $I=1/2$ and $S=0$, 1, 2, are intriguingly similar to a previous study of the ${D}^{*}\ensuremath{\rho}$ system based on effective field theories, supporting in this way a molecular picture for the resonances ${D}^{*}(2640)$ and ${D}_{2}^{*}(2460)$, while the results for $I=3/2$ hint towards the existence of exotic mesons since a multiquark configuration is required to get the quantum numbers of the states found.

EXPERIMENTAL REVIEW OF STRUCTURES IN THE J/ψϕ MASS SPECTRUM

The discovery of numerous new charmonium-like structures since 2003 have revitalized interest in exotic meson spectroscopy. These structures do not fit easily into the conventional charmonium model, and proposals like four-quark states, hybrids and rescattering effects have been suggested as explanations. Since 2009, several new structures were reported in the J/ψϕ mass spectrum with the following characteristics: they are the first ones reported decaying into two heavy mesons which contain both a [Formula: see text] pair and a [Formula: see text] pair; and their masses are well beyond the open charm pair threshold. Conventional [Formula: see text] states with a mass beyond the J/ψϕ threshold are not expected to decay into this channel and the width is expected to be large, thus they are good candidates for exotic mesons. The focus of this paper is to review the recent developments on the structures in the J/ψϕ mass spectrum from CDF, Belle and LHCb.

Tetraquark Mesons in Large-NQuantum Chromodynamics

It is argued that exotic mesons consisting of two quarks and two antiquarks are not ruled out in quantum chromodynamics with a large number N of colors, as generally thought. Tetraquarks of one class are typically long-lived, with decay rates proportional to 1/N.

Excited spectroscopy of charmed mesons from lattice QCD

We present spectra of highly excited D and Ds mesons up to around 3.8 GeV determined using dynamical lattice QCD. We employ novel computational techniques and the variational method with a large basis of carefully constructed operators in order to extract and reliably identify the continuum spin of an extensive set of excited states. These include states with high spin and states identified as having an explicit gluonic contribution. Calculations were performed on two volumes, both with a pion mass of approximately 400 MeV, achieving a high statistical precision for both ground and excited states. We discuss our results in light of experimental observations, comment on the phenomenological implications and identify the lightest `supermultiplet' of hybrid mesons in each sector.

Exotic Mesons with Hidden Bottom Near Thresholds

We discuss exotic meson spectroscopy near open bottom thresholds. Assuming the exotic mesons as \({B^{(\ast)}\bar{B}^{(\ast)}}\) molecular states, we study the interaction among two heavy mesons in terms of the one boson exchange potential model. It is shown that masses of Z b (10610) and Z b (10650) are reproduced as \({B^{(\ast)}\bar{B}^{(\ast)}}\) bound and resonance states. Besides, we also show that \({B^{(\ast)}\bar{B}^{(\ast)}}\) molecular states having various exotic quantum numbers can exist around the thresholds. By contrast, there are no \({D^{(\ast)}\bar{D}^{(\ast)}}\) molecular states having exotic quantum numbers.

Exotic mesons with double charm and bottom flavor

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 total angular momentum J \leq 2. By solving the coupled channel Schrodinger equations, due to the strong tensor force of one pion exchanging, we find bound and/or resonant states of various quantum numbers.

Investigating photoproduction of scalar mesons at medium energies

In this letter we study the photoproduction of scalar mesons in the intermediate energies considering distinct mixing scenarios in the description of meson physical states. The differential and integrated total cross section are computed for the cases of the mesons a_0(980), f_0(1500) and f_0(1710) focusing the GlueX energy regime with photon energy equal to 9 GeV. Our results indicate that light-quark scalar meson photoproduction is well suited for studying hybrid mesons structure.

Exotic mesons with hidden bottom near thresholds

We study heavy hadron spectroscopy near open bottom thresholds. We employ $\mathrm{B}$ and ${\mathrm{B}}^{*}$ 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 $\mathrm{B}$ and ${\mathrm{B}}^{*}$ mesons are studied up to the total angular momentum $J\ensuremath{\le}2$. We consider, as exotic states, isosinglet states with exotic ${J}^{PC}$ quantum numbers and isotriplet states. We solve numerically the Schr\"odinger equation with channel couplings for each state. The masses of twin resonances ${\mathrm{Z}}_{\mathrm{b}}(10\text{ }610)$ and ${\mathrm{Z}}_{\mathrm{b}}(10\text{ }650)$ recently found by Belle are reproduced. We predict several possible bound and/or resonant states in other channels for future experiments.

Excited and exotic charmonium spectroscopy from lattice QCD

We present a spectrum of highly excited charmonium mesons up to around 4.5 GeV calculated using dynamical lattice QCD. Employing novel computational techniques and the variational method with a large basis of carefully constructed operators, we extract and reliably identify the continuum spin of an extensive set of excited states, states with exotic quantum numbers (0+-, 1-+, 2+-) and states with high spin. Calculations are performed on two lattice volumes with pion mass approximately 400 MeV and the mass determinations have high statistical precision even for excited states. We discuss the results in light of experimental observations, identify the lightest 'supermultiplet' of hybrid mesons and comment on the phenomenological implications of the spectrum of exotic mesons.

Hybrid baryons in QCD

We present the first comprehensive study of hybrid baryons using lattice QCD methods. Using a large basis of composite QCD interpolating fields we extract an extensive spectrum of baryon states and isolate those of hybrid character using their relatively large overlap onto operators which sample gluonic excitations. We consider the spectrum of Nucleon and Delta states at several quark masses finding a set of positive parity hybrid baryons with quantum numbers N_1/2+, N_1/2+, N_3/2+, N_3/2+, N_5/2+ and Delta_1/2+, Delta_3/2+ at an energy scale above the first band of `conventional' excited positive parity baryons. This pattern of states is compatible with a color octet gluonic excitation having J^P=1+ as previously reported in the hybrid meson sector and with a comparable energy scale for the excitation, suggesting a common bound-state construction for hybrid mesons and baryons.

Investigation of rainbow-ladder truncation for excited and exotic mesons

Ground-state, radially excited, and exotic scalar, vector, and flavored-pseudoscalar mesons are studied in rainbow-ladder truncation using an interaction kernel that is consonant with modern Dyson-Schwinger equation and lattice-QCD results. The inability of this truncation to provide realistic predictions for the masses of excited and exotic states is confirmed and explained. However, its application does provide information that is potentially useful when working beyond this leading-order truncation, e.g.: assisting with the development of projection techniques that ease the computation of excited-state properties; placing qualitative constraints on the long-range behavior of the interaction kernel; and highlighting and illustrating some features of hadron observables that do not depend on details of the dynamics.

Dynamical implications of gluonic excitations in meson-meson systems

We study meson-meson interactions using an extended $q^2\bar{q}^2(g)$ basis that allows calculating coupling of an ordinary meson-meson system to a hybrid-hybrid one. We use a potential model matrix in this extended basis which at quark level is known to provide a good fit to numerical simulations of a $q^2\bar{q}^2$ system in pure gluonic theory for static quarks in a selection of geometries. We use a combination of resonating group method formalism and Born approximation to include the quark motion using wave functions of a $q\bar{q}$ potential within a cluster. This potential is taken to be quadratic for ground states and has an additional smeared $\frac{1}{r}$ (Gaussian) for the matrix elements between hybrid mesons. For the parameters of this potential, we use values chosen to 1) minimize the error resulting from our use of a quadratic potential and 2) best fit the lattice data for differences of $\Sigma_{g}$ and $\Pi_{u}$ configurations of the gluonic field between a quark and an antiquark. At the quark (static) level, including the gluonic excitations was noted to partially replace the need for introducing many-body terms in a multi-quark potential. We study how successful such a replacement is at the (dynamical) hadronic level of relevance to actual hard experiments. Thus we study effects of both gluonic excitations and many-body terms on mesonic transition amplitudes and the energy shifts resulting from the second order perturbation theory (i.e. from the respective hadron loops). The study suggests introducing both energy and orbital excitations in wave functions of scalar mesons that are modelled as meson-meson molecules or are supposed to have a meson-meson component in their wave functions.

The quintessential exotic X(3872)

The experimental evidence for the exotic meson, X(3872), is summarized and the various proposed theoretical models for it are reviewed. It is pointed out that several important experimental measurements have marginal statistical significance and disagreements between different sources. Until these problems are resolved, the understanding of the structure of X(3872) is likely to remain controversial.

GlueX: Photoproduction of Hybrid Mesons

The goal of the GlueX experiment is to provide critical data to help understand the soft gluonic field responsible for binding quarks in hadrons. Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. Photoproduction is expected to be particularly effective in producing exotic hybrids but there is little data on the photoproduction of light mesons. GlueX will use the new 12-GeV electron beam to produce a 9-GeV beam of linearly polarized photons using the technique of coherent bremsstrahlung. A solenoid-based hermetic detector is under construction, which will be used to collect data on meson production and decays. These data will also be used to study the spectrum of conventional mesons, including the poorly understood excited vector mesons. This talk will describe the latest theoretical developments to help understand how the data of hybrid mesons can provide insights into the fundamental theory of strong interactions.

The Strong Decay Properties of the 1−+Hybrid Mesons

We calculate the coupling constants of the decay modes 1 + ! ,f1 , b1 , , 0 , a1 , f1 within the framework of the light-cone QCD sum rule. Then we cal- culate the partial widths of these decay channels, which di er greatly from the existing calculations using phenomenological models. For the isovector 1 + state, the dominant decay modes are ; f1 . For its isoscalar partner, its dominant decay mode is a1 . We also discuss the possible search of the 1 + state at BESIII, for example through the decay chains J= ( 0 )! 1 + or J= ( 0 )! 1 + where 1 can be reconstructed through the decay modes 1 ! ! + 0 or 1 ! f1(1285) 0 . Hopefully the present work will be helpful to the experimental establishment of the 1 + hybrid meson.