Hybrid Mesons Research Articles

Light hybrid meson mixing and phenomenology

A simple constituent model of gluodynamics that is motivated by lattice field theory and the QCD Hamiltonian in Coulomb gauge is applied to descriptions of hybrid meson flavor mixing and vector hybrid configuration mixing. Good agreement with lattice gauge computations is obtained for flavor multiplet masses, while mixing angles are in approximate agreement, given large errors. The configuration mixing results are also in rough agreement with lattice NRQCD calculations. Thus the viability of constituent gluon models of hybrid hadrons and glueballs is supported. The results suggest that a flavor multiplet of vector hybrids should appear with masses of approximately 2100, 2200, and 2300 MeV and that the isovector vector hybrid decay constant is about 20 MeV. Similarly, the ${\ensuremath{\pi}}_{1}$ exotic hybrid should have isospin partner states near 1760 and 1900 MeV, and it is suggested that the recently seen ${\ensuremath{\eta}}_{1}$ hybrid signal is the latter state.

1−+ hybrid meson in J/ψ radiative decays from lattice QCD

We present the first theoretical prediction of the production rate of $1^{-+}$ light hybrid meson $\eta_1$ in $J/\psi$ radiative decays. In the $N_f=2$ lattice QCD formalism with the pion mass $m_\pi\approx 350$ MeV, the related electromagnetic multipole form factors are extracted from the three-point functions that involve necessarily quark annihilation diagrams, which are calculated through the distillation method. The partial width of $J/\psi\to \gamma \eta_1$ is determined to be $2.29(77)~\mathrm{eV}$ at the $\eta_1$ mass $m_{\eta_1}=2.23(4)$ GeV. If $\eta_1$ corresponds to the recently observed $\eta_1(1855)$ in the process $J/\psi\to \gamma\eta_1(1855)\to \gamma \eta\eta'$ by BESIII, then the branching fraction $\mathrm{Br}(J/\psi\to \gamma\eta_1(1855))$ is estimated to be $6.2(2.2)\times 10^{-5}$, which implies $\mathrm{Br}(\eta_1(1855)\to \eta\eta')\sim 4.3\%$.

On the η1(1855), π1(1400) and π1(1600) as Dynamically Generated States and Their SU(3) Partners

In this work, we interpret the newly observed η1(1855) resonance with exotic JPC=1−+ quantum numbers in the I=0 sector, reported by the BESIII Collaboration, as a dynamically generated state from the interaction between the lightest pseudoscalar mesons and axial-vector mesons. The interaction is derived from the lowest order chiral Lagrangian from which the Weinberg–Tomozawa term is obtained, describing the transition amplitudes among the relevant channels, which are then unitarized using the Bethe–Salpeter equation, according to the chiral unitary approach. We evaluate the η1(1855) decays into the ηη′ and KK¯*π channels and find that the latter has a larger branching fraction. We also investigate its SU(3) partners, and according to our findings, the π1(1400) and π1(1600) structures may correspond to dynamically generated states, with the former one coupled mostly to the b1π component and the latter one coupled to the K1(1270)K¯ channel. In particular, our result for the ratio Γ(π1(1600)→f1(1285)π)/Γ(π1(1600)→η′π) is consistent with the measured value, which supports our interpretation for the higher π1 state. We also report two poles with a mass about 1.7 GeV in the I=1/2 sector, which may be responsible for the K*(1680). We suggest searching for two additional η1 exotic mesons with masses around 1.4 and 1.7 GeV. In particular, the predicted η1(1700) is expected to have a width around 0.1 GeV and can decay easily into KK¯ππ.

Heavy-light hybrid mesons with different spin-parities

The spectroscopic parameters of the heavy-light hybrid mesons with different spin-parities and different quark contents are investigated in the framework of the QCD sum rule method. The mass and current coupling of these states are calculated by taking into account the quark, gluon and mixed vacuum condensates up to dimension 10. The obtained results are compared with the existing QCD Laplace sum rule predictions. The results of mass and current coupling for all the considered channels are obtained to be stable and reliable. Our results can be useful for future experimental searches as well as theoretical studies of different parameters related to the hybrid mesons and their decays and interactions with other particles.

Initial performance of the GlueX DIRC detector

The GlueX experiment at Jefferson Laboratory aims to perform quantitative tests of non-perturbative QCD by studying the spectrum of light-quark mesons and baryons. A Detector of Internally Reflected Cherenkov light (DIRC) was installed to enhance the particle identification (PID) capability of the GlueX experiment by providing clean π/K separation up to 3.7 GeV/c momentum in the forward region (θ < 11°), which will allow the study of hybrid mesons decaying into kaon final states with significantly higher efficiency and purity. The new PID system is constructed with radiators from the decommissioned BaBar DIRC counter, combined with new compact photon cameras based on the SuperB FDIRC concept. The full system was successfully installed and commissioned with beam during 2019/2020. The initial PID performance of the system was evaluated and compared to one from Geant4 simulation.

Monopoles, exotic states and muon g-2 in TeV scale trinification

We study the low energy implications of a trinification model based on the gauge symmetry G= SU(3)_c times SU(3)_L times SU(3)_R, without imposing gauge coupling unification. A minimal model requires two Higgs multiplets that reside in the bi-fundamental representation of G, and this is shown to be adequate for accommodating the Standard Model (SM) fermion masses and generate, via loop corrections and seesaw mechanism, suitable masses for the heavy neutral leptons as well as the observed SM neutrinos. We estimate a lower bound of around 15 TeV for the masses of the new down- type quarks that are required by the SU(3)_L times SU(3)_R symmetry. We examine the resonant production at the LHC of the new gauge bosons, which leads to a lower bound of 16 TeV for the symmetry breaking scale of G. We also show how the muon g-2 anomaly can be resolved in the presence of these new gauge bosons and the heavy charged leptons present in the model. Finally, the model predicts the presence of a topologically stable monopole carrying three quanta (6 pi /e) of Dirac magnetic charge and mass gtrsim 160 TeV. If new matter fields lying in the fundamental representations of G are included, the model predicts the presence of exotic leptons, mesons and baryons carrying fractional electric charges such as pm e/3 and pm 2e/3, fully compatible with the Dirac quantization condition.

Next-to-leading order (NLO) perturbative effects in QCD sum-rule analyses of light tetraquark systems: A case study in the scalar-isoscalar channel

QCD sum-rule mass predictions for tetraquark states provide insights on the interpretations and internal structure of experimentally-observed exotic mesons. However, the overwhelming majority of tetraquark QCD sum-rule analyses have been performed at leading order (LO), which raises questions about the underlying theoretical uncertainties from higher-loop corrections. The impact of next-to-leading order (NLO) perturbative effects are systematically examined in scalar (JPC=0++) isoscalar light-quark tetraquark systems where comprehensive LO sum-rule analyses have been performed and NLO perturbative corrections to the correlators have previously been calculated. Using the scalar-isoscalar state as a detailed case study to illustrate the differences between LO and NLO analyses, it is shown that NLO effects in individual Laplace sum-rules are numerically significant and have an important role in improving the reliability of the sum-rule analyses by widening the Borel window. However, ratios of sum-rules are found to be less sensitive to NLO effects with the additional advantage of cancelling the anomalous dimension that emerges from the NLO corrections. NLO mass predictions based on these sum-rule ratios are thus remarkably robust despite the slow perturbative convergence of the underlying correlator. The mass predictions 0.52GeV<mσ<0.77GeV for the lightest scalar-isoscalar σ state are in good agreement with the four-quark interpretation of the f0(500), and the relative coupling strengths of the f0(980) and f0(500) to the tetraquark current agree with the pattern found in chiral Lagrangian analyses. Effects of the σ resonance width are studied for different models, including resonance shapes inspired by chiral Lagrangians.

The low-lying hidden- and double-charm tetraquark states in a constituent quark model with instanton-induced interaction

Spectrum of the low-lying hidden- and double-charm tetraquark states are investigated in a nonrelativistic quark potential model, where the instanton-induced interaction is taken as the residual spin-dependent hyperfine interaction between quarks. The model parameters are fixed by fitting the spectrum of the ground hadron states. Our numerical results show that masses of several presently studied tetraquark states are close to those of the experimentally observed candidates of exotic meson, which indicates that the corresponding compact tetraquark components may take considerable probabilities in those observed exotic states.

Amplitude analysis of ηπ final states at glueX

The primary goal of the GlueX experiment at Jefferson Lab is to map the spectrum of light hybrid mesons. GlueX, which has a linearly polarized photon beam and a large acceptance for both charged and neutral particles, has access to both the neutral, γp → ηπ0 p, and charged, γp → ηπ−∆++, final states. These proceedings will discuss the amplitude analysis of ηπ channels at GlueX with a focus on the study of the production of the a2(1320) meson as function of Mandelstam t.

Accessing glue through photoproduction measurements at GlueX

&#x0D; &#x0D; &#x0D; Photoproduction experiments are a key tool in the investigation of the spectrum of hadronic states and the way gluons contribute to this spectrum. The GlueX experiment, located at Jefferson Lab, features a linearly polarized tagged photon beam and its detector system is optimized to measure a wide range of neutral and charged final states. GlueX offers unique capabilities to study the spectrum of hadrons and is dedicated to the search for hybrid mesons, states with gluonic degrees of freedom. This talk presents first results from our initial campaign of data taking which finished in 2018.&#x0D; &#x0D; &#x0D;

Photo- and hadron-production of mesons

We discuss the πη(‘) production in the double Regge model. We also identify the dominant exchanges necessary to describe the πη(‘) system for energies above the resonance region. The model explains the forward-backward asymmetry observed in this reaction by associating it with the exotic P−wave, which in turn is related to the production of the putative π1 hybrid meson. We also discuss the π +π − production through two complementary mechanisms - direct resonance production (with subsequent π +π − decay) and the Deck mechanism. The interference of the Deck and direct production explains the di-pion mass distribution for invariant masses below 2 GeV. Model predictions are compatible with the qq¯ nature of the lightest P− and D−wave resonances, whereas they hint towards substantial molecular or tetraquark component of the f0(980).

Mass predictions of vector ( 1−− ) double-gluon heavy quarkonium hybrids from QCD sum rules

In this work, we study the ${1}^{\ensuremath{-}\ensuremath{-}}$ double-gluon charmonium ($\overline{c}ggc$) and bottomonium ($\overline{b}ggb$) hybrids in terms of QCD sum rules. We find that the mass of $\overline{c}ggc$ hybrid lies in ${M}_{{H}_{c}}=5.33--5.90\text{ }\text{ }\mathrm{GeV}$, while in the bottom sector the mass of $\overline{b}ggb$ hybrid may be situated in ${M}_{{H}_{b}}=11.20--11.68\text{ }\text{ }\mathrm{GeV}$. The contributions up to dimension eight at leading order of ${\ensuremath{\alpha}}_{s}$ (LO) in the operator product expansion are taken into account in the calculation. The double-gluon charmonium hybrid meson predicted in this work can decay into a pair of charmed mesons or a pair of charmed mesons together with a light meson. Especially, we propose to search for $\overline{c}ggc$ hybrid with ${I}^{G}({J}^{PC})={0}^{\ensuremath{-}}({1}^{\ensuremath{-}\ensuremath{-}})$ in their decay channels $D\overline{D}/{D}^{*}\overline{D}/{D}^{*}{\overline{D}}^{*}$ with P wave and ${D}^{*}{\overline{D}}^{*}\ensuremath{\pi}/{D}^{*}{\overline{D}}^{*}\ensuremath{\eta}/D\overline{D}\ensuremath{\rho}/D\overline{D}\ensuremath{\omega}$ with S wave, which may be accessible in Belle II, PANDA, Super-B, GlueX, and LHCb experiments.

Masses of vector and pseudovector hybrid mesons in a chiral symmetric model

We enlarge the chiral model, the so-called extended Linear Sigma Model (eLSM), by including the low-lying hybrid nonet with exotic quantum numbers J^{PC}=1^{-+}JPC=1−+ and the nonet of their chiral partners with J^{PC}=1^{+-}JPC=1+− to a global U(3)_r \times U(3)_lU(3)r×U(3)l chiral symmetry. We use the assignment of the \pi_1^{hyb}= \pi_1(1600)π1hyb=π1(1600) as input to determine the unknown parameters. Then, we compute the lightest vector and pseudovector hybrid masses that could guide ongoing and upcoming experiments in searching for hybrids.

Impact of a thermal medium on newly observed Z_{cs}(3985) resonance and its b-partner

Motivated by the very recent discovery of the strange hidden-charm exotic state Z_{cs}(3985) by the BESIII Collaboration, we study the possible interpretation of this exotic state the both at T=0 and Tne 0 . We analytically compute the mass and meson-current coupling constant of this resonance with spin-parity J^{P} = 1^+ at finite temperature approximation up to the sixth order of the thermal operator dimension including non-perturbative contributions. Extracting thermal mass and meson-current coupling constant sum rules, the modifications on properties of the Z_{cs}(3985) state in hot medium are determined. As a by-product, the hadronic parameters of the bottom partner of Z_{cs}(3985) is estimated as well. The search for temperature effects on the hadronic parameters of the hidden-charm meson Z_{cs}(3985) and the bottom partner enable us to understand the phase transitions, chiral symmetry breaking, and the properties of hot-dense matter in QCD. Moreover, the full width of the resonance Z_{cs}(3985) is calculated as (12.0pm 0.8)~{mathrm {MeV}} using the strong decay in the tetraquark picture. Results for width and mass are in reasonable agreement with existing experimental data and results of other theoretical works. The information obtained about the parameters of the considered states is useful for experimental investigations of exotic mesons.

QCD Axial Anomaly Enhances the ηη′ Decay of the Hybrid Candidate η 1(1855)

We study the hybrid mesons with the exotic quantum number I G J PC = 0+1−+ and investigate their decays into the η η′, a 1(1260) π, f 1(1285) η, f 1(1420) η, , , and channels. It is found that the QCD axial anomaly enhances the decay width of the η η′ channel although this mode is strongly suppressed by the small p-wave phase space. Our results support the interpretation of the η 1(1855) recently observed by BESIII as the hybrid meson of I G J PC = 0+1−+. The QCD axial anomaly ensures the η η′ decay mode to be a characteristic signal of the hybrid nature of the η 1(1855).

Mass of 1−+ four-quark–hybrid mixed states

We calculate the masses of $J^{PC}=1^{-+}$ light exotic mesons by QCD sum rules; the masses are extracted from four-quark--hybrid mixing correlation functions. We construct several $1^{-+}$ four-quark currents and hybrid currents, and get two masses around $1.2\text{-}1.4\text{GeV}$ and $1.45\text{-}1.67\text{GeV}$; they can be identified as $\pi_1(1400)$ and $\pi_1(1600)$.

Masses, Radii and Regge Trajectories of $$\Sigma _{u}^{-}$$ State Hybrid Charmonium

In this paper, masses, radii and $$E_1$$ radiative transitions of $$\Sigma ^-_u$$ state hybrid charmonium mesons for radial and orbital excitations are calculated by numerically solving the Schrödinger equation with non-relativistic potential model. Results for calculated masses of $$\Sigma ^-_{u}$$ states charmonium hybrid mesons are found to be close to the results obtained through lattice simulations. Calculated masses are used to construct Regge trajectories. It is found that the trajectories are almost linear and parallel.

Hybrid static potentials in SU(3) lattice gauge theory at small quark-antiquark separations

We compute the ${\mathrm{\ensuremath{\Pi}}}_{u}$ and ${\mathrm{\ensuremath{\Sigma}}}_{u}^{\ensuremath{-}}$ hybrid static potentials in SU(3) lattice gauge theory using four different lattice spacings ranging from $a=0.040\text{ }\text{ }\mathrm{fm}$ to $a=0.093\text{ }\text{ }\mathrm{fm}$. We provide lattice data points for quark-antiquark separations as small as 0.08 fm, where the $a$-dependent self-energy as well as lattice discretization errors at tree level of perturbation theory and at leading order in ${a}^{2}$ have been removed. We also investigate and exclude possibly present systematic errors from topological freezing, due to the finite spatial lattice volume and from glueball decays. Moreover, we provide corresponding parametrizations of the potentials, which can e.g. be used for Born-Oppenheimer predictions of heavy hybrid mesons.

From the line shape of the X(3872) to its structure

From a study of the lineshape of the $X(3872)$, the LHCb collaboration measures a sizeable negative effective range. This cannot be reconciled with a shallow $D\bar{D}^*$ bound state hypothesis. Based on Weinberg's compositeness criterion, together with a theorem by Smorodinsky, it follows that the $X$ has to have a compact hidden charm structure, most likely a tetraquark, interacting with unbound $D\bar{D}^*$ pairs via short-distance color forces. This conclusion is strengthened by the general pattern recently emerging for exotic mesons.

Study of light tetraquark spectroscopy

We calculate the masses of the $qq\overline{q}\overline{q}$ tetraquark ground state and first radial excited state in a constituent quark model where the Cornell-like potential and one-gluon exchange spin-spin coupling are employed. The three coupling parameters for the Cornell-like potential and one-gluon exchange spin-spin coupling are proposed mass-dependent in accordance with lattice QCD data, and all model parameters are predetermined by studying light, charmed, and bottom mesons. The theoretical predictions for light tetraquarks are compared with the observed exotic meson states in the light-unflavored meson sector, and tentative assignments are suggested. The work suggests that ${f}_{0}(1500)$ and ${f}_{0}(1710)$ might be ground light tetraquark states with $J=0$.