Hybrid Mesons Research Articles

Exotic hybrid pseudopotentials at finite temperature and chemical potential

Abstract Using gauge/gravity duality, we study the exotic hybrid pseudopotentials at finite temperature and chemical potential. The $\Sigma$ hybrid meson can be described by a model including an object called ``defect'' on a string linking the quark and antiquark. It was first proposed by Andreev and perfectly described the $\Sigma_u^-$ hybrid potential at zero temperature and chemical potential. In this paper, we would like to extend this model to finite chemical potential and compare the separate distance and pseudopotentials of $\Sigma_g^+ $ and $\Sigma_u^-$. Unlike $\Sigma_g^+$ ground state, the $\Sigma_u^-$ hybrid pseudopotentials no longer behave as Coulomb-like at short distances. In addition, temperature and chemical potential have a significant impact on the $\Sigma_u^-$ hybrid pseudopotentials. The screen distances and hybrid pseudopotentials of $\Sigma_u^-$ significantly decrease with the increase of temperature and chemical potential. At last, we draw the melting diagram of $\Sigma_g^+ $ and $\Sigma_u^-$ in the $T- \mu$ plane, and confirm that the quark-antiquark pair in $\Sigma_u^-$ excited state is easier to melt than in $\Sigma_g^+$ ground state.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

Towards heavy double-gluon hybrid mesons with exotic quantum numbers in QCD sum rules

The double-gluon hybrid meson configuration was recently proposed and investigated within QCD sum rules. In this talk, we discuss the color structures of the double-gluon hybrid meson and construct current operators with exotic quantum numbers JPC=1−+ and 2+− for two of the structures. In the framework of QCD sum rules, we consider the condensates up to dimension-8 at the leading order of αs for both charmonium and the bottomonium systems. The results indicate that the masses of the 1−+ and 2+− charmonium double-gluon hybrid mesons are approximately 6.1−7.2 GeV and 6.3−6.4 GeV, respectively. As for the bottomonium systems, their masses fall within the range of 13.7−14.3 GeV and 12.6−13.3 GeV for the 1−+ and 2+− channels, respectively. Additionally, the charmonium hybrids could be produced in the radiative decays of bottomonium mesons in BelleII experiment.

Born-Oppenheimer potentials for SU(3) gauge theory

We develop parametrizations of eight of the lowest Born-Oppenheimer potentials for quarkonium hybrid mesons as functions of the separation r of the static quark and antiquark sources. The parameters are determined by fitting results calculated using pure SU(3) lattice gauge theory. The parametrizations have the correct limiting behavior at small r, where the potentials form multiplets associated with gluelumps. They have the correct limiting behavior at large r, where the potentials form multiplets associated with excitations of a relativistic string. There is a narrow avoided crossing in the small-r region between two potentials with the same Born-Oppenheimer quantum numbers. Published by the American Physical Society 2024

Meson mass and width: Deep learning approach

It is fascinating to predict the mass and width of the ordinary and exotic mesons solely based on their quark content and quantum numbers. Such prediction goes beyond conventional methodologies traditionally employed in hadron physics for calculating or estimating these quantities. The relation between the quantum numbers and the properties of the mesons, such as the mass and width, is complicated in the world of particle physics. However, the deep neural network (DNN) as a subfield of machine learning techniques provides a solution to this problem. By analyzing large datasets, deep learning algorithms can automatically identify complex patterns among the particles’ quantum numbers, and their mass and width, that would otherwise require complex calculations. In this study, we present two approaches using the DNNs to estimate the mass of some ordinary and exotic mesons. Also for the first time, the DNNs are trained to predict the width of ordinary and exotic mesons, whose widths have not been experimentally known. Our predictions obtained through the DNNs, will be useful for future experimental searches. Published by the American Physical Society 2024

Parameters of the tensor tetraquark [formula omitted]

The mass and width of the tensor tetraquark T=bbc‾c‾ with spin-parity JP=2+ are calculated in the context of the QCD sum rule method. The tetraquark T is modeled as a diquark-antidiquark state built of components bTCγμb and c‾γνCc‾T with C being the charge conjugation matrix. The mass m=(12.795±0.095)GeV of the exotic tensor meson T is found by means of the two-point sum rule approach. Its full width Γ is evaluated by considering processes T→Bc−Bc−, Bc−Bc⁎−, and Bc⁎−Bc⁎−. Partial widths of these decays are computed by means of the three-point sum rule approach which is used to determine the strong couplings at relevant tetraquark-meson-meson vertices. Predictions obtained for the width ΓT=55.5−9.9+10.6MeV, as well as the mass of the tetraquark T can be useful in investigations of fully heavy four-quark mesons.

Revisit the heavy quarkonium double-gluon hybrid mesons with exotic quantum numbers

We revisit the masses of heavy quarkonium double-gluon hybrid mesons with exotic quantum numbers JPC = 1−+ and 2+− in the framework of the QCD sum rules. Considering the double-gluon hybrid meson operators in the octet-octet color structure, we have constructed two independent interpolating currents with JPC = 1−+ and five independent currents with JPC = 2+−. For the interpolating currents with antisymmetric glueball operator, there exist non-local divergences in one kind of additional Feynman diagrams of the tri-gluon condensate, which will give important contributions to the sum rule stabilities and mass predictions. We use the diagrammatic renormalization to cancel out such divergences. At the leading order of αs, the two-point correlation functions and spectral densities can be expressed in the analytic form of the generalized hypergeometric functions and Meijer’s G-functions. After performing the numerical analysis, we predict the masses of the 1−+ and 2+− charmonium double-gluon hybrid mesons to be around 6.1 − 7.2 GeV and 6.3 − 6.4 GeV, respectively. For the bottomonium systems, their masses are predicted to be 13.7 − 14.3 GeV and 12.6 − 13.3 GeV for the 1−+ and 2+− channels, respectively. Besides, it is possible to hunt for these charmonium hybrids in the radiative decays of bottomonium mesons in BelleII experiment. Further investigations on these hybrid states in various theoretical and phenomenological methods are also anticipated in the future.

Light quarkonium hybrid mesons

We investigate the light quarkonium hybrid mesons of various spin parities in QCD. Considering different interpolating currents made of the valence light quarks and single gluon, we calculate the mass and current coupling of the strange and nonstrange members of light hybrid mesons by including into computations the nonperturbative quark and gluon condensates up to ten dimensions in order to increase the accuracy of the results. The obtained results may be useful for future experimental searches of these hypothetical states. They can also be used in the calculations of different parameters related to the decays/interactions of light hybrid mesons to/with other states. Published by the American Physical Society 2024

Mass spectra of heavy hybrid quarkonia and [formula omitted] mesons

Masses and current couplings of the charmonium and bottomonium hybrids c‾gc and b‾gb with spin-parities JPC=0++,0+−,0−+,0−− and 1++,1+−,1−+,1−− are calculated using QCD two-point sum rule method. Computations are performed by taking into account gluon condensates up to dimension 12 including terms ∼〈gs3G3〉2. The parameters of the bottom-charm hybrids b‾gc with quantum numbers JPC=0+,0−,1+, and 1− are calculated as well. In computations the dominance of the pole contribution to sum rule results is ensured. It is demonstrated that all charmonia hybrids decay strongly to two-meson final states. The bottomonium hybrids 0−+ and 1−+ as well as the bottom-charm hybrid mesons 0−(+) and 1−(+) may be stable against strong two-meson decay modes. Results of the present work are compared with ones obtained using the sum rule and alternative approaches. Our predictions for parameters of the heavy hybrid mesons may be useful to study their various decay channels which are important for interpretation of ongoing and future experiments.

Constituent model of light hybrid meson decays

A model of light hybrid mesons and their strong decays is developed. The model employs a gluonic quasiparticle to describe low energy gluodynamics and uses the QCD Hamiltonian in Coulomb gauge to guide the construction of states and decay amplitudes. We compute the partial widths of the twelve low lying isovector and vector hybrids. Implications of these results on hybrid searches are also made, with the chief conclusions being that direct observation of the vector states will be difficult, that a hybrid π(1800) has distinctive decay characteristics, a narrow η(1900) hybrid should exist, an η1(1750) should be sought, and that the exotic nature of JPC=2−+ hybrid mesons should be discernible with sufficient data. We argue that the isovector π2 hybrid has been discovered, giving a total of four possible hybrid mesons, π1(1600), η1(1855), π(1800), and π2(2360), which appear to be filling out the low lying hybrid supermultiplet in the expected fashion. Published by the American Physical Society 2024

Gluelump masses and mass splittings from SU(3) lattice gauge theory

We compute gluelump masses and mass differences using SU(3) lattice gauge theory. We study states with total angular momentum up to J=3, parity P=+,−, and charge conjugation C=+,−. Computations on four ensembles with rather fine lattice spacings in the range from 0.040 to 0.093 fm allow continuum extrapolations of gluelump mass differences. We complement existing results on hybrid static potentials with the obtained gluelump masses, which represent the limit of vanishing quark-antiquark separation. We also discuss the conversion of lattice gluelump masses to the renormalon subtracted scheme, which is e.g. important for studies of heavy hybrid mesons in the Born-Oppenheimer approximation. Published by the American Physical Society 2024

Decays of fully beauty scalar tetraquarks to BqB¯q and Bq*B¯q* mesons

Decays of the fully beauty four-quark structures X4b and T4b to B meson pairs are investigated in the framework of the QCD three-point sum rule method. We model the scalar exotic mesons X4b and T4b as diquark-antidiquark systems composed of the axial-vector and pseudoscalar diquarks, respectively. The masses m=(18540±50) MeV and m˜=(18858±50) MeV of these compounds calculated in our previous articles fix possible decay channels of these particles. In the present work, we consider their decays to BqB¯q and Bq*B¯q* (q=u, d, s, c) mesons. In the case of X4b, the mass of which is below the 2ηb threshold, these channels determine essential part of its full width Γ4b. The tetraquark T4b can decay to the pair ηbηb; therefore, partial widths of processes with B(B*) mesons in the final state permit us to refine our estimate for the full width of this particle. The predictions Γ4b=(9.6±1.1) MeV and Γ˜4bFull=(144±29) MeV obtained in this article can be used in future experimental investigations of four b-quark mesons. Published by the American Physical Society 2024

Search for Hybrid Mesons at GlueX

Recent lattice QCD calculations predict a multiplet of hybrid mesons, which are mesons with gluonic degrees of freedom. By mapping out the hybrid meson spectrum, we can gain insight into how the gluon contributes to the properties of bound states in QCD. The π1(1600) is a candidate for the lightest hybrid meson. This state has exotic quantum numbers of JPC = 1−+, which are forbidden for conventional mesons. The GlueX experiment has collected high statistics photoproduction data, which we are using to search for the π1(1600). These proceedings will summarize the search strategy for the π1(1600) at GlueX, including the most recent results in the ωππ, ηπ, and η′π final states.

Exploring Photoproduction with the GlueX Experiment

The GlueX experiment at Jefferson Lab (Newport News, VA USA) is designed to explore the spectrum of mesons up to about 3 GeV. We present results on the production of light-quark resonances with linearly polarized photons. These results enhance our understanding of photoproduction mechanisms, which is valuable in subsequent searches for exotic hybrid mesons. Measurements of the J/ψ photoproduction cross section at threshold are also presented.

Exotic Meson candidates in COMPASS data

Abstract. One of the prime goals of the COMPASS experiment at CERN is the study of the light meson spectrum, with a particular emphasis on the search for exotic states. The focus of this paper is on signals of the lightest hybrid can didate π1(1600) with spin-exotic quantum numbers JPC = 1−+ in several decay channels such as π−π+π−, rwπ−, ωπ−π0, π−π+π−η, and KSKSπ. In addition, we highlight new results for the K−π+π− final state, which indicate a supernumerary state with respect to the constituent quark model with JP = 0−.

Spectroscopy of heavy exotic mesons using lattice QCD static potentials and the Born-Oppenheimer approximation

I discuss the investigation of heavy exotic mesons using lattice QCD static potentials and the Born-Oppenheimer approximation. I summarize selected recent results for b¯b¯qq tetraquarks, for I = 0 bottomonium and for I = 1 bottomonium.

Fully charmed resonance X(6900) and its beauty counterpart

The fully heavy scalar tetraquarks T4Q=QQQ‾Q‾, (Q=c,b) are explored in the context of QCD sum rule method. We model T4Q as diquark-antidiquark systems composed of pseudoscalar constituents, and calculate their masses m(′) and couplings f(′) within the two-point sum rule approach. Our results m=(6928±50)MeV and m′=(18858±50)MeV for masses of the tetraquarks T4c and T4b prove that they can decay to hidden-flavor heavy mesons. The full width Γ4c of the T4c is evaluated by taking into account the decay channels T4c→J/ψJ/ψ, J/ψψ′, ηcηc, ηcηc(2S), ηcχc1(1P), and χc0χc0. The partial widths of these processes depend on strong couplings gi at vertices T4cJ/ψJ/ψ, T4cJ/ψψ′ etc., which are computed using the QCD three-point sum rule method. The decay T4b→ηbηb is used to find the width Γ4b of the T4b. The predictions for m and Γ4c=(128±22)MeV are compared with parameters of the fully charmed resonances reported by the LHCb, ATLAS, and CMS Collaborations. Based on this analysis, we interpret the tetraquark T4c as a candidate to the resonance X(6900). The mass m′ and width Γ4b=(94±28)MeV of the exotic meson T4b can be used in future experimental investigations of these states.

Ultra-peripheral nuclear collisions

This article presents a very brief review of the physics of Ultra-Peripheral Collisions (UPC) at the Large Hadron Collider (LHC) and other nuclear facilities. I discuss several processes of interest such as electron-position pair production, the anti-hydrogen atom, giant resonances, exotic meson production and parton distribution functions.

Revealing the inner structure of the newly observed η 1(1855) via photoproduction

Very recently, a new hadronic exotic state η 1(1855) at the invariant mass spectrum of was observed by the BESIII Collaboration. According to its properties, such as the mass and decay width, the η 1(1855) have been suggested to be a compact multi-quark state, a hadron molecule, or a hybrid meson. In order to distinguish the various interpretations of the η 1(1855), a Reggeized model combined with the vector dominance model for η 1(1855) photoproduction on the proton target is presented. The η 1(1855) photoproduction is dominated by the t-channel vector mesons ρ, ω, and ϕ exchange. If the η 1(1855) is a hybrid meson, the Primakoff effect also should be included. Our calculations show that the total cross section of the η 1(1855) production via γ p reaction can reach up to 0.276 pb for θ = 15° or 0.119 pb for θ = 0°, about 0.0297% or 0.0128% of the total cross section obtained by considering the η 1(1855) as an S-wave molecule. We also find their line shapes are sizably different. If the η 1(1855) is a molecular state, the photoproduction of η 1(1855) near the threshold offers a nice place to test its molecular nature. However, it should be better to take high energy, at least above E γ = 16.97 GeV, to observe the production of η 1(1855) if η 1(1855) is a hybrid meson. These results can be measured in the GlueX experiment or electron-ion collider in China to test the nature of the η 1(1855).

Kinematical higher-twist corrections in γ*→M1M2γ : Neutral meson production

We carry out the calculation of kinematical higher-twist corrections to the cross section of $\gamma^* \to M_1 M_2 \gamma$ up to twist 4, where $M_i$ is a scalar or pseudoscalar neutral meson. The three independant helicity amplitudes are presented in terms of the twist-2 generalized distribution amplitudes (GDAs), which are important non-perturbative quantities for understanding the 3D structure of hadrons. Since this process can be measured by BESIII in $e^+ e^-$ collisions, we perform the numerical estimate of the kinematical higher-twist corrections by using the kinematics of BESIII. We adopt the $\pi \pi$ GDA extracted from Belle measurements and the asymptotic $\pi \pi$ GDA to study the size of the kinematical corrections in the case of pion meson pair, and a model $\eta \eta$ GDA is used to see the impact of target mass corrections $\mathcal O(m^2/s)$ for $\gamma^* \to \eta \eta \gamma$. Our results show that the kinematical higher-twist corrections account for $\sim 20\%$ of the cross sections at BESIII on the average, and it is necessary to include them if one tries to extract GDAs from experimental measurements precisely. We also comment the case of $\pi^0 \eta$ production which is important for the search of hybrid mesons.

Solving bound state equation for scalar and hybrid QCD in two-dimensional spacetime

Abstract We investigate the bound-state equations in two-dimensional QCD in the $N_c\to \infty$ limit. We consider two types of hadrons, an exotic ``meson" (which is composed of a bosonic quark and a bosonic anti-quark), and an exotic ``baryon" (composed of a fermionic quark and a bosonic antiquark). Using the operator approach, we derive the corresponding bound-state equations for both types of hadrons from the perspectives of light-front quantization and equal-time quantization and confirm the known results. We also present a novel diagrammatic derivation for the exotic meson bound-state equation in the equal-time quantization.Our bound-state equation for the exotic baryons in the equal-time quantization is new. We also numerically solve various bound-state equations, obtain the hadron spectrum and the bound-state wave functions of the lowest-lying states. We explicitly demonstrate the pattern that as the hadron is boosted to infinite-momentum frame, the forward-moving bound-state wave function approach the light-front wave function. 
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.