Search

Inspired by recent measurement of possible fully charmed tetraquarks in LHCb Collaboration, we investigate the mass spectra of fully heavy tetraquarks $QQ \bar Q \bar Q$ in an extended relativized quark model. Our estimations indicate that the broad structure around 6.4 GeV should contain one or more ground $cc \bar c \bar c$ tetraquark states, while the narrow structure near 6.9 GeV can be categorized as the first radial excitation of $cc \bar c \bar c$ system. Moreover, with the wave functions of the tetraquarks and mesons, the strong decays of tetraquarks into heavy quarkonium pair are qualitatively discussed, which can be further checked by the LHCb and CMS Collaborations., Comment: 7 pages, 3 figures, comments and suggestions are welcome

In this paper, we adopt an extended relativized quark model to investigate the triply-heavy tetrquarks systematically. The mass spectra are obtained by solving the four-body relativized Hamiltonian including the Coulomb potential, confining potential, spin-spin interactions, and relativistic corrections. We find that all of the triply-heavy tetraquarks lie above the corresponding meson-meson thresholds, and thus no stable one exists. In particular, besides the spin-spin interactions, the Coulomb and confining potentials also contribute to the mass splittings in the $cb\bar c \bar q$ and $cb\bar b \bar q$ systems. Moreover, the whole mass spectra for triply heavy tetraquarks show quite similar patterns, which preserve the light flavor SU(3) symmetry and heavy quark symmetry well. Through the fall-apart mechanism, the triply-heavy tetraquarks may easily decay into the heavy quarkonium plus heavy-light mesons, which are good candidates for investigation in future experiments., Comment: 15 pages, 4 figures. Minor modifications, accepted version

In present work, we systematically calculate the mass spectra of open charm and bottom tetraquarks $q q\bar q \bar Q$ within an extended relativized quark model. The four-body relativized Hamiltonians including the Coulomb potential, confining potential, spin-spin interactions, and relativistic corrections are solved by using the variational method. It is found that the predicted masses of four $0^+$ $ud\bar s \bar c$ states are 2765, 3065, 3152, and 3396 MeV, which disfavor the assignment of the newly observed $X_0(2900)$ as a compact tetraquark. Moreover, the whole mass spectra of the open charm and bottom tetraquarks show quite similar patterns, which preserve the light flavor SU(3) symmetry and heavy quark symmetry well. In addition, our results suggest that the flavor exotic states $nn\bar s \bar c$, $nn\bar s \bar b$, $ss\bar n \bar c$, and $ss\bar n \bar b$ and their antiparticles can be searched in the heavy-light meson plus kaon final states by future experiments. More theoretical and experimental efforts are needed to investigate these singly heavy tetraquarks., 11 pages, 3 figures, comments and suggestions are welcome

The ρ meson gravitational form factors are studied based on a light-front constituent quark model which has been successfully employed to calculate its generalized parton distributions and some low-energy observables. The distributions of energy, spin, pressures, and shear forces inside the ρ meson are explicitly given.

The newly confirmed pentaquark state $P_c(4312)$ has been treated as a weakly bound $(\Sigma_c\bar{D})$ state by a well-established chiral constituent quark model and by a dynamical calculation on quark degrees of freedom where the quark exchange effect is accounted for. The obtained mass $4308$ MeV agrees with data. In this work, the selected strong decays of the $P_c(4312)$ state are studied with the obtained wave function. It is shown that the width of the $\Lambda_c\bar{D}^*$ decay is overwhelmed and the branching ratios of the $p\,\eta_c$ and $p\,J/\psi$ decays are both less than 1 percentage., Comment: 13 pages, 2 figures, 3 tables

We report progress, in last decade in description of heavy exotic resonances using phenomenological Lagrangians — of a hadronic molecular approach developed by the Tubingen–Beijing group. In particular, we present a comprehensive analysis of decays properties of exotic heavy mesons and baryons.

Recently, the BESIII Collaboration reported a new measurement of the ${\ensuremath{\eta}}_{c}\ensuremath{\rho}$ decay mode of ${Z}_{c}^{(\ensuremath{'})}$, which motivated us to study the inner structure of ${Z}_{c}^{(\ensuremath{'})}$ via investigating the hidden charm decays of these two ${Z}_{c}$ states. We consider the triangle loop mechanism contribution in the hidden charm decays of ${Z}_{c}^{(\ensuremath{'})}$. Our estimations indicate that the triangle loop mechanism plays an important role in the decays of the ${Z}_{c}^{(\ensuremath{'})}$, where our results are in agreement with the experimental observations in a reasonable parameter range. Furthermore, we point out that the ${Z}_{c}^{(\ensuremath{'})}$ can be interpreted as the hadronic molecules, while the tetraquark scenario is less favored.

$d^\ast(2380)$ was observed by WASA-at-COSY collaborations in the nuclear reaction recently. Its particularly narrow width may indicate the new QCD-allowed hadronic structure. To further confirm the existence of this peculiar particle in a totally different kind of reaction, we study the opportunity for searching $\bar{d}^\ast$ in the $\Upsilon(nS)$ (with n=1, 2, 3) decay. As a phenomenological study, our framework is based on SU(3) chiral quark model. By virtue of the unitarity of $S$-matrix and crossing symmetry, we study the imaginary part of the forward scattering amplitudes between $\Upsilon(nS)$ and $d^\ast$. The scattering process is mainly governed by the quark-meson interaction. We examine both the pseudoscalar and vector meson contribution in the intermediate state. Hopefully, our results show that it's quite possible to find $\bar{d}^\ast$ in this decay mode in the future.

Careful observation of the experimental spectra of heavy-light mesons tells us that heavy-light mesons with the same angular momentum $L$ are almost degenerate. The estimate is given how much this degeneracy is broken in our relativistic potential model, and it is analytically shown that expectation values of a commutator between the lowest order Hamiltonian and ${\vec L}^{~2}$ are of the order of $1/m_Q$ with a heavy quark mass $m_Q$. It turns out that nonrelativistic approximation of heavy quark system has a rotational symmetry and hence degeneracy among states with the same $L$. This feature can be tested by measuring higher orbitally and radially excited heavy-light meson spectra for $D/D_s/B/B_s$ in LHCb and forthcoming BelleII., 4 pages, 1 table

The ρ meson polarized generalized parton distribution functions, its structure functions g1 and g2 and its axial form factors G˜1,2 are studied based on a light-front quark model for the first time. Comparing our obtained moments of g1 to the lattice QCD calculation, we find that our results are reasonably consistent to the lattice predictions.

We investigate the ρ meson unpolarized Generalized Parton Distributions based on a Light-Front Constituent Quark Model. Moreover, by employing a Gaussian form wave packet, we apply the obtained generalized parton distributions to get the Impact Parameter dependent parton Distributions.

Using the Regge-like formula $(M-m_Q)^2=\pi\sigma L$ between hadron mass $M$ and angular momentum $L$ with a heavy quark mass $m_Q$ and a string tension $\sigma$, we analyze all the heavy-light systems, i.e., $D/D_s/B/B_s$ mesons and charmed and bottom baryons. Numerical plots are obtained for all the heavy-light mesons of experimental data whose slope becomes nearly equal to 1/2 of that for light hadrons. Assuming that charmed and bottom baryons consist of one heavy quark and one light cluster of two light quarks (diquark), we apply the formula to all the heavy-light baryons including recently discovered $\Omega_c$'s and find that these baryons experimentally measured satisfy the above formula. We predict the average mass values of $B$, $B_s$, $\Lambda_b$, $\Sigma_c$, $\Xi_c$, and $\Omega_c$ with $L=2$ as 6.01, 6.13, 6.15, 3.05, 3.07, and 3.34 GeV, respectively. Our results on baryons suggest that these baryons can be safely regarded as heavy quark-light cluster configuration. We also find a universal description for all the heavy-light mesons as well as baryons, i.e., one unique line is enough to describe both of charmed and bottom heavy-light systems. Our results suggest that instead of mass itself, gluon flux energy is essential to obtain a linear trajectory.

Using the Regge-like formula $(M-m_Q)^2=\pi\sigma L$ between hadron mass $M$ and angular momentum $L$ with a heavy quark mass $m_Q$ and a string tension $\sigma$, we analyze all the heavy-light systems, i.e., $D/D_s/B/B_s$ mesons and charmed and bottom baryons.Numerical plots are obtained for all the heavy-light mesons of experimental data whose slope becomes nearly equal to 1/2 of that for light hadrons. Assuming that charmed and bottom baryons consist of one heavy quark and one light cluster of two light quarks (diquark), we apply the formula to all the heavy-light baryons including recently discovered $\Omega_c$'s and find that these baryons experimentally measured satisfy the above formula. We predict the average mass values of $B$, $B_s$, $\Lambda_b$, $\Sigma_c$, $\Xi_c$, and $\Omega_c$ with $L=2$ as 6.01, 6.13, 6.15, 3.05, 3.07, and 3.34 GeV, respectively. Our results on baryons suggest that these baryons can be safely regarded as heavy quark-light cluster configuration. We also find a universal description for all the heavy-light mesons as well as baryons, i.e., one unique line is enough to describe both of charmed and bottom heavy-light systems. Our results suggest that instead of mass itself, gluon flux energy is essential to obtain a linear trajectory., Comment: 10 pages, 8 figures, 5 tables

Spin-1 hadrons have additional structure functions not present for spin 1/2 hadrons. These could probe novel aspects of hadron structure and QCD dynamics. For the deuteron, the tensor structure function $b_1$ inherently mixes quark and nuclear degrees of freedom. These proceedings discuss two standard convolution models applied to calculations of the deuteron $b_1$ structure functions. We find large differences with the existing HERMES data and other convolution model calculations. This leaves room for non-standard contributions to $b_1$ in the deuteron. We also discuss the influence of higher twist nuclear effects in the model calculations and data extraction at kinematics covered in HERMES and Jefferson Lab., Proceedings of 25th International Workshop on Deep Inelastic Scattering and Related Topics, 3-7 April 2017 University of Birmingham

The strong decays of the two resonances $\ensuremath{\eta}(2225)$ and $\ensuremath{\phi}(2170)$ are discussed for selected decay channels. The two resonances are considered as the $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ bound states in the molecular scenario. The phenomenological hadronic molecular approach is employed for the calculation of respective decay modes using effective Lagrangians. Our results show that the decay modes $\ensuremath{\eta}(2225)\ensuremath{\rightarrow}{K}^{*}K$ and $\ensuremath{\phi}(2175)\ensuremath{\rightarrow}KK$ dominate over the partial decay widths of $\ensuremath{\eta}(2275)\ensuremath{\rightarrow}VV(\ensuremath{\phi}\ensuremath{\phi},\ensuremath{\omega}\ensuremath{\omega},{K}^{*}{K}^{*})$ and $\ensuremath{\phi}(2175)\ensuremath{\rightarrow}VS(\ensuremath{\omega}\ensuremath{\sigma},{K}^{*}{K}_{0}^{*}(800),\ensuremath{\phi}{f}_{0}(980))$ due to phase space and couplings.

We study $\ensuremath{\rho}$ meson unpolarized generalized parton distributions based on a light-front constituent quark model where the quark-antiquark-meson vertex is constructed under the symmetric loop momentum convention. The form factors and some other low-energy observables of the $\ensuremath{\rho}$ meson are calculated. Moreover, the contributions to the form factors and generalized parton distributions from the valence and nonvalence regimes are discussed and analyzed in detail. In the forward limit, the usual structure functions are estimated as well. In addition, by evolving the moments of the obtained structure functions to the scale of the lattice calculation, we give the factorization scale of our quark model. It is found that the present phenomenological model is reasonable to describe the general properties of $\ensuremath{\rho}$ meson.

The width of three-body single-pion decay process d ⁎ → N N π 0 , ± is calculated by using the d ⁎ wave function obtained from our chiral SU(3) constituent quark model calculation. The effect of the dynamical structure on the width of d ⁎ is taken into account in both the single ΔΔ channel and coupled Δ Δ + C C two-channel approximations. Our numerical result shows that in the coupled-channel approximation, namely, the hidden-color configuration being considered, the obtained partial decay width of d ⁎ → N N π is about several hundred keV, while in the single ΔΔ channel it is just about 2 ∼ 3 MeV . We, therefore, conclude that the partial width in the single-pion decay process of d ⁎ is much smaller than the widths in its double-pion decay processes. Our prediction may provide a criterion for judging different interpretations of the d ⁎ structure, as different pictures for the d ⁎ may result quite different partial decay width.

The transverse charge density of pions is calculated based on relativistic quantum mechanics, where the pion is regarded as a quark-antiquark bound state. Corrections from the two spin-1/2 constituents and from the wave function of a quark and antiquark inside the bound system are discussed. The calculated results are compared to the results with a realistic effective Lagrangian approach as well as to that with a simple covariant model where the pion is regarded as a composite system with two scalar particles.

An effective potential in a meson-meson system is discussed based on the SU(3) chiral constituent quark model, and the analytic form of the potential is explicitly given. In addition, the effective potential is employed to study the bound state problem of $\omega\phi$, which is related to the new resonance of $f_0(1810)$ observed in BESII very recently., Comment: 7 pages, 4 figures

The discrepancy between the PQCD calculation and the CLEO data for $\chi_{c1}\to \gamma V$ ($V=\rho^0,\,\omega,\,\phi$) stimulates our interest in exploring extra mechanism of $\chi_{c1}$ decay. In this work, we apply an important non-perturbative QCD effect, i.e., hadronic loop mechanism, to study $\chi_{c1}\to \gamma V$ radiative decay. Our numerical result shows that the theoretical results including the hadronic loop contribution and the PQCD calculation of $\chi_{c1}\to \gamma V$ are consistent with the corresponding CLEO data of $\chi_{c1}\to \gamma V$. We expect further experimental measurement of $\chi_{c1}\to \gamma V$ at BES-III, which will be helpful to test the hadronic loop effect on $\chi_{c1}$ decay., Comment: 7 pages, 2 figures. Accepted for publication in Eur. Phys. J. C

In this paper, the $\rho$ meson impact parameter dependent parton distributions and the impact parameter dependent form factors are introduced and discussed. By employing a Gaussian form wave packet, we calculate the impact parameter distributions of $\rho$ meson based on a light-front constituent quark model.

We study the radiative decays of the Z(c)(3900)(0) in a hadronic molecule picture, where the Z(c)(3900) is treated as a DD + c.c hadronic molecule. The partial widths of Gamma(Z(c)(3900)(0) -> eta(c)(2S)) and Gamma(Z(c)(3900)(0) -> gamma chi(c0)) are predicted to be of order 10 keV, and the cross sections for sigma(e+e- -> pi(0)Z(c)(3900) -> pi(0)gamma eta(c)(2S)) and sigma(e(+)e(-) -> pi(0)Z(c)(3900) -> pi(0)gamma chi(c0)) are of order 0.1 pb at 4.23 GeV, which may be accessible for the BESIII and forthcoming Belle II.

Bloom-Gilman quark-hadron duality of nucleon spin-structure functions g1,2 is discussed. The occurrence of the duality is also estimated. Moreover, target mass corrections to the nucleon spin-structure functions as well as to the quark-hadron duality are studied.

Target mass corrections to nucleon spin structure functions are analyzed. Our results show that the corrections are important to the structure functions in a large x region. Moreover, they play a remarkable role to the local quark–hadron duality of the nucleon spin structure functions in three individual inelastic resonance production regions.

The results for the elastic nucleon form factors and the electromagnetic transition amplitudes to the Delta(1232) resonance, obtained with the Hypercentral Constituent Quark Model with the inclusion of a meson cloud correction are briefly presented. The pion cloud effects are explicitly discussed., Comment: 7 pages, 7 figures, talk at 5th International Conference on Perspectives in Hadronic Physics: International Conference on Particle-Nucleus and Nucleus-Nucleus Scattering at Relativistic Energies, Trieste, Italy, 22-26 May 2006

Bloom-Gilman quark-hadron dualities of proton spin structure functions g(1), g(2) and g(T), in inelastic resonance region, are analyzed. The contribution of the nucleon elastic peak to the local duality and the local duality of each resonance region are also discussed. It is found that the occurrences of the quark-hadron dualities of g(1)(p) and g(2)(p) are similar and are expected at about 2 GeV2. Moreover, our results show that the nucleon elastic peak and Delta(1232) resonance remarkably break the quark-hadron dualities. (c) 2006 Elsevier B.V. All rights reserved.

Target mass corrections to proton spin structure functions in deep inelastic scattering region are analysed. Moreover, Bloom-Gilman quark-hadron dualities of proton spin structure functions g(1) and g(2), in inelastic resonance region, are studied. The onsets of the dualities are discussed. (c) 2006 Elsevier B.V. All rights reserved.

The recent measurement of quark-hadron duality is shortly discussed by using a simple constituent quark model. Comparing with the moments of the structure functions in the resonance region and in the deep inelastic scattering region, we study the occurances of Bloom-Gilman quark-hadron duality for the nucleon structure function F2 and g1. Local duality is also analyzed.

We study the Nachtmann moments and Bloom-Gilman quark-hadron duality of nucleon spin structure function. The target mass corrections to the moments of g(1) are discussed. Moreover, the onset of Bloom-Gilman quark-hadron duality of g, is estimated to be similar to 2 GeV2. (C) 2004 Elsevier B.V. All rights reserved.

Constituent quark model is employed to estimate nucleon structure functions in the resonance region. Comparing with scaling structure functions given by QCD calculations or parameterizations in deep inelastic scattering region, we study onsets of quark-hadron duality for the nucleon structure functions F-2 and g(1). Local dualities in the first resonance region and second one are also analyzed. Moreover, we discuss local duality in elastic region. (C) 2004 Published by Elsevier B.V.

To extract the polarized gluon distribution Δg, we study the large–pTlight hadron pair productions in polarized semi-inclusive reaction, which are dominantly produced via 2 mechanisms: photon–gluon fusion(PGF) and QCD Compton. The PGF gives us a direct information on Δg in the nucleon, whereas QCD Compton is background to the signal process for extracting Δg. By using symmetry relation among fragmentation functions and taking an appropriate combination of light hadron pair production processes, we can remove an effect of QCD Compton from those cross sections. Here we propose a new formula for extracting Δg from those processes.

Pion cloud effects on Delta-N mass splitting are studied based on quark models. Pseudo-scalar pion-quark coupling is discussed in the relativistic and nonrelativistic frameworks. We separately calculate the pion cloud effects by the one-pion exchange potential and by another method which is consistent with the baryon chiral perturbation theory. Remarkable discrepancy in the mass splitting between the two methods is shown.

Based on a relativistic quark model approach, the transition properties of the first nucleon resonance Delta(1232), and the coupling constants gpiNN, gDeltapiN are investigated. Two different ways to remove the center of mass motion are considered. The results of the relativistic approaches with and without center of mass correction are compared with those of nonrelativistic constituent quark model. Moreover, pion meson cloud effect on these calculated observables is explicitly addressed. Better results are obtained by taking the pion meson cloud into account.

We apply a phenomenological Lagrangian approach to the radiative decay $Y(4260)\ensuremath{\rightarrow}X(3872)+\ensuremath{\gamma}$. The $Y(4260)$ and $X(3872)$ resonances are considered as composite states containing both molecular hadronic and charmonium components. Having a leading molecular component in the $X(3872)$ and a sole molecular configuration for the $Y(4260)$ results in a prediction compatible with present data.

Based on a relativistic potential quark model approach for the nucleon and Δ(1232) , the electroproduction amplitudes of the Δ(1232) resonance are calculated. The important effect of the pion meson cloud is stressed. Different predictions for helicity amplitudes and electroproduction amplitudes are obtained compared to the nonrelativistic constituent quark model.

Mass spectra of heavy meson ground states D, D s , B and B s are calculated based on a relativistic quark potential model with mean field approach. Radiative and hadronic pion decays and Isgur–Wise functions of semileptonic decays of these heavy states are investigated. In our approach, both light and heavy quark wave functions are treated independently by solving Dirac equation self-consistently. Linear confinement and one-gluon exchange interaction are taken into account. The calculations show that the mass splitting between the triplet 3 S 1 and singlet 1 S 0 heavy meson states affects the Isgur–Wise functions. Our results also illustrate the success of heavy quark effective theory in the mass spectra of the heavy mesons. Our calculations for the radiative and strong decays of the heavy mesons are in agreement with the data.

We investigate the structure of the nucleon and the Δ resonance employing a constituent chiral quark model. We propose a variational approach which allows the nucleon to have a [21] flavor-spin symmetry as well as the usual [3] symmetry. This means one gives also up the requirement that the spatial wave function must be symmetric [3]. One then has also to admix the [21] spatial symmetry for a total antisymmetrization and thus one allows quark-diquark correlations, where two quarks are more closely bound than the other two possible pairs. It is found that a quark pair with isospin T = 0 and spin S = 0 in the nucleon becomes spatially very close compared with other pairs in order to gain a strong attraction due to the pion exchange in the chiral quark model, which implies a diquark correlation in the nucleon. We also calculate the proton and neutron charge square radii.

In this letter, the predictions of nucleon polarized structure functions based on constituent quark model and Breit-Wigner resonance properties are employed to determine the higher-twist QCD effects in the resonance region. (C) 1998 Elsevier Science B.V. All rights reserved.

The newly observed hidden-charm meson Z(c)(+)(3900) and a possible partner state Z(c)'(+) with quantum numbers J(P) = 1(+) are considered as hadronic molecules composed of (D) over barD* and (D) over bar *D*, respectively. We give predictions for the decay widths of the strong two-body transitions Z(c)(+) -> H + pi(+) and Z(c)'(+) -> H + pi(+) with H = Psi(nS), h(c)(mP) in a phenomenological Lagrangian approach.

Stimulated by the proton size puzzle and the proposal of mu p scattering at the Paul Scherrer Institute (PSI), we study the two-photon exchange corrections to the massive-lepton-proton scattering. The leptonic helicity broken caused by lepton mass introduces new form factors. We estimate the two-photon exchange corrections to these form factors as well as to the unpolarized differential cross sections.

In present work the Y(4260) resonance is considered as a weakly bound state of a pseudoscalar D and an axial D1 charm meson. We consider the two-body decay Y(4260) -> Zc(3900) + pi where Zc(3900) is treated as hadron molecule as well. Moreover we compute the Y(4260) decay modes J/psi pi(+) pi(-), recently observed by the BESIII Collaboration, and psi(2S) pi(+) pi(-). In the last process both the contact diagram with D D1 -> psi(nS) pi(+) pi(-) and the resonance diagram with D D1 -> Zc(3900) + pi -> psi(nS) pi(+) pi(-) are taken into account., 11 pages

We perform a systematic study of $B\bar{B}^*$, $B^*\bar{B}^*$, $D\bar{D}^*$ and $D^*\bar{D}^*$ systems by using effective interaction in our chiral quark model. Our results show that the interactions of $B\bar{B}^*$, $B^*\bar{B}^*$, $D\bar{D}^*$ and $D^*\bar{D}^*$ states are attractive, which consequently result in $B\bar{B}^*$, $B^*\bar{B}^*$, $D\bar{D}^*$ and $D^*\bar{D}^*$ bound states. The recent observed exotic-like hadrons of $Z_b(10610)$ and $Z_b(10650)$ are, therefore in our approach, interpreted as loosely bound states of $B\bar{B}^*$ and $B^*\bar{B}^*$, while X(3872) and Y(3940) are $D\bar{D}^*$ and $D^*\bar{D}^*$ molecule states, respectively., Comment: 8 pages, 4 figures

The ${\ensuremath{\Lambda}}_{c}(2940{)}^{+}$ baryon with quantum numbers ${J}^{P}={\frac{1}{2}}^{+}$ is considered as a hadronic molecule composed of a nucleon and ${D}^{*}$ meson. We give predictions for the width of the strong three-body decay processes ${\ensuremath{\Lambda}}_{c}(2940{)}^{+}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}_{c}(2286{)}^{+}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\Lambda}}_{c}(2286{)}^{+}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ in this interpretation. Upcoming experimental facilities like a Super $B$ factory at KEK or LHCb might be able to provide data on these decay modes.

An off-shell potential is derived from the effective interaction Harniltonian which appears in the relativistic Pauli-Schrodinger equation for quark-antiquark bound states. This potential gives considerable theoretical improvements on meson spectra.

The transverse charge density of the pion is studied based on a realistic effective Lagrangian approach, where the pion is regarded as a quark-antiquark pair-bound state. Corrections from the two spin-1/2 constituent particles and from the correlation function of quark and antiquark inside the pion are addressed.

A general Lorentz structure of the confining interaction between a quark and an antiquark is examined by using the linear potential and determined through fitting the experimental meson masses. The Lorentz structure thus determined gives a qq-bar confining potential which contains proper spin-dependent terms. Use of this potential in calculations of meson spectra leads to results which are in good agreement with the observed mass levels of light and heavy quark mesons.

By considering the retardation effect of gluon propagation and a general Lorents structure of the confinement, a retarded and off-shell one-gluon exchange potential and a harmonic oscillator-dominant confining potential with a certain relativistic corrections are derived based on the relativistic Pauli-Schrodinger equation for quark-antiquark bound states. Use of these potentials to calculate meson spectra leads to the results which are in good agreement with the experimental data.

Based on the exact three-dimensional relativistic equation for qq bound states, an off-shell retarded one-gluon exchange potential is derived and employed to calculate meson spectra. The calculated results show that the retardation effect is important for the improvement of the theoretical values of lighter meson masses.

The $X(3872)$ with quantum numbers ${J}^{PC}={1}^{++}$ is considered as a composite hadronic state comprised of the dominant molecular ${D}^{0}{D}^{*0}$ component and other hadronic pairs---${D}^{\ifmmode\pm\else\textpm\fi{}}{D}^{*\ensuremath{\mp}}$, $J/\ensuremath{\psi}\ensuremath{\omega}$, and $J/\ensuremath{\psi}\ensuremath{\rho}$. Applying the compositeness condition we constrain the couplings of the $X(3872)$ to its constituents. We calculate two- and three-body hadronic decays of the $X(3872)$ to charmonium states ${\ensuremath{\chi}}_{cJ}$ and pions using a phenomenological Lagrangian approach. Next using the estimated $XJ/\ensuremath{\psi}\ensuremath{\omega}$ and $XJ/\ensuremath{\psi}\ensuremath{\rho}$ couplings we calculate the widths of $X(3872)\ensuremath{\rightarrow}J/\ensuremath{\psi}+h$ transitions, where $h={\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$, ${\ensuremath{\pi}}^{0}\ensuremath{\gamma}$, and $\ensuremath{\gamma}$. The obtained results for the decay pattern of the $X(3872)$ in a molecular interpretation could be useful for running and planned experiments.

In a simple hadronic model, the two-photon exchange contributions to the single spin asymmetries for the nucleon and the $^3$He are estimated. The results show that the elastic contributions of two-photon exchange to the the single spin asymmetries for the nucleon are rather small while those for the $^3$He are relatively large. Besides the strong angular dependence, the two-photon contributions to the single spin asymmetry for the $^3$He are very sensitive to the momentum transfer., Comment: 11 pages, 8 figures