Narrow Results

We compute the longitudinal structure function of the proton (F_L) at the next-to-next-to-leading order (NNLO) approximation. For this purpose, we should know the flavor-singlet, non-singlet and gluon distribution functions of the proton. We use the chiral quark model (χQM) to determine these distributions. Finally, we compare the results of F_L with the recent ZEUZ and H1 experimental data and some fitting parametrizations. Our results are in good agreement with the data and the related fittings.

In the extended chiral SU(3) quark model, we studied the structure of ${({\mathrm{\Sigma }}^{*}\mathrm{\Delta })}_{ST=0\frac{5}{2}}$ dibaryon with strangeness $s=-1$. Our research mainly focuses on three aspects: the contribution of vector mesons, the influence of the hidden-color channel (CC), and the stability of the system under the color screening effect. We found that the ${({\mathrm{\Sigma }}^{*}\mathrm{\Delta })}_{ST=0\frac{5}{2}}$ can form a bound dibaryon with a broad width. In the short-range interaction, the repulsive effect provided by vector $\rho$, $\omega$ fields is quite similar to one-gluon-exchange (OGE) in the chiral SU(3) quark model. The CC effect promotes the formation of ${({\mathrm{\Sigma }}^{*}\mathrm{\Delta })}_{ST=0\frac{5}{2}}$ bound state. The binding properties are stable when different confining potentials are considered.

We study on-shell decays of light vector meson resonances ρ, K* and ϕ in the framework of chiral constituent quark model using resummation calculations. Such studies are necessary for showing that dynamics described by chiral Lagrangian works well at this energy scale. The effective action is derived by a proper vertex method, where resummation of all orders of momentum expansion is accomplished. Also studied are the loop effects of pseudoscalar meson, which play an important role at this energy scale. The numerical results agree well with the experimental data. A new method to explore the chiral symmetry spontaneously breaking (CSSB) is proposed. It is found that the unitarity of the effective meson theory resulted from resummation derivations demands an upper-limit to the momentum of vector meson. This upper-limit, being critical point, is just the energy scale of CSSB, and is found to be flavor-dependent.

The chiral quark model posits that pions couple to the axial current of constituent quarks. This chiral pion coupling governs the pion decays of baryon resonances and also implies pion and multipion interactions between the quarks. The qualitative features of this model for the description of pionic decay widths and baryon spectra are outlined. When the covariant quark model wave function for the nucleon is chosen so as to describe the empirical nucleon form factors the pion decay widths of the positive parity resonances are underpredicted. This indicates the presence of significant multiquark components in these resonances.

We report the correction from the asymmetric strange-antistrange sea of the nucleon by using both the light-cone baryon-meson fluctuation model and the chiral quark model, and show that a significant part of the NuTeV anomaly can be explained by the strange-antistrange asymmetry. We also show that the calculated asymmetry are compatible with the NuTeV data by including some additional symmetric quark contribution.

The effective chiral Lagrangian in both nonlocal form $L_{\mathrm{\text{ECCL}}}$ and standard local form $L_{\mathrm{\text{ECL}}}$, derived in the vacuum correlator formalism in QCD, is studied in the chiral perturbation theory. It is shown, that all coefficients of $L_{\mathrm{\text{ECL}}}$ can be computed via $q\bar{q}$ Green’s functions. In the $p^2$ order of $L_{\mathrm{\text{ECL}}}$ one obtains GOR relations and quark decay constants $f_a$ are calculated $a=1,\dots ,8$, while in the $p^4$ order the coefficients $L_1$, $L_2$, $L_3$, $L_4$, $L_5$, $L_6$ are obtained in good agreement with the values given by data. The chiral quark model is shown to be a simple consequence of $L_{\mathrm{\text{ECCL}}}$ with defined coefficients. It is demonstrated that $L_{\mathrm{\text{ECCL}}}$ gives an extension of the limiting low-energy Lagrangian $L_{\mathrm{\text{ECL}}}$ to arbitrary momenta.

The spectrum of the SU(2) flavor baryons is studied in the frame of a relativistic chiral quark potential model based on the one-pion and one-gluon exchange mechanisms. All the N* and Δ* resonances appearing in the πN scattering data are identified with the orbital configurations (1S_1/2)²(nlj) with a single excited valence quark. The obtained selection rules allow one to construct a schematic periodic table of baryon resonances, consistent with the experimental data and yielding no "missing resonances". A new original method for the center of mass correction problem of the zero-order three-quark core energy values of the excited baryon resonances is suggested, which is based on the separation of the three-quark Dirac Hamiltonian into the parts, corresponding to the Jacobi coordinates. The numerical estimations for the energy positions of the Nucleon and Delta baryons, obtained within the field-theoretical framework, yield an overall good description of the experimental data at a level of the relativized Constituent quark model.

In this talk, we report our main results on the study of the low-energy reactions K^-p → Σ⁰π⁰, Λπ⁰, , ηΛ in a quark model. This model might be a powerful approach to deal with these low-energy reactions. In the analysis, we find that the u- and t-channel backgrounds play crucial roles in the low-energy scatterings. Significant evidence of some low-lying Λ and Σ hyperon resonances in these processes are found. Furthermore, we obtain some properties of the low-lying Λ and Σ hyperon resonances. For example, Λ(1670)S₀₁ should have a much stronger coupling to ηΛ, while having a weaker coupling to than that derived in the SU(6) ⊗ O(3) limit; the coupling of Λ(1690)D₀₃ to ηΛ is very small. The configuration mixing effects might be crucial to understand the strong interaction properties of the low-lying negative Λ resonances.

In this talk, we summarize the present calculations for the newly observed dibaryon resonance d*(2380) with the help of a chiral constituent quark model. Due to its binding and its narrow width, we interpret it as a deeply bound 6-quark state. The important role of quark-exchange is addressed. Moreover, the calculated decays as well its electromagnetic form factors are reported. Comparing to the available experimental data, we conclude that the d^*(2380) resonance can be reasonably assigned as a compact hexaquark dominant state.

Recent indications for high neutron star masses (M ~ 2 M_⊙) and large radii (R > 12 km) could rule out soft equations of state and have provoked a debate whether the occurence of quark matter in compact stars can be excluded as well. We show that modern quantum field theoretical approaches to quark matter including color superconductivity and a vector meanfield allow a microscopic description of hybrid stars which fulfill the new, strong constraints. For these objects color superconductivity turns out to be essential for a successful description of the cooling phenomenology in accordance with recently developed tests. We discuss QCD phase diagrams for various conditions thus providing a basis for a synopsis for quark matter searches in astrophysics and in future generations of nucleus-nucleus collision experiments such as low-energy RHIC and CBM @ FAIR.

To understand the nature of the low-lying negative parity Λ resonances, we carried out a combined study of the reaction K⁻p → Λη at low energies and their strong decays with a chiral quark model. It is found that the low-lying negative parity Λ resonances, such as Λ(1670), Λ(1405), Λ(1520) and Λ(1690), are most likely mixed states between different configurations.