Narrow Results

In this paper we calculate spin dependence of parton distributions and proton structure function. The polarized valon distributions in a proton and the polarized parton distributions inside the valon are necessary to obtain the polarized parton distributions in a proton. The results of our calculations for the proton structure function, are in good agreement with the available experimental data. We obtain the spin contribution and angular momentum of the valons to the proton.

Next-to-leading order approximation of the quark helicity distributions are used in the framework of polarized valon model. The flavor-asymmetry in the light-quark sea of the nucleon can be obtained from the contributions of unbroken sea quark distributions. We employ the polarized valon model and extract the flavor-broken light sea distributions which are modeled with the help of a Pauli-blocking ansatz. Using this ansatz, we can obtain broken polarized valon distributions. From there and by employing convolution integral, broken sea quark distributions are obtainable in this framework. Our results for δu, δd, and are in good agreement with recent experimental data for polarized parton distribution from HERMES experimental group and also with GRSV model. Some information on orbital angular momentum as a main ingredient of total nucleon spin are given. The Q² evolution of this quantity, using the polarized valon model is investigated.

Sea quark distributions in the NLO approximation, based on the phenomenological valon model or constituent quark model are analyzed. We use the parametrized inverse Mellin transform technique to perform a direct fit with available experimental data and obtain the unknown parameters of the distributions. We try to extend the calculation to the NLO approximation for the singlet and nonsinglet cases in DIS phenomena. We do also the same calculation for electron–positron annihilation. The resulting sea distributions are effectively independent of the process used. The approach of complete RG improvement (CORGI) is employed and the results are compared with the standard approach of perturbative QCD in the scheme with a physical scale. The comparisons with data are in good agreement. As is expected, the results in the CORGI approach indicate a better agreement to the data than the NLO calculation in the standard approach.

Sea quark densities in the nucleon, based on the constituent quark model are analyzed. To model the asymmetry of these densities, the meson cloud or alternatively chiral quark model (χQM) is used. Valence quark densities of the meson which are required to extract the sea quark densities in the constituent quarks are obtained using the phenomenological valon model. In addition to the standard perturbative QCD approach which uses the scheme with a physical choice of renormalization scale, the calculations are also performed using the complete RG-improvement (CORGI) approach. To avoid a physically unacceptable Q² behavior of the sea densities inside the constituent quarks, we assume that the free parameter which exists in the vertex function of the boson–quark splitting function, is Q²-dependent. Using the unsymmetrized sea densities of the nucleon which result from convoluting the constituent density in a nucleon with the quark density in the constituent quark, the Gottfried sum rule (GSR) is calculated using the standard perturbative and CORGI approaches. The CORGI result is closer to the reported experimental value for the GSR. The extracted sea and valence quark density in a nucleon, using χQM and also the CORGI approach, have been compared with available experimental data and what was obtained, based on χQM in the standard approach. This comparison confirms the anticipated better agreement of the CORGI approach with the data.

We employ the polarized chiral constituent quarks to extract the polarized structure function of the nucleon. The polarized valon model is used to calculate the spin dependence of parton distribution functions of meson. The connection between the polarized structure of the proton and the Goldstone bosons, using the chiral quark model (χQM) is analyzed and the spin dependence of the parton distribution functions for pion and kaon, is obtained thoroughly. These functions are evolved to high Q² values, using the singlet, nonsinglet and quark–gluon moments (ΔM_S, ΔM_NS, ΔM_gq) which are convoluted with the polarized valon distributions. The polarized valon distributions for meson are computed, based on a phenomenological method and a comparison between polarized and unpolarized parton distribution functions for pion and kaon are performed. As a consequence of the χQM, the SU(3)_f symmetry breaking for the spin dependent of the nucleon sea distributions is achieved. The required polarized parton distributions of the proton will be obtained from the parton distribution functions of the polarized meson via the related convolution integral which are existed in the χQM. Following that the analytical result for the proton's spin structure function, , is obtained and compared with experimental data. Finally, the parton orbital angular momentum of meson are introduced and the total spin of the meson, based on this quantity and the first moment of distributions for gluon and singlet sectors, are obtained.

We employ the statistical approach to obtain the nucleon parton distributions. Statistical distributions are considered as well for partons in the valon model in which a nucleon is assumed to be a state of three valence quark clusters (valon). Analytic expressions of the x-dependent of parton distribution functions (PDFs) in the valon model are obtained statistically in the whole x region [0, 1] in terms of the statistical parameters such as temperature, chemical potential and accessible volume. Since PDFs are obtained by taking the required sum rules including Gottfried sum rule at different energy scales, the Q²-dependence of these parameters can be obtained. Therefore the parton distributions as a function of Q² will be resulted. To make the calculations more precise, we extend our results to contain three flavors rather than two light u and d quarks.

We have calculated the orbital angular momentum of quarks and gluons in the nucleon. The calculations are carried out in the next to leading order utilizing the so-called valon model. It is found that the average quark orbital angular momentum is positive, but small, and the average gluon orbital angular momentum is negative and large. We also report on some regularities about the total angular momentum of the quarks and the gluon, as well as on the orbital angular momentum of the separate partons. We have also provided partonic angular momentum, $L^{q,g}$ as a function of $Q^2$.