Study of Nucleon Spin Structure Functions based on an Effective Model of QCD

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We have completed the theoretical calculation of the unpolarized and longitudinally polarized twist-2 parton distribution functions within the framework of the chiral quark soliton model. It was shown, that, without any adjustable parameter except for the initial-energy scale of the Q^2-evolution, the model can explain all the qualitatively noticeable features of the recent high-energy deep-inelastic Scattering observables. They include the NMC data for F^p_2(x) - F^n_2 (x), F^n_2(x)/F^p_2(x), the HERMES and E866 data for d^^-(x) - u^^-(x) data, and the EMC and SMC data for g^p_1(x), g^n_1(x) and g^d_1(x). We have also given theoretical predictions for the light-flavor sea-quark distributions in the nucleon including the strange quark one by extending the model to flavor SU(3). A particular emphasis of study is put on the light-flavor sea-quark asymmetry as exemplified by the observables d^^-(x) - u^^-(x), d^^-(x)/u^^-(x), Δu^^-(x) - Δd^^-(x) as well as on the particle-antiparticle asy … More mmetry of the strange quark distributions represented by s(x) - s^^-(x), s(x)/s^^-(x), Δs(x) - Δs^^-(x) etc. We also investigated the spin-dependent structure function g_2(x, Q^2). It turns out that the twist-3 part of g_2(x, Q^2) gives nonnegligible contributions to the total distributions at the energy scale of Q^2 - 5 GeV^2 but mainly in the small x region only, so that the corresponding third moment ∫^1_0 x^2 g^^-_2(x, Q^2) dx are pretty small for botb of the proton and neutron in conformity with the recent E155 data. Also given are the predictions for the twist-2 and twist-3 chirally-odd distribution functions of the nucleon. The importance of the vacuum polarization effects, which are properly taken into account within the framework of the chiral quark soliton model, is demonstrated by showing that the so-called Soffer inequality holds not only for the quark distributions but also for the antiquark ones. We have also succeeded in proving that the nonvanishing quark condensate as a signal of the spontaneous chiral symmetry breaking of the QCD vacuum brings about a delta-function singularity at x = 0 in the chirally-odd twist-3 distribution functions e (x) of the nucleon. This singularity in e (x), which would be observed as a sizable violation of the 1st moment sum rule, is then be interpreted as giving a very rare case that the nontrivial vacuum structure of QCD manifests in an obsrvable of a localized QCD excitation, i.e. the nucleon. Less