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Register a new userFlavor asymmetry of polarized and unpolarized sea quark distributions in the large-N_c limit
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We summarize recent attempts to calculate the flavor asymmetry of the nucleons sea quark distributions in the large-$N_c$ limit, where the nucleon can be described as a soliton of an effective chiral theory. We discuss the leading-twist longitudinally polarized and transversity antiquark distributions, $Deltabar u (x) - Deltabar d (x)$ and $deltabar u (x) - deltabar d (x)$, as well as the unpolarized one, $bar u (x) - bar d (x)$, which appears only in the next-to-leading order of the $1/N_c$-expansion. Results for $bar u (x) - bar d (x)$ are in good agreement with the recent Drell-Yan data from the FNAL E866 experiment. The longitudinally polarized antiquark asymmetry, $Deltabar u (x) - Deltabar d (x)$, is found to be larger than the unpolarized one.
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It is now widely recognized that a key to unravel the nonperturbative chiral-dynamics of QCD hidden in the deep-inelastic-scattering observables is the flavor structure of sea-quark distributions in the nucleon. We analyze the flavor structure of the nucleon sea in both of the unpolarized and longitudinally polarized parton distribution functions (PDFs) within a single theoretical framework of the flavor SU(3) chiral quark soliton model (CQSM), which contains only one adjustable parameter $Delta m_s$, the effective mass difference between the strange and nonstrange quarks. A particular attention is paid to a nontrivial correlation between the flavor asymmetry of the unpolarized and longitudinally polarized sea-quark distributions and also to a possible particle-antiparticle asymmetry of the strange quark distributions in the nucleon. We also investigate the charge-symmetry-violation (CSV) effects in the parton distribution functions exactly within the same theretical framework, which is expected to provide us with valuable information on the relative importance of the asymmetry of the strange and antistrange distributions and the CSV effects in the valence-quark distributions inside the nucleon in the resolution scenario of the so-called NuTeV anomaly in the extraction of the Weinberg angle.
We propose a unified new approach to describe polarized and unpolarized quark distributions in the proton based on the gauge-gravity correspondence, light-front holography, and the generalized Veneziano model. We find that the spin-dependent quark distributions are uniquely determined in terms of the unpolarized distributions by chirality separation without the introduction of additional free parameters. The predictions are consistent with existing experimental data and agree with perturbative QCD constraints at large longitudinal momentum $x$. In particular, we predict the sign reversal of the polarized down-quark distribution in the proton at $x=0.8pm0.03$, a key property of nucleon substructure which will be tested very soon in upcoming experiments.
We study the helicity distributions of light flavor quark-antiquark ($q bar{q}$) pairs in the nucleon sea. The valence quarks are handled by adopting the light-cone SU(6) quark-spectator-diquark model and the sea $q bar{q}$ pairs are treated from statistical consideration by introducing the helicity suppression factors $l_q(x)$ and $bar l_q(x)$ to parametrize the helicity distributions of q-flavor sea quark and antiquark respectively, while $Delta l_q(x)=l_q(x)-bar l_q(x)$ represents a combined effect of helicity contribution due to sea $q bar{q}$ pairs. From fitting the nucleon polarization asymmetries $A^N_1$ in inclusive deep inelastic scattering processes and the single-spin asymmetries $A^{W^{pm}}_L$ in Drell-Yan type processes, we find a significant asymmetry between the quark and antiquark helicity distributions of the nucleon sea. Therefore the quark-antiquark asymmetry of helicity distributions of nucleon sea $q bar{q}$ pairs, i.e., $Delta q_s(x) eq Delta bar q_s(x)$, plays an important role for a comprehensive understanding of the nucleon spin content.
We use the meson cloud model to calculate $bar{d}(x) - bar{u}(x)$ and $ bar{d}(x)/bar{u}(x)$ in the proton. We show that a modification of the symmetric, perturbative part of the light quark sea provides better agreement with the ratio $ bar{d}(x)/bar{u}(x).
Although the distributions of sea quarks and antiquarks generated by leading-twist QCD evolution through gluon splitting $g rightarrow bar q q$ are necessarily CP symmetric, the distributions of nonvalence quarks and antiquarks which are intrinsic to the nucleons bound state wavefunction need not be identical. In this paper we investigate the sea quark/antiquark asymmetries in the nucleon wavefunction which are generated by a light-cone model of energetically-favored meson-baryon fluctuations. The model predicts striking quark/antiquark asymmetries in the momentum and helicity distributions for the down and strange contributions to the proton structure function: the intrinsic $d$ and $s$ quarks in the proton sea are predicted to be negatively polarized, whereas the intrinsic $bar d$ and $bar s$ antiquarks give zero contributions to the proton spin. Such a picture is supported by experimental phenomena related to the proton spin problem and the violation of the Ellis-Jaffe sum rule. The light-cone meson-baryon fluctuation model also suggests a structured momentum distribution asymmetry for strange quarks and antiquarks which could be relevant to an outstanding conflict between two different determinations of the strange quark sea in the nucleon. The model predicts an excess of intrinsic $d bar d$ pairs over $u bar u$ pairs, as supported by the Gottfried sum rule violation. We also predict that the intrinsic charm and anticharm helicity and momentum distributions are not identical.
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