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Register a new userFlavor structure of the unpolarized and longitudinally-polarized sea-quark distributions in the nucleon
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Masashi Wakamatsu
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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.
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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.
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 present the first direct lattice calculation of the isovector sea-quark parton distributions using the formalism developed recently by one of the authors. We use $N_f=2+1+1$ HISQ lattice gauge ensembles (generated by MILC Collaboration) and clover valence fermions with pion mass 310 MeV. We are able to obtain the qualitative features of the nucleon sea flavor structure even at this large pion mass: We observe violation of the Gottfried sum rule, indicating $overline{d}(x) > overline{u}(x)$; the helicity distribution obeys $Delta overline{u}(x) > Delta overline{d}(x)$, which is consistent with the STAR data at large and small leptonic pseudorapidity.
We discuss two topics related to the flavor structure of the nucleon sea. The first is on the identification of light-quark intrinsic sea from the comparison between recent data and the intrinsic sea model by Brodsky et al. Good agreement between the theory and data allows a separation of the intrinsic from the extrinsic sea components. The magnitudes of the up, down, and strange intrinsic seas have been extracted. We then discuss the flavor structure and the Bjorken-x dependence of the connected sea (CS) and disconnected sea (DS). We show that recent data together with input from lattice QCD allow a separation of the CS from the DS components of the light quark sea.
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.
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