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Register a new userdbar/ubar Asymmetry and the Origin of the Nucleon Sea
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The Drell-Yan cross section ratios, $sigma(p+d)/sigma(p+p)$, measured in Fermilab E866, have led to the first determination of $bar d(x) / bar u(x)$, $bar d(x) - bar u(x)$, and the integral of $bar d(x) - bar u(x)$ for the proton over the range $0.02 le x le 0.345$. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates these non-perturbative processes play an important role in the origin of the $bar d$, $bar u$ asymmetry in the nucleon sea.
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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.
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.
A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on hydrogen and deuterium targets is reported. Over 140,000 Drell-Yan muon pairs with dimuon mass M_{mu+ mu-} >= 4.5 GeV/c^2 were recorded. From these data, the ratio of anti-down (dbar) to anti-up (ubar) quark distributions in the proton sea is determined over a wide range in Bjorken-x. A strong x dependence is observed in the ratio dbar/ubar, showing substantial enhancement of dbar with respect to ubar for x<0.2. This result is in fair agreement with recent parton distribution parameterizations of the sea. For x>0.2, the observed dbar/ubar ratio is much nearer unity than given by the parameterizations.
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 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.
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