Flavor Asymmetry of Light Quarks in the Nucleon Sea.pdf

a r X i v : n u c l - e x / 0 1 0 9 0 1 0 v 1 1 4 S e p 2 0 0 1 Flavor Asymmetry of Light Quarks in the Nucleon Sea Gerald T. Garvey and Jen-Chieh Peng Physics Division Los Alamos National Laboratory Los Alamos, New Mexico, 87545 USA February 8, 2008 Abstract A surprisingly large asymmetry between the up and down sea quark distributions in the nucleon has been observed in recent deep inelastic scattering and Drell-Yan experiments. This result strongly suggests that the mesonic degrees of freedom play an important role in the description of the parton distributions of the hadronic sea. In this article, we review the current status of our knowledge of the flavor structure of the nucleon sea. The implications of various theoretical models as well as possible future measurements are also discussed. 1 Introduction The first direct evidence for point-like constituents in the nucleons came from the observation of scaling phenomenon in Deep-Inelastic Scattering (DIS) experiments [1, 2] at SLAC. These point-like charged constituents, called partons, were found to be spin-1/2 fermions. These partons were initially identified as the quarks in the Constituent Quark Models (CQM). However, it was soon realized that valence quarks alone could not account for the large enhancement of the cross sections at small Bjorken-x, the fraction of nucleon momentum carried by the partons. This swarm of low-momentum partons, dubbed wee-partons by Feynman [3], was interpreted as the quark and antiquark sea of the nucleon. DIS experiments therefore provided the first evidence for the existence of antiquarks in the nucleon. The observation of partons in DIS experiments paved the road to the formulation of Quantum Chromodynamics (QCD) as the theory for strong interactions. Nevertheless, the exact form of the parton distribution functions (PDF) can not be deduced from perturbative QCD. Like many other static properties of hadrons, the parton distribution functions belong to the domain of non-p