Validity of flavor symmetry and charge symmetry for parton distributions

Abstract

Recent experimental measurements of the Gottfried Sum Rule, and pp and pD Drell-Yan processes, suggest significant violation of flavor symmetry in the proton sea. This interpretation rests on the assumption of parton charge symmetry. Our model calculations suggest charge symmetry violation [CSV] for parton valence distributions of a few percent. Precision measurements of structure functions in muon and neutrino reactions allow us to set rather stringent experimental limits on CSV in certain kinematic regions. In another region, these experiments suggest substantial CSV effects. We suggest experiments which could test parton CSV. Flavor Symmetry in Parton DistributionsThe basic features of parton distributions have been well established through measurements of deep inelastic scattering [DIS], Drell-Yan processes and direct photon experiments. Precision tests of approximate symmetries allow us to understand the details of nucleon parton distributions. For example, we know that the strange quark distribution is substantially smaller than the light nonstrange sea, due to the relatively large mass of the s quark (this is sometimes termed SU(3) flavor symmetry violation)1 . A new generation of precise high energy experiments allows us to examine finer details of parton distributions. An example of this is the NMC experiment 2 , which measured µp and µD DIS, and accurately tested the Gottfried Sum Rule S G by comparing F µp 2 and F µn 2 . Assumingd p (x) =ū p (x) one predicts S G = 1/3. pQCD predicts very small deviations from 1/3. The NMC result S G = 0.235 ± 0.026 was four standard deviations lower than the "naive" prediction, apparently indicating significant flavor symmetry violation [FSV] in the proton sea.This was followed by a comparison of pp and pD Drell-Yan [DY] processes 3 . For large x F the ratio of DY cross sections will be larger than one ifd p (x) > u p (x), as observed in the E866 experiment (for a detailed discussion see the talk by W. Melnitchouk at this conference). The most promising theoretical model to date is the "meson-cloud" picture. In these models one includes a quark "core" for the nucleon plus a "cloud" of baryon-meson Fock components, and the virtual photon scatters from any of these components. Melnitchouk showed that quantitative agreement with E866 data can be achieved with a model including nucleon, pion and ∆ components.