Omega Meson Cloud and the Proton’s Light Anti-Quark Distributions

We use the meson cloud model of the nucleon to calculate distribution functions for (d̄− ū) and d̄/ū in the proton. Including the effect of the omega meson cloud, with a coupling constant g ω/4π ≈ 8, allows a reasonably good description of the data. There has been considerable interest in the flavor dependence of the proton’s quark distributions. About ten years ago, the NMC measurement of the integral of the (d̄− ū) provided convincing evidence for the flavor asymmetry of the sea [1]. The meson cloud model and the Sullivan process were used to explain the momentum fraction distribution of (d̄ − ū). For reviews, see Refs. [2, 3]. Interest in the light sea quark distributions was heightened by the recent measurement of the ratio d̄/ū by means of the Drell-Yan process [4, 5]. Similar results for (d̄− ū) were obtained by HERMES [6]. In the context of previous work on the light antiquark asymmetry in the nucleon, a good description of the data was obtained for the x-dependence (momentum fraction) of (d̄ − ū), but not for the ratio of d̄/ū [5]. The meson cloud models provide ratios that either increase monotonically with x or turn back towards unity too slowly [7]. Various explanations have been advanced for the discrepancy, e.g., effects of the ∆ [8]-[11], and the influence of the Pauli exclusion principle [11], adjustment of parameters [12], but none of these changes provides a satisfactory description of the d̄/ū distribution function. The use of a soft πN∆ form factor enables a reasonable description of (d̄ − ū) [5], but then d̄/ū is too large . The importance of understanding the ratio d̄/ū was emphasized recently [13]. In this Letter we show that the inclusion of the ω meson along with pion cloud effects, provides a good description of the present data for both (d̄ − ū) and d̄/ū. The importance of the ω in providing the repulsive short-range part of the nucleon-nucleon and consequently attractive nucleon-antinucleon force has been known for a long time. The Compton wavelength of this meson is small, but the large (almost puzzlingly large g ω 4π ≈ 20) coupling constants universally used to describe the nucleon-nucleon scattering data cause the potential to be non-vanishing over a fairly large range. The ω meson also