Light quark distributions in the proton sea

Flavor asymmetry in the light quark sea of the proton has been well-established by experiment [1–4]. The violation of the Gottfried sum rule found by NMC [1] indicated that D ≡ ∫ 1 0 dx (d̄(x) − ū(x)) = 0.148 ± 0.039. In Drell-Yan experiments, d̄/ū was determined to be greater than 2 at x = .18 by NA51 [2], and the x-dependence of this ratio has recently been measured by E866 [3] in the region 0.02 ≤ x ≤ 0.345. From their data and global parton distributions [5] E866 also determined the difference d̄(x) − ū(x) and D = 0.100 ± 0.018. Flavor asymmetry consistent with these results has also been seen in the measurement of d̄(x)− ū(x) by HERMES [4]. It was first suggested by Thomas [6], and later by Henley and Miller [7] that a natural explanation for this asymmetry is the meson cloud of the proton. The net positive charge of the cloud leads to an excess of d̄ over ū. Other causes for the asymmetry have been invoked, such as antisymmetrization [8,9], but these have been insufficient to describe the data. For reviews, see Refs. [10,11]. We wish to emphasize that the E866 measurements provide a critical test of our understanding of the flavor-symmetric (FS) contributions to the light quark sea, as well as the flavor-asymmetric (FA) contributions. Meson cloud models [9,12–15] provide a reasonably good description of the d̄(x)− ū(x) asymmetry, which depends on FA contributions alone, but fail to explain the broad maximum in the ratio d̄(x)/ū(x) at x ≈ 0.18 and its return to unity, or even lower values, at larger x. FS terms also contribute to this ratio