Flavor Dependence of Sea Quark Structure Functions

Quark masses are shown to play an important role in the nucleon sea. Our analysis using massive QCD splitting functions demonstrates the existence of two Feynman-x sea regimes. For small x the strange sea is larger while at large x the non-strange light sea is dominant. This crossover effect has been observed in a phenomenological analysis by the CTEQ Collaboration. We also investigate the charm component in the nucleon. Permanent address: Department of Physics, Yonsei University, Seoul 120-749, Korea Present address: Department of Physics, University of Minnesota, Minneapolis, MN 55455 Investigations of hadron structure functions normally assume some form of SU(3) flavor symmetry of the sea-quark distributions. In a recent analysis of neutrino production of unlike-sign dileptons the CCFR Collaboration [1] found the strange-sea distributions at 〈Q〉 ≃ 20 GeV to be about one half the light-quark sea. A measure of the relative strange sea content as a function of Feynman x is given by κs(x) = 2s(x) ū(x) + d̄(x) . (1) If 〈q〉 is the momentum fraction carried by a quark, ∫ 1 0 xq(x)dx, the CCFR conclusion is 〈s〉 ∼ 1 2 〈ū or d̄〉 and hence 〈κs〉 = 2〈s〉/〈ū+ d̄〉 ∼ 1 2 . Following the CCFR result, a number of structure function analyses have been proposed with 〈κs〉 ∼ 1 2 at Q = Q0 and s(x) < ū(x)or d̄(x) at all Q, as shown in Fig. 1 with dot-dashed curve for MRS-D0 [2], for example. A recent global analysis by the CTEQ Collaboration [3] using all relevant data finds an improved fit by a more flexible sea parameterization. They find a crossover point for the relative sea content. For small x the strange sea is larger (and κs > 1) while at larger x the situation reverses, as shown in Fig. 1 by the dashed curve. In addition, because of increased s(x) in the small-x region, 〈κs〉 ≃ 0.9 instead of 0.5. In this paper we propose a natural explanation for the seemingly unexpected CTEQ result. Our main assertion is that quark-mass threshold effects persist to surprisingly large Q values, although they eventually disappear. The CTEQ results concerning the strange sea can be qualitatively accounted for within a straight-forward perturbative QCD framework. We begin with a brief discussion of our assumptions and method. In the QCD-improved parton model, the evolution of a quark density q(x,Q) by the Altarelli-Parisi evolution equation [4] becomes dq d lnQ = αs 2π ∫ 1