Polarized parton distribution functions reexamined

The hard-gluonic contribution to the first moment of the polarized proton structure function gp1(x) is dependent on the factorization convention chosen in defining the quark spin density and the hard cross section for photon-gluon scattering. Two extremes of interest, namely, gauge-invariant and chiral-invariant factorization schemes, are considered. We show that in order to satisfy the positivity constraint for sea and gluon polarizations, the polarized valence quark distributions should fully account for the observed gp1(x) at x approx-gt 0.2. This together with the first-moment and perturbative QCD constraints puts a pertinent restriction on the shape of Δuv(x) and Δdv(x). The spin-dependent sea distribution in the gauge-invariant factorization scheme is extracted from the data of gp1(x). It is shown in the chiral-invariant scheme that it is possible to interpret the gp1(x) data with anomalous gluonic contributions, yet a best least χ2 fit to the data implies a gluon spin distribution which violates the positivity condition parallel ΔG(x)parallel ≤G(x). We then propose a more realistic set of parton spin distributions with sea polarization and with a moderate value of ΔG. The polarized parton distributions in this paper are presented in the next-to-leading order of QCD at the scale Q2=10 GeV2. Predictions for the polarized structure functions gn1(x) of the neutron and gd1(x) of the deuteron are given. copyright 1996 The American Physical Society