  We present a next-to-next-to-leading order (NNLO) global QCD analysis of the
proton's helicity parton distribution functions (PDFs), the first of its kind.
To obtain the distributions, we use data for longitudinal spin asymmetries in
inclusive and semi-inclusive lepton-nucleon scattering as well as in weak-boson
and hadron or jet production in proton-proton scattering. We analyze the data
using QCD perturbation theory at NNLO accuracy, employing approximations
provided by the threshold resummation formalism in cases where full NNLO
results for partonic hard-scattering functions are not readily available. Our
numerical results suggest a remarkable perturbative stability of the extracted
helicity PDFs.
