  We perform the first simultaneous global QCD analysis of spin-averaged and
spin-dependent parton distribution functions (PDFs), including single jet
production data from unpolarized and polarized hadron collisions. We critically
assess the impact of SU(3) flavor symmetry and PDF positivity assumptions on
the quark and gluon helicity PDFs, and find strong bias from these,
particularly on the gluon polarization. The simultaneous analysis allows for
the first time extraction of individual helicity-aligned and antialigned PDFs
with a consistent treatment of uncertainties.
