NNLO Determination of Polarized Parton Distribution Functions with Higher-Twist and Target-Mass Corrections
We present a next-to-next-to-leading order (NNLO) QCD analysis of polarized parton distribution functions (PDFs) based on the world data set of inclusive polarized deep-inelastic scattering (DIS). The resulting PDF set, denoted as \texttt{KLS26}, includes a consistent treatment of target-mass corrections (TMCs), higher-twist (HT) contributions, and positivity constraints within a fully NNLO framework. To estimate the residual theoretical uncertainty associated with nonperturbative power corrections, both additive and multiplicative HT parametrizations are considered. Although these two approaches lead to different higher-twist coefficients, the current results yield very similar fit qualities and compatible polarized PDFs within uncertainties. The impact of NNLO corrections is assessed through a comparison with the KLSS21 polarized PDF determination at NLO, showing the most visible changes in the polarized strange-quark distribution, together with moderate modifications in the gluon sector and smaller effects in the up- and down-quark helicity distributions. Additional fits treating the nonsinglet axial charges as free parameters, together with different higher-twist implementations, induce small but flavor-dependent modifications in the PDFs, with the most noticeable effects in the polarized strange-quark distribution and moderate sensitivity in the gluon. The \texttt{KLS26} set establishes an updated determination of helicity-dependent parton distributions, enabling future precision studies of nucleon spin structure.