Spin Structure Functions of the Nucleon and Twist-3 Operators in QCD

Abstract

We investigate the twist-3 spin-dependent parton distribution functions h_L(x; Q²) and g_T (x; Q²). We discuss the physical relevance of the parton distributions from the view point of the factorization theorem in QCD. A unique feature of the ‘measurable’ higher-twist distributions h_L and g_T is emphasized. We investigate the Q² -evolution of h_L and g_T in the framework of the renormalization group and standard QCD perturbation theory. We calculate the anomalous dimension matrix for the twist-3 operators for h_L and g_T in the one-loop order. The operator mixing among the relevant twist-3 operators, including the operators proportional to the QCD equations of motion, is treated properly in a consistent scheme. Implications for future experiments are also discussed.