Evolution equation for the structure function g(2)(x, Q(2))

We perform an extensive study of the scale dependence of flavor-singlet con tributions to the structure function g(2)(x, Q(2)) in polarized deep-inelas tic scattering. We find that the mixing between quark-antiquark-gluon and t hree-gluon twist-3 operators only involves the three-gluon operator with th e lowest anomalous dimension and is weak in other cases. This means, effect ively, that only those three-gluon operators with the lowest anomalous dime nsion for each moment are important, and allows to formulate a simple two-c omponent parton-like description of ga(x, Q(2)) in analogy with the convent ional description of twist-2 parton distributions. The similar simplificati on was observed earlier for the nonsinglet distributions, although the reas on is in our case different. (C) 2001 Elsevier Science B.V. All rights rese rved.