Evolution of twist-three parton distributions in QCD beyond the large-Nc limit

We formulate a consistent 1/N-c(2) expansion of the QCD evolution equations for the twist-three quark distributions g(2)(x,Q(2)), h(L)(x,Q(2)) and e(x,Q(2)) based on the interpretation of the evolution as a three-particle quantum-mechanical problem with hermitian Hamiltonian. Each distribution amplitude can be decomposed in contributions of partonic components with DGLAP-type scale dependence. We calculate the 1/N-c(2) corrections to the evolution of the dominant component with the lowest anomalous dimension - the only one that survives in the large-N-c limit - and observe a good agreement with the exact numerical results for N-c = 3. The 1/N-c(2) admixture of operators with higher anomalous dimensions is shown to be concentrated at a few lowest partonic components and in general is rather weak. (C) 2000 Elsevier Science B.V. All rights reserved.