Collinear Factorization and Splitting Functions for   Next-to-next-to-leading Order QCD Calculations
  We consider the singular behaviour of tree-level QCD amplitudes when the momenta of three partons become simultaneously parallel. We discuss the universal factorization formula that controls the singularities of the multiparton matrix elements in this collinear limit and present the explicit expressions of the corresponding splitting functions. The results fully include spin (azimuthal) correlations up to ${\cal O}(\as^2)$. In the case of spin-averaged splitting functions we confirm similar results obtained by Campbell and Glover. 
S. Catani and M. Grazzini