Factorization violation in the multiparton collinear limit
  We present an all-order generalized factorization formula for QCD scattering amplitudes in kinematical configurations where two or more momenta of the external partons become collinear. The singular behaviour of the scattering amplitudes in the collinear limit is encoded by collinear splitting matrices. In the space-like region and beyond the tree level, the collinear splitting matrices depend also on the momenta and quantum numbers of the non-collinear partons, thus breaking strict collinear factorization. Although the factorization breaking contribution partly cancels for squared amplitudes, due to its one-loop absorptive origin, remaining effects at high perturbative orders have implications on the non-abelian structure of logarithmically-enhanced terms in perturbative calculations and on various factorization issues of mass singularities. 
Stefano Catani, Daniel de Florian, German Rodrigo
hep-ph hep-th