Factorization violation in the multiparton collinear limit

Understand

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.