Infrared singularities in QCD amplitudes

Understand

We review recent progress in determining the infrared singularity structure of on-shell scattering amplitudes in massless gauge theories.

• We present a simple ansatz where soft singularities of any scattering amplitude of massless partons, to any loop order, are written as a sum over colour dipoles, governed by the cusp anomalous dimension.
• We explain how this formula was obtained, as the simplest solution to a newly-derived set of equations constraining the singularity structure to all orders.
• We emphasize the physical ideas underlying this derivation: the factorization of soft and collinear modes, the special properties of soft gluon interactions, and the notion of the cusp anomaly.