Basics of generalized unitarity

Abstract

We review generalized unitarity as a means for obtaining loop amplitudes from\non-shell tree amplitudes. The method is generally applicable to both\nsupersymmetric and non-supersymmetric amplitudes, including non-planar\ncontributions. Here we focus mainly on N=4 Yang-Mills theory, in the context of\non-shell superspaces. Given the need for regularization at loop level, we also\nreview a six-dimensional helicity-based superspace formalism and its\napplication to dimensional and massive regularizations. An important feature of\nthe unitarity method is that it offers a means for carrying over any identified\ntree-level property of on-shell amplitudes to loop level, though sometimes in a\nmodified form. We illustrate this with examples of dual conformal symmetry and\na recently discovered duality between color and kinematics.\n