Basics of Generalized Unitarity

TL;DR

This paper reviews generalized unitarity as a versatile method for deriving loop amplitudes from on-shell tree amplitudes, applicable to various theories and regularization schemes, and discusses its ability to transfer tree-level properties to loop level.

Contribution

It provides a comprehensive overview of generalized unitarity, including its application to non-supersymmetric theories, regularization techniques, and the transfer of tree-level symmetries to loop amplitudes.

Findings

Applicability to both supersymmetric and non-supersymmetric amplitudes.

Inclusion of non-planar contributions.

Demonstration of transferring tree-level properties like dual conformal symmetry to loop level.

Abstract

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