TASI Lectures on Scattering Amplitudes
Clifford Cheung
TL;DR
Cheung's TASI lectures present an on-shell program for scattering amplitudes, starting from spinor-helicity kinematics and bootstrapping three- and four-point data to build higher-point amplitudes. The methods include on-shell recursion (BCFW and all-line), soft theorems, and a detailed exploration of color-kinematics duality and the double-copy construction, demonstrating that gravity and various EFTs are intimately connected to gauge theory data. The work highlights that symmetry, locality, and analyticity tightly constrain the S-matrix, enabling a unified view across YM, gravity, and scalar EFTs, with implications spanning from quantum amplitudes to classical double-copy structures. Overall, the lectures argue that the on-shell viewpoint reveals deep, broadly applicable structures that simplify computations and suggest a deeper unity among fundamental interactions.
Abstract
These lectures are a brief introduction to scattering amplitudes. We begin with a review of basic kinematical concepts like the spinor helicity formalism, followed by a tutorial on bootstrapping tree-level scattering amplitudes. Afterwards, we discuss on-shell recursion relations and soft theorems, emphasizing their broad applicability to gravity, gauge theory, and effective field theories. Lastly, we report on some of the new field theoretic structures which have emerged from the on-shell picture, focusing primarily on color-kinematics duality.