Introduction to Scattering Amplitudes Lecture 1: QCD and the Spinor-Helicity Formalism

Scattering amplitudes take surprisingly simple forms in theories such as quantum chromodynamics (QCD) and general relativity. This simplicity indicates deep symmetry. Recently, it has become possible to explain some of this symmetry. I will describe these insights and show how to derive amplitudes efficiently and elegantly. Key new ideas involve using complexified momentum, exploring singular behavior, and seeking clues in so-called twistor geometry. Complete amplitudes can be produced recursively. This streamlined approach is being applied in searches for new physics in high-energy particle colliders. Lecture 1: Color quantum numbers can be set aside. Helicity amplitudes have elegant expressions in terms of spinors. MHV (maximally helicity violating) amplitudes are the simplest.