Massive Spinor Helicity Amplitudes, Cross Sections, and Coalescence

We examine recent advancements of the spinor helicity formalism of massive particles. Technical aspects about the formulation of massive helicity spinors are presented in detail to analyze the projective-geometry kinematics of helicity spinors as well as the diagrammatical and analytical structure of their interactions. Two new methods for calculating massive cross sections are derived and tested on Bhabha and Compton processes: a quasi-high-energy limit and an assembly of partial cross sections. The acquisition of mass, where ultrarelativistic amplitudes coalesce at low energy, is given a physical interpretation as the localization of particle worldlines in twistor theory. By subsuming the spinor helicity formalism in this way, both spacetime and particle content can emerge from null, lightlike, and timelike twistors.