Unitarity methods and on-shell particles in scattering amplitudes

The Standard Model of particle physics describes all known elementary particles and their interactions. Important tests of this theory are performed with high-energy particle scattering experiments, for instance at the Large Hadron Collider. Such scattering processes are impressively well described by the Standard Model, although there is room for small differences between theory and experiment. Investigation of this possibility requires large experimental and theoretical precision, and may hold the key to better understanding the world of elementary particles.
This thesis presents new tools for making precise theoretical predictions. Each tool imposes on-shell restrictions on scattering amplitudes, which tremendously simplifies the calculation of key ingredients for theoretical predictions. The theory of unitarity forms the basis of this method and is explained in chapter two of this thesis. The third chapter describes a computer program for modeling the decay of unstable particles in a general and realistic manner. Chapter four uses the on-shell restriction to extract information from scattering amplitudes about intriguing effects of infrared gluon/photon radiation. Finally, the fifth chapter demonstrates for the first time that unitarity can be used to compute the cross section of the important Drell-Yan scattering process in a direct way.