Abstract

Although the field of one-loop computations is mature, parton multiplicities in hard LHC events are so high that traditional computational techniques become inefficient. Recently new approaches based on unitarity have been developed for calculating one-loop scattering amplitudes in quantum field theory. These methods are especially suitable for the description of multi-particle processes in QCD and are amenable to numerical implementations. They are also quite peculiar in that on-shell tree scattering amplitudes, rather than Feynman diagrams, can be used as an input. I will discuss the new methods in a pedagogical fashion and will explain conceptual and technical aspects of the new methods.