Abstract

Although the field of one-loop computations is mature, parton multiplicities in hard LHCevents 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.