Résumé:
The growing precision of our experimental apparatus requires relevant advances in the accuracy of
our theoretical cross-section computation. Implying that we need to be able to calculate multi-leg
and/or multi-loop corrections to various amplitudes. The purpose of this thesis is to come to this
aim and to demonstrate the affinity of the Spinor Helicity Formalism in dealing with Next-to-leading
order (NLO) computations related to the QCD sector. We start by employing Helicity spinors
in writing scattering amplitudes resulting in expressions with a high potential for simplifications,
then introduce complex momenta to find a recursive method for building Helicity tree amplitudes
and at last we used Generalized Unitarity to connect amplitudes in an order by order manner.
The results were a generic one-loop formulation for Helicity amplitudes that is entirely determined
through cut-coefficients along with a procedure for the extraction of box and triangle coefficients
generalizable to the remaining ones. We found that cut-coefficients were built out of the product of
Helicity tree amplitudes which themselves are constructed using lower-point amplitudes recursively,
meaning that the NLO was ultimately linked to the kinematic 3-points of the theory and that even
higher orders will necessarily employ available lower order found amplitudes.
Keywords : High-energy physics, SM, QCD, Spinor Helicity Formalism, Helicity Amplitude,
Complex momenta, Recursion formulae, NLO, Generalized Unitarity, On-shell cut-coefficients Extraction
