The Large Hadron Collider (LHC) has provided, and will continue to provide, data for collisions at the highest energies ever seen in a particle accelerator. A strong knowledge of the properties of amplitudes for Quantum Chromodynamics in the High Energy Limit is therefore important to interpret this data. We study this limit in the context of the High Energy Jets (HEJ) formalism. This formalism resums terms in the perturbative expansion of the cross-section that behave like αn/s log (s/-t)ⁿ¯¹, which are enhanced in this limit. Understanding this region is particularly important in certain key analyses at the LHC: for example, Higgs-boson- plus-dijet analyses where cuts are applied to pick out events with a large mjj and in many searches for new physics. In this thesis, we discuss two directions in which HEJ's accuracy has been improved. Firstly, we look at adding descriptions of partonic subprocesses which are formally sub-leading in the jet cross-section but Leading Logarithmic (LL) in the particular subprocess itself. This required the derivation of new effective vertices that describe the emission of a quark/anti-quark pair in a way that is consistent with the resummation procedure. The inclusion of such processes reduces HEJ's dependence on fixed-order calculations and marks an important step towards full Next-to-Leading Logarithmic (NLL) accuracy in the inclusive dijet cross-section. The second extension was to improve our description of events involving the emission of a Higgs boson along with jets. Specifically, we derive new effective vertices which keep the full dependence on the quark mass that appears in the loops that naturally arise in such amplitudes. The formalism is also simple enough to allow for any number of extra nal state jets in the process. Therefore, HEJ is unique in its ability to provide predictions for high-multiplicity Higgs-plus-jets processes with full nite quark mass e ects. Such a calculation is far beyond the reach of any xed order approach.

Lorsqu'un boson Z de haute impulsion se désintègre en paire e+e-, l'électron et le positron laissent des traces très rapprochées l'une de l'autre, ce qui rend la reconstruction de ce processus peu efficace. L'identification de signature leptoniques d'évènements massifs issus de théories telles l'extension du groupe de jauge électrofaible SU(2)_1 x SU(2)_2 x U(1) pour ajouter des bosons massifs W' et Z' se couplant directement aux bosons vectoriels et aux fermions du modèle standard est important dans l'optique de recherche de nouvelle physique puisque les états finaux leptoniques fournissent le canal de détection de plus pur. Ici, un outil d'identification des paires d'électron-positron issues de bosons Z à haute impulsion basé uniquement sur les variables décrivant les jets est développé en se basant sur des simulations d'évènements massifs produits dans le détecteur ATLAS du LHC au CERN. Une étude topologique des évènements candidats est portée dans un ensemble d'échantillons Monte Carlo. Une série de coupures rectangulaires est optimisée sur un échantillon de bosons Z produits avec une haute impulsion. L'efficacité de reconstruction des paires d'électron-positron est maximisée. Cinq sources de bruit de fonds ont été soumises aux critères de sélection pour l'étude de la suppression du bruit de fonds. Finalement l'outil développé est comparé à l'identification de paires d'électron-positron basé sur la reconstruction usuelle d'objets leptoniques au LHC. Cette comparaison met en évidence le gain significatif de l'outil développé, atteignant une efficacité de reconstruction de 93% dans des échantillons Monte Carlo à haute énergie, avec un bruit de fond négligeable. / When a Z boson with high transverse momentum desintegrates into an e+e- pair, the electron and the positron leave nearly superimposed tracks which makes the detection process very difficult. The identification of massive events arising from theories like the extension of the electroweak gauge group SU(2)_1 x SU(2)_2 x U(1) leading to the existence of massive W' and Z' bosons coupling directly to standard model vector bosons and fermions is more readily performed through leptonic channel. Here, a series of rectangular cuts comprising a purely jet substructure based electron positron pair arising from the decay of a Z boson identification in boosted topologies (p_T(Z)>1TeV) is presented. The selection process of the cuts is done through a topology study of candidate events in various Monte Carlo simulation samples. The reconstruction efficiency of the electron-positron pairs is maximised. Five background sources were tested against the selection criteria to study the background rejection efficiency of the methodology. Finally, it is found that the reconstruction tool developped nearly triples electron positron identification, reaching 93% efficiency in high energy Monte Carlo samples, with insignificant background noise.