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Perturbative study of selected exclusive QCD processes at high and moderate energies

Abstract : At high enough energies, QCD processes can be factorized into a hard part, which can be computed by using the smallness of the strong coupling to apply the perturbative Feynman diagram method, and a non-perturbative part which has to be fitted to experimental data, modeled or computed using other tools like for example lattice QCD. However the smallness of the strong coupling in the perturbative part can be compensated by large logarithms which arise from the cancellation of soft or collinear divergences, or by the presence of multiple kinematic scales. Such logarithmically-enhanced contributions must be resummed, leading to the DGLAP evolution at moderate energies and to the BFKL or B-JIMWLK equation in the high energy limit. For the largest energies gluon recombination effects lead to saturation, which can be described in the color glass condensate (CGC) or shockwave formalism. In this thesis, we propose to study several exclusive perturbative QCD processes in order to get a better understanding of factorization, resummation and saturation effects. In the first part we perform the first computation of an exclusive quantity at Next-to-Leading-Order (NLO) accuracy using the QCD shockwave formalism. We calculate the NLO amplitude for the diffractive production of an open quark-antiquark pair, then we manage to construct a finite cross section using this amplitude by studying the exclusive diffractive production of a dijet. Precise phenomenological and experimental analysis of this process should give a great insight on high energy resummation due to the exchange of a Pomeron in diffraction, which is naturally described by the resummation of logarithms emerging from the soft divergences of high energy QCD. Our result holds as the center of mass energy grows towards the saturation scale or for diffraction off a dense target so one could use it to study saturation effects. In the second part we show how the experimental study of the photoproduction of a light meson and a photon at moderate energy should be a good probe for Generalized Parton Distributions (GPDs), one of the generalizations of the non-perturbative building blocks in collinear factorization. In principle such a study would give access to both helicity-conserving and helicity-flip GPDs. We give numerical predictions for this process at JLAB@12GeV.
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Submitted on : Wednesday, February 15, 2017 - 3:26:06 PM
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  • HAL Id : tel-01468540, version 1



Renaud Boussarie. Perturbative study of selected exclusive QCD processes at high and moderate energies. High Energy Physics - Phenomenology [hep-ph]. Université Paris Saclay (COmUE), 2016. English. ⟨NNT : 2016SACLS280⟩. ⟨tel-01468540⟩



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