Nuclear effects in high-energy proton-nucleus collisions : transverse momentum broadening of energetic parton systems and soft anomalous dimension matrices

Abstract : In the Standard Model of particle physics,the theory of the strong interaction, Quantum Chromodynamics (QCD), is a gauge theory of symmetry group SU (Nc) with respect to the color quantum number. QCD obeys the property of asymptotic freedom, allowing the computation of high-energy physical observables using perturbative QCD (pQCD). This thesis deals with the pQCD description of hadron production rates in high-energy hadronic collisions, in view of applications to the phenomenology of proton-nucleus and nucleus-nucleus collisions at hadron colliders (RHIC,LHC), where so-called nuclear effects (shadowing, parton energy loss, transverse momentum broadening) come into play. In a first part, I study the transverse broadening of an energetic parton system crossing a nucleus, putting emphasis on the color structure of the process. A theoretical setup based on the dipole formalism is used,and a kinetic equation is derived for the parton pair transverse momentum distribution, requiring the parton pair to be in a given color state (SU (Nc) irreducible representation) both in the initial and final state. The color structure is encoded in a color evolution operator, which is obtained for any type of parton pair. For a small-size compact pair, the derivation yields a transparent physical interpretation of the pair transverse broadening process. In a second part, I discuss the soft anomalous dimension matrix Q, which is formally analogous to the previous evolution operator, and which appears when studying soft gluon radiation associated to 2 −> 2 hard parton scattering. It has been noticed that the Q-matrix associated to gg −> gg has a surprising symmetry (relating external and internal degrees of freedom). I developed tools to derive the Q-matrices associated to2 −> 2 scatterings involving generalized partons, in order to explore if the symmetry observed for gg −> gg is fortuitous or not.
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Florian Cougoulic. Nuclear effects in high-energy proton-nucleus collisions : transverse momentum broadening of energetic parton systems and soft anomalous dimension matrices. High Energy Physics - Phenomenology [hep-ph]. Ecole nationale supérieure Mines-Télécom Atlantique, 2018. English. ⟨NNT : 2018IMTA0086⟩. ⟨tel-02011152⟩

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