In the Standard Model of particle physics, the theory of the strong interaction, Quantum Chromodynamics (QCD), is a gauge theory of symmetry group SU(N) with respect to the color degree of freedom. 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 (pA) and nucleus-nucleus (AA) 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 SU(N) 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(N) 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 theQ-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 verify if the symmetry observed for gg → gg is fortuitous or not.
Keywords : pQCD, transverse momentum broadening, soft anomalous dimension matrix.
