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

Florian Cougoulic

Subatech (groupe Théorie)

vendredi 21 septembre 2018

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.