PhD in theoretical physics (M/F) – Quark-gluon tomography of nuclei at high energy

This PhD project explores the three-dimensional structure of hadrons and the emergence of gluon saturation in Quantum Chromodynamics (QCD). It focuses on Transverse Momentum Dependent (TMD) parton distributions at small longitudinal momentum fraction (x), a regime where theoretical understanding remains limited. The project combines next-to-leading order calculations within the Colour Glass Condensate (CGC) effective theory with phenomenological studies of key processes at the Large Hadron Collider, such as forward particle production in proton–nucleus collisions. A dedicated numerical framework will be developed to confront theoretical predictions with experimental data. The goal is to provide new insights into parton dynamics at high energies and to identify clear signatures of gluon saturation.

This PhD project tackles some of the most fundamental open questions in our understanding of the strong interaction, one of the four fundamental forces of Nature. The strong force governs phenomena across a vast range of scales—from the binding of atomic nuclei at low energies to the intricate internal structure of protons and nuclei explored in high-energy collisions at major facilities such as the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC).

At short distances, the strong interaction is described by Quantum Chromodynamics (QCD), the theory of quarks and gluons — collectively known as partons — which combine to form hadrons. Despite its remarkable success, QCD still presents profound theoretical challenges. This project focuses on two closely intertwined questions at the forefront of modern research: how partons are distributed inside hadrons, and the emergence of gluon saturation at high energies.

A central goal of the project is to build a three-dimensional picture of hadron structure, encoding both the longitudinal momentum and transverse motion of quarks and gluons. This information is captured by Transverse Momentum Dependent (TMD) parton distribution functions. While parton tomography is essential for understanding how hadrons acquire their mass and spin, the behavior of TMDs in the small longitudinal momentum fraction (low-x) regime remains largely unexplored. Since first-principles lattice QCD approaches are currently unable to access this region, progress relies on high-precision perturbative QCD calculations combined with experimental data. However, beyond leading-order accuracy, such studies are still in their infancy at low x.

In parallel, the project addresses gluon saturation, a striking phenomenon expected to arise at very high collision energies. In this regime, hadrons are dominated by a dense system of low-x gluons, where nonlinear recombination effects tame the growth of gluon densities and ensure consistency with fundamental unitarity constraints. This highly occupied gluonic matter is described by the Colour Glass Condensate (CGC), an effective field theory of QCD in the presence of strong gluon fields. Although theoretically compelling, gluon saturation has yet to be unambiguously established experimentally, and its quantitative features remain poorly constrained.

The objective of this PhD project is to perform next-to-leading order (NLO) calculations within the CGC framework for a set of key processes whose cross sections can be factorized in terms of gluon and sea-quark TMD distributions. These observables are directly relevant to current and future experiments at the LHC, including forward particle production in proton–nucleus (pA) collisions, a flagship physics program of LHCb and the future ALICE upgrade with the FoCal detector. Representative processes include forward photon–jet correlations, as well as inclusive and diffractive dijet or heavy-meson production in ultra-peripheral collisions. In addition, the project will involve the development of a dedicated numerical code to implement these calculations and perform detailed phenomenological studies. 

The ultimate goal is to identify clear and robust signatures of gluon saturation through the extraction of gluon and quark TMD distributions from experimental observables.