La chromodynamique quantique (QCD) rend compte de l'intéraction forte qui a lieu entre des degrés fondamentaux de quarks et gluons (i.e. partons) qui portent une charge nommée "couleur". À cause de la croissance de la force avec la distance on n'observe jamais de quarks libres, mais ils sont confinés à l'intérieur des hadrons (e.g. protons, neutrons, mésons). Si par contre la matière est soumise à des conditions extrêmes de pression et température, l'interaction de grandes distances est masquée par l'effet d'écrantage et les quarks et gluons sont déconfinés. Selon le modèle du Big Bang, la température pendant les premières phases de l'histoire de l'univers a été assez haute pour que la matière existe sous la forme d'un plasma de quarks et les gluons (QGP) déconfinés. Lors de l'expansion et du refroidissement de l'univers il y avait une transition de phases entre des degrés de liberté partoniques et hadroniques, où 99\% de la masse visible de notre univers a été créé.

À cause du couplage fort de la QCD les méthodes pertubatives ne peuvent pas être appliqué aux températures autour de la transition de phases. En ayant recours aux calculs sur réseau, il est montré qu'à partir d'une densité d'énergie de l'ordre 1-2GeV/fm³ l'état de QGP serait réalisé. La construction récente d'accélerateurs très puissants, comme le RHIC et le LHC, a permis de tester ces prédictions. Ceci se fait grâce à la collision d’ions lourds (Pb et Au) ultrarelativiste, qui génère un milieu extrêmement dense en atteignant des températures de quelques billions de degrés. Or, ces observations expérimentales suggèrent que la densité au centre de la boule de feu créée par ces collisions est plus élévée que la valeur critique.
 
Mes activités de recherche ont pour but de révéler le comportement des quarks et des gluons dans des milieus fortement couplés et la nature de la transition de phases de QCD. Dès mon doctorat, j'ai développé des vues globales sur différents aspects de la matière QCD à haute température et densité. J'ai ainsi étudié les effets dynamiques lors de la transition de phases et les sondes caractérisant les propriétés de QGP. Cela nécessite d'une compréhension de la phénoménologie, la connaissance des expériences, la maîtrise de la physique théorique fondamentale et la mise en oeuvre des méthodes numériques avancés.

Dynamical net-baryon fluctuations at the QCD critical point

The search for the critical point of QCD is one of the main goals of the beam energy scan at RHIC, the CERN-SPS and future facilities like FAIR at GSI and NICA in Dubna. In equilibrium, correlations diverge at the critical point leading to large event-by-event fluctuations in conserved quantities. For expanding systems like in heavy-ion collisions it is important to study the dynamical formation of long-range correlations in the critical region. The critical mode is the diffusive baryon current and can be described fluid dynamically. We include the propagation of fluctuations directly in the fluid dynamical equations. Using an equation of state with a 3D Ising model critical point we study the evolution of critical fluctuations, Gaussian and non-Gaussian. We solve the stochastic diffusion equation on a discretized numerical grid and compare to known analytical results for static systems. The nonlinear terms of the equation of state lead to the emergence of non-Gaussian correlations from underlying white noise. Finally, moving toward more realistic scenarios of heavy-ion collisions, the model to be established will be able to treat the formation of critical fluctuations in expanding systems. In future work, we will develop fully Fluctuating Dissipative Fluid Dynamics to be applied to a variety of dynamical critical systems.

Nonequilibrium Chiral Fluid Dynamics (NχFD)

NχFD focuses on the explicit stochastic propagation of the order parameter of chiral symmetry, the sigma field, and the order parameter of the confinement-deconfinement transition, the Polyakov-loop, coupled to a fluid dynamical expansion of the medium. Due to the stochastic evolution of the fields, the medium evolution is itself stochastic and fluctuations in fluid  dynamical quantities can be observed.  The numerical implementation of this dynamical model has shown the characteristic effects of critical slowing down at a critical point and supercooling, reheating and domain formation at a first-order phase transition at finite net-baryon density.

I have further developed this model during the past years in collaboration with Dr. Christoph Herold. We have in particular tested the impact of various equations of state with different descriptions of the low-temperature hadronic phase.
In order to investigate the experimental observables, we have recently included a particlization scheme for producing all hadronic particles and study net-proton fluctuations in comparison to experimental data. First results show promising features of nonequilibrium dynamics of critical fluctuations during the evolution of a heavy-ion collision. Systematic improvements, however, need to be addressed in future works. Beyond particlization it will be important to study the impact of final hadronic interactions and see if the dynamically formed fluctuations survive late stage effects.

Heavy-flavor dynamics in the QGP produced in heavy-ion collisions

Due to the short lifetime of the quark-gluon plasma (QGP) created in heavy-ion collisions no external probes can be used to explore its properties and one has to rely on probes, which are created during the collision itself and which distinguish themselves from the bulk by their high momenta and/or masses, such as jets and heavy quarks, or their decoupling from the strongly interacting medium, such as electromagnetic probes. The production of these probes and/or their subsequent interactions with constituents of the bulk medium illuminate the thermodynamic and transport properties of strongly interacting matter.
During my postdoctoral research at SUBATECH in Nantes, I coupled a Monte-Carlo Boltzmann transport model of heavy-quark dynamics, taking into account collisional interactions, incoherent and coherent gluon emission, to a 3 + 1 dimensional fluid dynamical evolution subsequent to the EPOS initial conditions. This approach yields very good agreement with the experimental data for observables like the nuclear modification factor and the elliptic flow of D mesons simultaneously. This simultaneous description remains challenging, although a couple of other models with to some extent very different ingredients and
transport coefficients can also successfully describe the same data. It is therefore important to go beyond these observables in AA collisions and look in particular at D Dbar (B Bbar) and D(B)-hadron correlations as well as higher-order flow harmonics. In exploratory studies I have shown that these observables are very promising for better constraining the interaction mechanisms, the transport coefficients of heavy quarks and their mass dependence. It is now compulsory to turn these first works into solid predictions. In accordance with the experimental situation heavy-quark observables need to be studied in an event-by-
event setup and coupled to a realistic evolution of the bulk matter during all stages of the collision, ranging from the initialization and the expansion of the plasma to the hadronization and hadronic reinteractions.

These projects are ongoing work in the EPOS-HQ project in collaboration with my colleagues at SUBATECH.

Bayesian model-to-data analysis of heavy-flavor transport coefficients

In collaboration with the QCD theory group at Duke University, North Carolina, USA, we are working on a rigorous application of Bayesian statistical methods to obtain the temperature and momentum dependence of the heavy-flavor transport coefficients in the QGP by comparing model calculations to a large set of experimental data.

Our most recent results were presented by Y. Xu at the Strangeness in Quark Matter 2017 conference in Utrecht.