Non-Equilibrium QCD

Non-Equilibrium Dynamics & Matter under Extreme conditions

We investigate the theoretical description of quantum many-body systems out of thermal equilibrium. By combining advanced numerical simulations with (semi-) analytical methods, we decode the complex, real-time dynamics of matter under extreme conditions—such as the early stages of ultrarelativistic heavy-ion collisions and the formation of the Quark-Gluon Plasma (QGP).

Key pillars of our research include:

• Universality Far From Equilibrium: We study how highly occupied quantum systems lose memory of their initial states and exhibit self-similar scaling behaviors. Our work uncovers universal attractor solutions that bridge seemingly unrelated fields, showing that the non-equilibrium evolution of non-Abelian plasmas shares a dual description with superfluid Bose gases (Berges et al., 2015).
• The Limits of Hydrodynamics: We determine precisely when macroscopically effective descriptions, like relativistic viscous hydrodynamics, become applicable to high-energy collisions. Using microscopic kinetic simulations, we map out pre-equilibrium boundaries and establish the range of validity for fluid dynamics in both large and small collision systems (Ambrus et al., 2023).
• Development of real-time methods: We develop functional methods, such as the real-time functional renormalization group (rFRG) and large scale numerical simulations to describe complex systems, such as stochastic fluids.

Further Reading:

• V. Ambrus, S. Schlichting, C. Werthmann Phys. Rev. Lett. 130 (2023) 15, 152301 
• X. Du, S. Schlichting Phys. Rev. D 104 (2021) 5, 054011 
• J. Berges, K. Boguslavski, S. Schlichting, R. Venugopalan Phys. Rev. Lett. 114 (2015) 6, 061601 

• J. Roth, Y. Ye, S. Schlichting, L. von Smekal arXiv:2603.17874 

Heavy-Ion Collisions & QGP Evolution

We develop comprehensive theoretical frameworks to model the entire spacetime evolution of ultrarelativistic heavy-ion collisions, tracing the system from the initial impact to the final freeze-out of hadrons. A core focus of our work is the "bottom-up" thermalization scenario, where we map how early-stage glasma fields transition into a chemically equilibrated Quark-Gluon Plasma. By connecting first-principles QCD dynamics with experimental observables, we also how to constrain the initial geometry and transport properties of the plasma in both large and small collision systems.

Review:

• S. Schlichting, D. Teaney Ann. Rev. Nucl. Part. Sci. 69 (2019) , 447--476 

Further Reading:

• M. Coquet, M. Winn, X. Du, J. Ollitrault, S. Schlichting Phys. Rev. Lett. 132 (2024) 23, 232301 
• A. Kurkela, A. Mazeliauskas, J. Paquet, S. Schlichting, D. Teaney Phys. Rev. Lett. 122 (2019) 12, 122302 
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Phase Transitions & Critical Dynamics

Strong interaction matter is expected to possess a critical point at finite net-baryon density, where it undergoes a second-order phase transition. Similarly, in the chiral limit of vanishing light-quark masses the transition between the hadronic phase with broken chiral symmetry and the chirally symmetric phase at vanishing net-baryon density in all likelihood also becomes of second order, which may have an imprint on the properties of real-world QCD, where the light quark masses are small. Near these critical points, static and dynamic properties of strong-interaction matter exhibit universal behavior, which can be characterized in terms of critical exponents and universal scaling functions. 

Further Reading:

• J. Roth, Y. Ye, S. Schlichting, L. von Smekal Phys. Rev. D 111 (2025) 11, L111901 
• L. Sieke, M. Harhoff, S. Schlichting, L. von Smekal Nucl. Phys. B 1011 (2025) , 116808 

Gluon Saturation & High-Energy Scattering in QCD

We investigate high-energy scattering processes in Quantum Chromodynamics (QCD) across a wide range of collision systems, encompassing proton-proton (p+p), proton-nucleus (p+A), nucleus-nucleus (A+A), and electron-nucleus (e+A) collisions. Our focus is the regime where rapid gluon growth leads to non-linear recombination and gluon saturation. Using the Color Glass Condensate (CGC) effective field theory, we study how this saturated state sets the universal initial conditions for high-energy interactions. Our work explores the impact of non-linear QCD dynamics on key experimental observables, such as forward particle production and particle correlations, providing clean insights into the nuclear wave function that are vital for both current hadron colliders and future e+A programs. Furthermore, we develop advanced numerical and analytical frameworks to track how these highly occupied gluon fields decorrelate and evolve immediately after the initial impact, bridging the gap between saturation physics and pre-equilibrium plasma dynamics.

Review:

• O. Garcia-Montero, S. Schlichting Eur. Phys. J. A 61 (2025) 3, 54 

Further Reading:

• O. Garcia-Montero, S. Schlichting, J. Zhu Phys. Rev. D 111 (2025) 7, 076029 
• O. Garcia-Montero, Y. Hoffmann, S. Schlichting arXiv:2511.22763 

QCD Jets

Highly energetic particles created at the early stages of heavy ion collisions act as tomographic probes of the QGP. These jets traverse the medium and lose energy through interactions, a phenomenon known as jet quenching, allowing us to extract fundamental properties such as transport coefficients.

Further Reading:

• Y. Mehtar-Tani, S. Schlichting, I. Soudi JHEP 05 (2023) , 091 
• S. Schlichting, I. Soudi Phys. Rev. D 105 (2022) 7, 076002