Quark-Gluon Plasma

Quantum chromodynamics (QCD) is the accepted theory of the strong interactions responsible for the binding of quarks into hadrons such as protons and neutrons, and the binding of protons and neutrons into atomic nuclei. The fundamental particles of QCD, the quarks and gluons, carry a new form of charge, which is called color because of its triplet nature in the case of the quarks (e.g. red, green, blue); gluons come in eight different colors which are composites of color and anticolor charges. However, quarks and gluons have never been observed as free particles. Nevertheless, because quarks have also electrical charge, they can literally be seen as constituents of hadrons by deep inelastic scattering using virtual photons. The higher the energy of the probing photon, the more do the quarks appear as particles propagating freely within a hadron. This feature is called "asymptotic freedom". It arises from so-called nonabelian gauge field dynamics, with gluons being the excitations of the nonabelian gauge fields similarly to photons being the excitations of the electromagnetic fields, except that gluons also carry color charges. Asymptotic freedom is well understood, and the Nobel prize was awarded to its main discoverers Gross, Politzer, and Wilczek in 2004.

Much less understood is the phenomenon of "confinement", which means that only color-neutral bound states of quarks and gluons can be observed. This confinement can be overcome when the temperature is very large, as, for example, in the first instances of the Early Universe. In this case, quarks and gluons form a quantum fluid that is known as the quark-gluon plasma. Its unique properties are studied on Earth in large collider facilities at LHC (CERN, Switzerland) or at RHIC (BNL, United States), where this plasma is created in ultrarelativistic heavy-ion collision experiments.

At our insitute, we investigate different thermal and nonthermal properties of the quark-gluon plasma, putting particular emphasis on its non-equilibrium early-time evolution shortly after the heavy-ion collision. Using a variety of different techniques, involving perturbative calculations, kinetic theory, hydrodynamics, holography, real-time lattice simulations and artificial neural networks, not only enables us to extract dynamical and universal key features of the plasma, but also to link to other fields of research like machine learning, gravity, the Early Universe and experiments with ultra-cold Bose gases.