Theoretical physics is the discipline that aims at describing how the world works in terms of fundamental equations. The goal is to abstract explicit phenomena by reducing them to underlying principles that are responsible for many different manifestations in nature. Physical processes are often the basis for other natural sciences, e.g. quantum mechanics is important to understand atoms and therefore chemistry. Establishing knowledge about the microscopic dynamics is crucial to understand macroscopic emergent phenomena. Theoretical physicists at FIAS are working on complex models on very different scales from the elementary particles in the universe via the nano-meso scales of protein structures to huge objects like neutron stars.

### Strongly Interacting Matter

The properties of strongly interacting matter under extreme conditions, governed by the theory of Quantum Chromo Dynamics (QCD), are still largely unknown. At high temperatures - accessible in heavy ion collisions - QCD predicts a phase transition from ordinary matter to the Quark-Gluon-Plasma, where the elementary constituents become unconfined. Other interesting new phases of strongly interacting matter, e.g. color superconductivity, at high densities and low temperatures might be realized in the interior of compact stars.

At FIAS, theoretical physicists focus on understanding the properties of strongly interacting matter using a broad range of methods and tools:

- Calculations of the equation of state and the phase diagram with effective models based on hadronic and partonic degrees of freedom, e.g. different versions of the relativistic mean field model, chiral models with Polyakov loop and dilaton field, resonance gas model
- Dynamical modeling of the complex many-body dynamics of heavy-ion collisions, using methods from classical and quantum mechanics, statistical mechanics, transport theory, quantum field theory, and gravity duality. The employed models include Relativistic Hydrodynamics, transport codes (e.g. Parton-Hadron String Dynamics PHSD) and Quantum Molecular Dynamics (e.g. Ultrarelativistic Quantum Molecular Dynamics UrQMD)
- Studies on the structure of exotic nuclei away from the band of stability including hypernuclei and antinuclei by solving the corresponding many-body problems. Binding energy and excitation spectra of these nuclei help understand the syntheses of heavy atoms in stellar explosions.
- Investigations of the interaction of ions with matter by advanced numerical simulations (GEANT4), especially relevant to heavy ion cancer therapy.

Interdisciplinary collaboration especially with computer scientists is essential for state-of-the-art research in theoretical physics. Theorists at FIAS closely collaborate with their experimental colleagues working at major accelerator centers like GSI (Darmstadt), CERN-LHC (Geneva), BNL-RHIC (Brookhaven) and are closely involved in the preparations for the future FAIR experiments within the program HIC for FAIR.