
Particle Physics  Themes 
The following is a selective list of themes, not meant as a full coverage of our research activities in this field, but to highlight for the nonexpert reader a few important notions or topics and to provide corresponding entry points for the research groups list, to which you could go directly by clicking here.
Analyzing Field Theories by Flow Equations
The functional renormalisation group (FRG) provides a way of studying field theoretical and statisticalmechanical models systematically as a function of a scale (e.g. a length scale or energy scale). The method has been applied to theories on all energy scales, ranging from ultracold gases to quantum gravity, both in equilibrium and for timedependent phenomena. In many cases it has provided a detailed understanding of phase diagrams and physical observables. Important contributions to renormalisation group theory have been made by people in Heidelberg, and our institute is one of the major centers worldwide in this area.
GroupsHiggs Particle
The hypothetical particle that is postulated to be the carrier particle, or boson, of the Higgs field, a theoretical field that permeates space and endows all elementary subatomic particles with mass through its interactions with them. The field and the particle  named after Peter Higgs , one of the physicists who first proposed this mechanism  provide a testable hypothesis for the origin of mass in elementary particles.
GroupsString Phenomenology
String Theory is the leading candidate for a unified quantum theory of gravity and particle physics. It describes elementary particles as onedimensional objects propagating in ten spacetime dimensions. Physics in four dimensions arises upon compactification of the six extra dimensions. String phenomenology studies the implications of this theory for particle interactions and cosmology. This covers a wide range of topics from more formal investigations of the mathematics of the compactification spaces to an analysis of the phenomenology of the resulting lowenergy theory.
GroupsSupersymmetry and Extra Dimensions
One of the challenges of the Standard Model is to explain the smallness of the electroweak scale as compared to the fundamental Planck scale. One possible answer is provided by Lowenergy Supersymmetry, where new particles at the TeV scale help stabilise the Higgs particle mass. This theory is also attractive from the viewpoint of Grand Unification and Dark Matter. In models with Large Extra Dimensions, the observed particle interactions are confined to a subspace in a higherdimensional space, while gravity propagates in all dimensions. The fundamental scale of gravity itself can thus be lowered to the TeV scale
GroupsDark Matter Particles
There is by now incontrovertible evidence from cosmology that by far the largest part
of matter in the Universe is made of particles that are not contained in the Standard Model of
Particle Physics, thus providing a strong argument that this model is really incomplete.
Among the proposed dark matter candidates, many share the property of being
heavy, neutral and weaklyinteracting. These particles are called WIMPs.
Prominent and probably most extensively discussed alternatives are the lightest supersymmetric particle,
and as light dark matter candidates the gravitino and the axion.
QCD Phase Transitions
QCD at finite temperature and density is governed by the phenomena of confinement and chiral symmetry breaking. Its phase structure is is relevant for our understanding of current and future experiments at LHC, RHIC and GSI.
Access to the phase diagram of QCD can only be obtained with nonperturbative methods, ranging from lattice computations, renormalisation group methods to stringinspired methods such as AdS/QCD correspondence.