**Research Focus**

- String Theory and and String Phenomenology
- D-Brane Models
- Heterotic Strings
- Compactifications

**String theory and string phenomenology**

String theory is the most promising framework for a unified description of all fundamental interactions, in other words it describes quantum field theory and quantised gravity simultaneously.

One dimensional vibrating objects, the “strings”, replace the conventional description of pointlike particles. There exist two different types: the omnipresent “closed strings”, on which the graviton arises as massless excitation with spin two, and the “open strings” which end on dynamical extended higher-dimensional object, the “D-branes”.

There exist five different superstring theories: the heterotic E8 X E8 and SO(32) string theories only have closed strings, whereas the Type I, IIA and IIB theories have both closed and open strings. These at first seemingly different string theories are connected by a web of symmetries, the “string dualities”.

All superstring theories are consistently defined in ten space-time dimensions, and their low-energy limit is supergravity and supersymmetric field theory. Supersymmetry relates fermionic and bosonic degrees of freedom with otherwise identical quantum numbers in quantum field theory, and supergravity extends this concept to gravitation. In order to relate string theory to particle physics in our four dimensional world, six of the extra space dimensions must be very small and compact, and part of the supersymmetry is broken by special choices of these compact dimensions.

Key questions in this context are: How is the Standard Model of particle physics embedded in string theory? How does the particle content arise in string compactifications? What can we say about the low-energy field theory, masses and interactions? How is the remaining supersymmetry broken in field theory? What is the string scale, and can there be direct experimental evidence for string theory? If string theory contains the Standard Model of particle physics, does this also explain cosmological data consistently?

The “Intersecting D-brane scenario” in Type IIA string theory has open strings ending on D6-branes, which span our four dimensional world plus half of the small extra dimension, where they intersect in points. This provides a geometrically very intuitive framework for engineering the Standard Model or Grand Unified Theory particle spectra, and for toroidal orbifolds as compact backgrounds the low-energy field theory can be computed exactly by means of conformal field theory.

The heterotic E8 X E8 string is historically the best studied one, but new model building options have opened up in the last years by combining techniques from Type II and heterotic strings. In view of the string duality relations, it is interesting to explore if heterotic models lead to the same low-energy physics as the intersecting brane scenario.