Research: Theory - Overview
Particle physics experiments at high-energy accelerators like the Large Hadron Collider (LHC) and the Tevatron allow us to study the basic laws of physics at the smallest length scales. The primary purpose of these machines is to look for the mechanism underlying electro-weak symmetry breaking - to test if the theoretically postulated Higgs sector of the Standard Model of particle physics exists. If it does not then these machines should give us insight into what else breaks electro-weak symmetry and thus gives mass to the elementary particles. In addition, there is hope that these experiments will be able to test ideas for completely new physics beyond the Standard Model, such as those strongly suggested by its incomplete mathematical structure. (-> Beyond the SM)
An equally important, but even less understood issue is the so-called flavour problem which, in the context of the Standard Model, translates into the question of why three generations of quarks and leptons exist, and why they transform in the way they do under the symmetries of the Standard Model. Most of the parameters of the Standard Model are related to its flavour structure and their origins are not yet theoretically understood. Despite this, the experimentally observed pattern of fermion masses and mixings shows tantalizing regularities, suggesting that some deeper explanation is behind them. Neutrino physics (-> Neutrinos) provides very valuable new insights in this context. In recent years the discovery of neutrino oscillations (-> GLoBES) and neutrino masses (-> MANITOP, REAP) has opened a new and unique window onto this issue. Observations at vastly different energy scales from various natural and artificial neutrino sources have become another valuable source of information for the study of the flavour problem and to probe new physics. In fact, the discovery of neutrino masses was the first solid evidence for new physics beyond the Standard Model. It showed for the first time that the leptons possess completely unexpected large mixings, and therefore changed our picture of the mechanisms responsible for the generation of the fermion masses significantly. Today it is widely believed that the smallness of the neutrino masses is caused by the so-called see-saw mechanism in one of its variants, but the observed mixing patterns require further ingredients. One particular implication of the see-saw mechanism is that the baryon asymmetry of the Universe (which can not be explained within the Standard Model) can be explained as a consequence of a lepton asymmetry produced in the very early universe. Further insight into the flavour problem and fundamental physics beyond the Standard Model might be gained from the search for very rare lepton flavour violating processes such as the decay μ → e + γ. The searches for this and similar processes are expected to reach new levels of sensitivity in the coming years.
It is a curious feature of nature that measurements on very large scales can also tell us something about the physics on the smallest scales. The evolution of astronomical objects, and indeed the whole Universe, depends in various ways, on the microscopic laws of nature. This lessen has been learned from the successfull theoretical prediction of observable cosmological properties, based on observations made in labaratory experiments and the physics inferred from these. The close connections between cosmology, astronomy and elementary particle physics allows for mutual benefit for these fields. For this reason the newer field of Astroparticle Physics combines knowledge from different fields to gain information which would otherwise not be available. A global, combined analysis of observational data in order to unveil fundamental physics requires a broad theoretical expertise in particle physics, astrophysics and cosmology. The goals of this program are to unveil the basic laws of nature, to understand the observed particles and their interactions, to uncover the nature of Dark Matter and Dark Energy (-> Dark Matter) and to study the role of neutrinos in astrophysics and cosmology. Examples for the ambitious theoretical efforts required for its success include the study of physics of matter at extreme temperatures and densities, in and out of equilibrium (conditions which prevail during the first few seconds after the big bang) and the study of unified theories and their implications. This broad theoretical enterprise also provides guidance for the long-term goals of our experimental activities as it inspires the search for new physics in laboratory experiments.