Precision supersymmetry at the LHC

Based on collider experiments in the last decades, the so-called 'Standard Model' of elementary particle physics has been established, which incorporates all elementary particles that have been observed so far and their interactions. It is based on symmetries called 'local gauge symmetries', which constrain the possible interactions of the fundamental particles. In fact, the predictive power of the Sandard Model is so strong that the existence of particles could be inferred which have been found only years later at collider experiments. Examples include the W and Z bosons which mediate the weak interactions and whose experimental discovery lead to the Nobel Prize in 1984, and the top quark whose mass could be inferred before its discovery by comparing precision predictions with precision measurements (Nobel Prize 2000). However, one ingredient of the Standard Model of elementary particle physics, called the 'Higgs boson' after the Scottish physicist Peter Higgs who first suggested it, has not been found at collider experiments up to now. It is a very important particle as it offers an explanation how particles can have mass without destroying the symmetries which are vital for the coherence of the theory. Further, we know from arguments of mathematical consistency that the Standard Model is most likely only a subpart of a more general theory which we do not know yet. A very appealing extension of the Standard Model is Supersymmetry, which predicts a 'supersymmetric partner' to each particle in the Standard Model and, due to its special symmetry properties, can cure the mathematical deficiencies of the Standard Model in an elegant way. Supersymmetry predicts the existence of five Higgs bosons, where the lightest one has similar properties as the one predicted by the Standard Model. From previous and present collider experiments, combined with precise theoretical calculations, we can deduce that the lightest Higgs boson has to lie in a certain mass range, which corresponds to an energy that could not be reached by collider experiments so far. However, a proton-proton collider, the 'Large Hadron Collider' (LHC) is being constructed at the moment at CERN, the European Laboratory for Particle Physics, which can probe an energy range where the Higgs boson cannot escape. The LHC will start operating in 2007 and is one of the biggest experimental endeavours physics has ever seen. In order to be able to detect Higgs bosons and supersymmetric particles among the wealth of 'ordinary' particles that will be produced at the LHC, very precise theoretical calculations are necessary. Only by knowing exactly the production rates and properties of the particles which are predicted by the different models, i.e. the Standard Model, different variants of supersymmetric models or non-supersymmetric extensions of the Standard Model, we can decide unambiguously which of our models is actually realised in nature. In our research programme we will develop and provide precise evaluations of supersymmetric observables. This is of high relevance for the success of LHC experiments in three ways. First it is useful to extract Higgs boson/new particle 'signals' from a huge 'background' of known particles. Second, precise theoretical predictions are the only way to discriminate between different models explaining the new findings. Finally, comparing these precision calculations with experimental data will allow to determine the new model parameters, to test links to cosmology (can one of the new particles be responsible for the dark matter in the universe?) and grand unification of all forces (do the model parameters display a pattern that is compatible with grand unification?) and thus to establish a new and more complete picture of fundamental interactions.