# Particle physics theory

The task of theoretical particle physics is to develop a description of the fundamental laws of nature, confronting theory predictions with the individual observations and combining them into a bigger picture, as well as suggesting new ways of probing nature. The research of the theory groups at DESY and KIT is closely tied to experiments such as the ones running at the LHC, Jefferson Lab, J-Parc, the fixed-target and neutrino experiments at CERN and Fermilab and the experiments Belle II and ALPS. On the other hand, with their broad scientific spectrum, including particle phenomenology, particle cosmology and lattice and string theory, the groups are able to address interrelated challenges and complex problems using the synergies between their different branches. Exploring and developing theories and finding signs of physics beyond the SM of particle physics, for example, benefits from the combination of work in particle phenomenology with input from lattice theory, cosmology and new developments from string theory. Similarly, precision computations in gauge theories are carried out by particle phenomenology and lattice field theory, with input of new ideas from string theory and modern mathematics.

The lively environment at DESY and KIT offers unique opportunities for research in particle physics as well as for the many young scientists that are being trained through the highly competitive Ph.D. and postdoc fellowship programmes.

There are a number of challenges for theoretical particle physics to address following the spectacular discovery of a Higgs boson last year. A crucial goal is to identify the nature of this particle and the role that it plays in the mechanism of electroweak symmetry breaking, which is responsible for providing the masses of elementary particles. The properties determined so far are compatible with the expectations for the Higgs boson of the SM, which raises the question whether the SM could be valid all the way up to the Planck scale, where the effects of gravity become comparable to those of the other three fundamental interactions. On the other hand, a large variety of other interpretations of the discovered particle is also possible, corresponding to very different underlying physics. The experimental results need to be confronted with the predictions of different models, taking into account existing limits from searches for physics beyond the SM, results from electroweak precision data and flavour physics, as well as cosmological data.

The interplay of particle physics and cosmology is the key for understanding the physics of the early Universe. The high energy densities accessible at particle physics collider experiments provide essential information for the conditions of the very early Universe. At the same time, cosmological data from satellites, telescopes and direct detection experiments provide new insights into the possible role of inflation in the early Universe, into the properties of Dark Matter and the problem of dark energy, and into the cosmological matter{antimatter asymmetry. One of the prerequisites for the generation of this asymmetry, the violation of charge-parity (CP) symmetry which cannot be explained within the SM, is precisely studied in terrestrial experiments involving flavour-changing transitions.

The quest for revealing the fundamental interactions of nature and the fabric of matter, space and time requires an exploration of theory space. Concepts such as supersymmetry and the unification of the fundamental interactions as well as additional space{time dimensions and their geometry play an important role. They have implications for the development of quantum field theory, for interpreting experimental results and for predicting new phenomena.

In order to reveal the underlying physics of observed phenomena it is crucial to confront high-precision measurements with theory predictions of at least the same level of accuracy. In this way it is possible to detect effects of New Physics, which often manifest themselves in small deviations from the SM, and to discriminate between different model descriptions. Precision tools based on perturbative and also non-perturbative methods are needed in particular for exploiting physics at the LHC and a future linear collider, for flavour physics, for instance B-meson decays in Belle II and at LHCb, for electroweak precision observables, for understanding hadron structure and also for describing cosmological observations. The required tools for providing sufficiently precise predictions for the observables mentioned above range from state-of-the-art Monte Carlo tools over advanced mathematical techniques to computer algebra methods and lattice field theory, making use of high-performance supercomputers.

From a conceptual point of view, standard perturbation theory is an enormously successful tool, but it is well known that it may become inadequate for capturing the relevant physics, either because of mathematical complexity or due to non-perturbative effects. Such conditions are in fact very common both in laboratory experiments and during the evolution of our Universe. In order to drive theoretical predictions into such regimes, new methods and concepts must be developed, such as e.g. instanton calculus, alternatives to the conventional Feynman graph computations and improved algorithmic technologies for large-scale lattice simulations within the framework of a "simulation lab".