According to our current understanding of particle physics, all matter directly observed in the universe so far – from astrophysical sources to earth-bound high-energy collider experiments – can be described in terms of fundamental fermionic particles, the quarks and leptons. Each fermion appears in three species (“generations” or “families”) which – a priori – are subject to identical interactions induced by electroweak or strong gauge bosons. The different quarks and leptons (“flavours”) are distinguished by their couplings to the ubiquitous, but still undiscovered Higgs particle, which eventually results in different mass values for the individual fermions and also leads to the phenomenon of particle–anti-particle asymmetry in weak interactions. The question why we observe exactly three fermion generations with identical gauge interactions but different (and strongly hierarchical) masses, is one of the biggest puzzles in contemporary particle physics. The flavour puzzle is closely related to the problem how the matter-antimatter asymmetry in the universe has emerged and how fundamental particles acquire their masses at all.
The discovery of flavour physics can be dated back to the first observation of weak interactions in the beta-decay of heavy nuclei in the late 19th century. Decisive hints at the structure of fundamental interactions came from flavour physics, such as the detection of strange and charmed particles, CP violation in strange particle decays and later, the discovery of the third generation of quarks and leptons. These discoveries, together with their theoretical interpretation, belong to the cornerstones of contemporary particle physics and several of them have been awarded with Nobel Prizes and other, highly prestigious awards.
This long development led to our current understanding which is encoded in the Standard Model (SM) of particle physics. It provides a fundamental explanation of how the observed interactions emerge from symmetry principles. However, the triplication of the particle spectrum as well as the mixing among the three fermion generations remains unexplained. Here the SM only provides a parameterization which, however, turned out to be phenomenologically very successful.
Over the last twenty years, dedicated large-scale experiments have been performed to test the flavour parameterization of the SM. At the level of the current precision, no significant deviations, which could unambiguously signal physics beyond the SM, have been observed. Still, the flavour sector of the SM has not yet been tested to the ultimate precision. Currently running and future experiments will explore a variety of rare flavour transitions with small systematic and statistical uncertainties and may give us a clue to a deeper understanding of the phenomenon of
The purpose of the Research Unit is to investigate the theoretical framework of flavour physics with a focus on the quark sector. The extraction of fundamental parameters from data is in most cases rather indirect, since only processes with hadrons are observable in nature. Hence a quantitative treatment of strong interaction effects is mandatory for reliable predictions.
The overall objectives of the Research Unit are:
- To increase the precision of the predictions within the SM as well as in a number of benchmark models for new physics;
- To improve the computational tools for quark flavour physics;
- To contribute significantly to our theoretical understanding of the phenomenon “flavour”;
- To complement the phenomenological investigations performed at high-energy experiments with predictions for precision flavour observables at low energies.