ELEMENTARY PARTICLE PHYSICS

In this project, a multi-instutional team of researchers investigated new strongly interacting theories beyond the Standard Model of particle physics. These theories share fascinating but still puzzling features with the strong interaction of nuclear forces. In addition, they offer new phenomena and exotic properties that make them interesting for a more general understanding of the foundations of particle physics.

The strong forces of nuclear matter have fascinating properties that are still not well understood. At low energies, the forces, which are described by the theory of quantum chromodynamics (QCD), strongly bind the quarks into protons and neutrons. This is the reason why the quarks are not directly observable in nature. At high energies, the quarks behave like free particles. The transition from the strong coupling to the weak coupling regime is characterized by the running coupling that tends towards zero at high energy, whereas it gets large in the low energy regime. Numerical simulations on high performance computers are currently the only possible approach to derive these main properties from the underlying fundamental theory.

Two very challenging questions are addressed in this project: How general are these features of quantum chromodynamics, and is it possible that additional strong forces provide a solution to unsolved problems of particle physics?

A first generalization of quantum chromodynamics appears if the number of quarks is increased. The quarks tend to screen the forces, therefore the running of the coupling gets weaker. At some critical number of quarks, the running saturates at a so-called infrared fixed point. The second generalization is the way the quarks are coupled to the force field. This project considers quarks that couple differenty to the strong forces, namely they transform in the adjoint representation of the gauge group. The critical number of quarks for the appearance of an infrared fixed point is reduced in this way. Furthermore, additional exotic low energy bound states that are not present in QCD are introduced.

These generalizations are not only motivated by the theoretical interest in the possible realisations of strong interactions. They also offer an interesting approach for an extension for the Standard Model of particle physics. In this approach, the Higgs particle is a composite state of a new strong interaction. It might provide a solution to the unresolved problem that the Higgs mass appears to be unreasonable small, compared to its large quantum corrections, and include a natural explanation for Dark Matter. However, the question whether these theories are consistent with the experimental data requires numerical investigations. Of particular interest are theories near to the appearance of an infrared fixed point. A second condition is the appearance of a light scalar bound state as a Higgs candidate. This project investigates whether these conditions are fullfilled in theories with a different number of quarks in the adjoint representation.