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The Conformal Window and Technicolour Theories with Adjoint Fermions

Principal Investigator: Georg Bergner, Theoretisch-Physikalisches Institut, Universität Jena (Germany)
Date published: January 2018
Project-ID: hfu08

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.

The Conformal Window and Technicolour Theories with Adjoint Fermions

Figure 1: This figure shows the running of the coupling in quantum chromodynamics (QCD) and related theories. If the number of quarks (Nf) is increased, the theory develops an infrared conformal fixed point at the coupling α*. A relevant theory for an extension of the Standard Model shows a walking behaviour indicated by the brown line. (c) University of Jena

The Conformal Window and Technicolour Theories with Adjoint Fermions

Figure 2: This Figure shows a theory, adjoint QCD with two light quark species, which behaves significantly different from QCD: the masses of all particles scale in the same way as a function of the quark mass (constant mass ratios), and there is a very light scalar particle (mass mS) in the theory. (c) University of Jena

Research Team:
Georg Bergner, PI (Friedrich-Schiller-Universität Jena)
Pietro Giudice (Westfälische Wilhelms-Universität Münster)
István Montvay (Deutsches Elektronen-Synchrotron/DESY, Hamburg)
Gernot Münster (Westfälische Wilhelms-Universität Münster)
Stefano Piemonte (Universität Regensburg)
Philipp Scior (Westfälische Wilhelms-Universität Münster)

Scientific Contact:

Dr. habil. Georg Bergner
Theoretisch-Physikalisches Insitut
Friedrich-Schiller-Universität Jena
Max-Wien-Platz 1, D-07743 Jena (Germany)
e-mail: georg.bergner[at]

January 2018

JSC project ID: hfu08