Physics Interest

Understanding the physics of the strong interaction

The Standard Model of particle physics describing three out of four fundamental interactions, the strong, electromagnetic, and weak interaction, is very well established. Within the Standard Model Quantum Chromodynamics (QCD), the theory of the strong interactions, is one of the most fascinating areas and at the same time one of the most challenging areas. The building blocks of QCD are the quarks (the matter fields) and the gluons (the force carriers). In contrast to e.g. the electromagnetic interaction, where the force carrier, the photon, does not carry any charge, the gluons carry themselves color charge and many fascinating aspects of QCD are related to the resulting gluonic self-interactions. They lead to the running (energy dependence) of the strong coupling constant resulting in non-trivial phenomena like asymptotic freedom and confinement. It is due to confinement that quarks can not be observed as free particles but are always bound in hadrons.

Even though a lot has been learned in recent years and new QCD-bound states have been discovered e.g. at our local electron accelerator ELSA, to reach a good understanding of the different forms of strongly interacting matter is still a challenge:

While it is known that the Higgs mechanism is responsible for the creation of the masses of quarks and leptons, a good understanding of the masses of the bound states of the strong interactions, the hadrons (baryons and mesons), is still not reached. The Higgs mechanism explains only a few percent of the mass of the nucleons and therefore of the matter surrounding us. The rest of the mass is due to the strong interaction itself (QCD field energy).

The question how QCD is producing its high mass bound states from almost massless quarks (u,d,s) and how one can understand the spectrum and the properties of the emerging strongly interacting particles (hadrons) is a central interest of our group.

What kind of bound states of the strong interactions do exist? Even though not understood, it was for long believed that all hadrons are of the simplest types allowed by QCD, namely mesons being quark-antiquark states and baryons being qqq-states. However, recently there have been exciting discoveries in the heavy quark sector, that indicate multi-quark configurations (LHCb-experiment).

Do such exotic states - multiquark or molecular states - also exist in the light quark sector? A question we will also investigate with the new INSIGHT experiment at ELSA in the strange quark sector.

It is further known that QCD can also form bound states from its force carriers alone, so-called glueballs. Here massive bound states are expected to exist. Whether or not
evidence for such states has already been found is subject of an ongoing debate. This question, is one of the questions which will be addressed by the PANDA-experiment
at FAIR investigating the interaction of anti-matter with matter.