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The Group

We are pursuing three major research areas. All of them are concerned with the problem of how particles behave, once interactions can no longer be treated as a small effect (perturbation). A description of this research can also be found in our popular science blog. The latest news on our research or from the field can also be found on Twitter.

 

Foundations of quantum-gauge-field theories

So-called gauge theories represent the most common version of particle physics theories. Especially the standard model, but also gravity, and most of its speculated extensions belong to this category. While they have been extremely successful in the description of nature, several very fundamental issues of them are still poorly understood.

One of them is how to choose suitable coordinates to describe the elementary particles (called fixing the gauge). This problem, known as the Gribov-Singer ambiguity, arises because the quantum effects forbid to choose your coordinates purely locally, but you have to include knowledge of all space-time. Though this problem can be fixed in principle, we still lack practical solutions.

We attempt to resolve this problem by finding suitable averages over coordinate system ensembles such that the averages can be easier treated than any individual coordinate system. As a byproduct, such ensembles over coordinate systems create new symmetries, which we try to understand and exploit.

 

QCD and the QCD phase diagram

QCD is the theory of strong nuclear interactions, giving us stable atomic nuclei. It also governs the interior of neutron stars, the most compact stellar objects still dominated by particle physics and not only gravity.

Such ultra-dense systems are not easily accessible numerically. This is the so-called sign problem. As a consequence, no satisfactory description of the interior of neutron stars was yet possible using QCD. Thus, our knowledge of these stars is still rather speculative.

Non-simulation methods are a possible alternatives for the description of neutron stars. However, such methods usually involve various approximations. To check these, we perform simulations in slightly modified versions of QCD, where such simulations are possible. The aim is to find a suitable set of approximations which can be translated back to QCD, and apply it to neutron stars.

 

Higgs physics

The discovery of the Higgs boson has been one of the greatest scientific discoveries of the recent years. It is the last element of the standard model of particle physics, and a possible gateway to whatever lies beyond. With this discovery, the properties of the Higgs came into the focus of investigations.

The theory underlying the Higgs sector has plenty of enigmas so far. One is that it is not even clear, whether it a real theory, or whether new physics is mandatory to make it a real theory. The other is that there are subtle effects which are not yet accounted for, but which could provide distinct new signatures, like states containing several Higgs bosons.

To fully understand Higgs physics, we simulate (generalized versions of) the Higgs theory numerically. We determine the possible multi-particle states, and deduce from them predictions for experiments, like at the LHC. We furthermore use our understanding of gauge theories to infer how Higgs theories work.

 

Physics beyond the standard model

There are many conceptual reasons why the standard model cannot be the ultimate theory of particle physics. Among those primarily interesting to us are: How can the gap to gravity be closed? What is a particle physics version of gravity at all? Why do seemingly unrelated features of the standard model, like the electric charges of quarks and leptons, are tightly linked?

There are many candidates to explain these features, developed to different degrees of sophistication. However, like with the standard-model Higgs sector, in many cases a multitude of subtleties pose conceptual questions. Answering these with a field-theoretical sound basis is our main aim. As abstract as this may seem, this can have quite explicit implications for experiments, and the interpretation of what is measured, just like in the Higgs case.

Theories we are currently investigating are of various types. Particular emphasis has been put on theories in which either the Higgs is a composite particle, so-called technicolor-type theories, theories which have additional Higgs degrees of freedom, and theories which unify the different forces. This covers not all the possibilities for which investigations are planned, and with every new piece of experimental evidence further theories may be added to this list.