# Research in Theoretical Particle Physics

Our field of research is theoretical physics with connection to experiment. We are interested in a variety of interesting topics both in and beyond the Standard Model, with connections to experiments in collider physics, astroparticle physics and cosmology. Our research range from quantum chromodynamics, which deals with the strong interactions of quarks and gluons, to Higgs physics beyond the Standard Model, and on high-energy neutrino astrophysics.

## QUANTUM CHROMODYNAMICS

The birth of QCD dates back to the beginning of the 50s where a vast and ever growing amount of new particles were discovered (baryons and mesons). Researchers in those days were baffled at the amount of seemingly fundamental particles, and there was a feeling that not all of these could truly be fundamental. This feeling came to fruition with the discovery by Yuval Ne'eman and Gell-Mann, which showed that all these newly discovered particles could be described in terms of three flavors of smaller particles which came to be known as the quarks. Starting initially with three quark flavours (up, down, strange) it was later found that it was necessary to include three more quark flavours (charm, bottom, top), and in addition these quarks had to carry “color” charge from which the name QCD is derived.

The theory of QCD has been hugely successful in describing the rich structure that we see in nature. Some of the noteworthy properties of QCD is that of confinement which states that quarks can not be individually observed at low energies but can only exist as bound states (baryons, mesons), while at a high enough energy the quarks may interact by themselves, for instance in deep-inelastic scattering (DIS). This property of QCD has the remarkable consequence that for high energies we can describe nature in terms of individual quarks, while at low energies the proper degrees of freedom is baryons and mesons. The big challenge is how to connect these two separated scales i.e. how to connect the description of the high energies with that of low energies.

The aim of our research is to understand this transition from high to low energies. The approach is to develop models and computational tools for various physical processes and compare with experiment in order to gain new insights to the nature of this region of intermediate scales. Our research in this area involves development and analysis of models, numerical and analytical calculations, and Monte Carlo simulation. Several Monte Carlo event generators have been developed in the group. You can look at our list of publications for further information on our present research.

## Higgs Physics

With the discovery of the Higgs boson by the LHC in 2012, a new era of theoretical electroweak physics was heralded. The discovered boson fits very well with the Standard Model prediction, but since we know that the Standard Model cannot be complete, it is worthwhile to ask the question if there in fact are more Higgs bosons out there.

One of the few ways to introduce more Higgs bosons at the electroweak scale, without contradicting earlier experiments, is by simply duplicating the structure of the Standard Model Higgs sector. These models are called Two Higgs Doublet Models, and they contain in total five different Higgs bosons. One reason these models are interesting is that they are able to explain the matter-antimatter asymmetry of our universe. Another reason is that to make the standard model supersymmetric, we would need two Higgs doublets.

In our research in Two Higgs Doublet Models we make predictions for particle colliders such as the LHC in conjunction with cosmological questions. For instance, is it possible to explain the matter-antimatter asymmetry with a Two Higgs Doublet Model while still explaining the data from LHC?