Research of the Theoretical High Energy Physics group

The research interest of the group is the phenomenology of elementary particle physics, both in and beyond the Standard Model of particle physics.

One activity is perturbative and non-perturbative QCD at high energy colliders, for instance, models for rapidity gap events and parton distributions in hadrons. Computer simulation methods in the tradition of the Lund Monte Carlos are often used. Another activity is physics beyond the Standard Model, with research interests in supersymmetry, Higgs physics, dark matter, neutrino physics etc. Applications to collider physics and in particular the Large Hadron Collider have been the main interest. A third activity is astroparticle physics, for example generation of high energy cosmic neutrinos and ways to detect them.

Below you'll find an overview of some of our research. For more detailed information, please take a look at our list of publications and the list of talks given by group members.

Beyond the Standard Model

The fundamental particles and forces in nature are described by the theory known as the Standard Model (SM), a very successful quantum field theory whose predictions have been thoroughly tested experimentally. However, there are both theoretical and experimental reasons to believe it is the low-energy approximation of some yet unknown more fundamental theory. For example, the SM has many free parameters that are not predicted by the theory, and the general structures are postulated rather than explained. We don't know how the electroweak symmetry breaking that gives the elementary particles their masses works—in the SM it is the minimal Higgs mechanism, but there is only indirect experimental evidence for this.

There is the hierarchy problem: why is there such a large difference in magnitude between the electroweak mass scale and the Planck scale of gravity?

There are many other questions: What is dark matter and dark energy? Why is the electric charge quantized? Where do the extremely small masses of the neutrinos come from? There is a widespread belief in the scientific community that we will start to see signs of new physics and possible answers to some of these questions when the LHC accelerator at CERN starts taking data in 2009.

The most studied candidate for physics beyond the Standard Model is supersymmetry (SUSY), which leads to the existence of superpartners of all SM particles. SUSY provides a solution to the hierarchy problem because the superpartners exactly cancel quadratic divergences in the Higgs mass. The minimal supersymmetric standard model (MSSM) is the simplest SUSY extension of the SM, and predicts additional Higgs bosons—two charged and two extra neutral bosons. If MSSM or similar theories are correct there are good chances of discovering such additional Higgs bosons and superpartners at the LHC. If the lightest SUSY particle is stable, it is also an attractive candidate for dark matter.

Much recent research in the group has been focused on aspects of SUSY; for example prospects of discovering SUSY at the LHC—in particular the physics of the additional charged Higgs particles. We have also been involved in organizing workshops on charged Higgs physics in Uppsala.

Research on dark matter is another area of activity, both SUSY dark matter and dark matter in other models for beyond the Standard Model physics. Prospects for direct and indirect detection, and experimental bounds on dark matter candidates are among the issues studied.

Quantum Chromodynamics

The main unsolved problems within the Standard Model are arguably finding the Higgs boson, exploring CP violation in the B-system, and understanding non-perturbative QCD (non-pQCD). The first two are largely experimental challenges while the third is essentially a theoretical one. An outstanding example is here the understanding of confinement of quarks and gluons which is manifested in hadron spectroscopy, parton density functions and the hadronization of quarks and gluons emerging from high energy processes. Non-pQCD is mostly treated by models with a varying content of theoretical input. The interest for non-pQCD has increased in recent years, both in the particle and nuclear physics communities.

Particle physicists often approach the problem from the perturbative QCD (pQCD) side and emphasize the interplay between hard and soft processes in QCD. Here, hard (soft) means large (small) momentum transfer giving a small (large) coupling strength alpha_s such that perturbation theory can (cannot) be applied. The hard-soft interplay is strongly related to the present activities of our group. This is due to its fundamental interest and its `practical' importance in our Monte Carlo event generators that are widely used in the particle physics community. To describe the production of the hadronic final state, one needs a description of both pQCD and non-pQCD processes and a smooth joining of the two.

An example is our model for soft colour interactions, i.e. for non-perturbative gluon exchange which alters the colour topology in an event and thereby the hadronic final state. This describes the recently discovered and now much studied "rapidity gap" (diffractive) events, as well as "normal" events in ep and pp collisions. The model belongs to a novel field of research concerning effects arising when a colour charged quark is moving through a colour background field provided by the initial hadron(s) in the collision. Other research groups have also considered such topics using other approaches.

Understanding the quark/gluon structure of the proton is another important topic. The large density of partons (=quarks and gluons) observed by the HERA experiments at DESY implies that the proton is a complicated many-body system. The theoretical description may therefore have relations to condensed matter physics. Furthermore, one may expect novel non-linear QCD effects mainly driven by gluon interactions at small x. The basic distributions in momentum fraction x of partons in the proton are presently given by parametrizations of data without physical motivation or understanding. We have introduced a new model of non-pQCD dynamics to provide these distributions.

At small x, non-linear effects start becoming important, leading to a phenomenon known as parton saturation. We have studied these effects theoretically both through numerical calculations and analytic studies. We have also taken such QCD effects into account in astrophysical problems (see below).

Something about heavy ions here...

Particle astrophysics

The application of elementary particle physics to astrophysical systems is a growing and very interesting research field. The group already made contributions to this field in the 1990s through the first PhD thesis in astroparticle physics at Uppsala University. We have also recently proposed new experimental ways to detect high-energy cosmic neutrinos through interactions in the moon, and we have studied production of such neutrinos in supernovae and gamma ray bursts as well as atmospheric neutrinos produced by cosmic rays hitting Earth's atmosphere.

Astroparticle physics has ties both to QCD and physics beyond the Standard Model. For example, the cross section for atmospheric neutrino production is sensitive to small-x QCD effects. Another example is that annihilation of dark matter in the Sun or in the galaxy can give rise to fluxes of neutrinos, positrons, or other particles, which may be detected by satellite experiments or large neutrino telescopes such as IceCube.

The present scarcity of observational data leave much room for speculations and crude approximations. With more data coming in the near future, models will be much more constrained and more detailed phenomenological studies be required. Monte Carlo simulations techniques will then become important and we could make significant contributions.

Conclusion

Finally, one should emphasize the possibility to start new projects depending on new theoretical ideas or experimental results that may come up. As a theorist one is not constrained to long-term commitments on specific research projects, like experimental particle physicists, so there is a freedom to follow the development of the field and enter new areas.

For example, we consider the fantastic future potential of the Free Electron Laser.