# Theoretical Physics, Particle Astrophysics and Cosmology

## Rainer Dick

My research in subatomic physics focuses on extensions of the Standard Model of particle physics, and in particular on the dark matter problem:

Astrophysical observations indicate that only about 15% of the matter in galaxies and galaxy clusters is composed of ordinary matter (essentially protons, neutrons, electrons, and their bound states), but the rest of the matter bound in large scale structures in the universe must be composed of particles which cannot be produced in accelerators so far.

Since this matter component reveals itself only through its gravitational interactions but does not produce luminous stars, it is denoted as **dark matter**.

One area of research that we pursue in this field is the investigation of particle physics candidates predicted by string theory and other extensions of the Standard Model of Particle Physics. We do this by calculating couplings, lifetimes and cosmic abundances of particle dark matter candidates, to test whether any of the candidates predicted by extensions of the standard model matches the requirements imposed by astrophysical observations. We also calculate corresponding nuclear recoil cross sections for direct dark matter search experiments and production cross sections for collider based dark matter searches.

We know that the Standard Model of particle physics cannot be the final theory, and dark matter provides a unique opportunity for collaboration between theory and experiment to push the boundary of knowledge in subatomic physics.

For more information, please contact me by email: rainer.dick@usask.ca

# Particle Physics Theory and Phenomenology

## Tom Steele

The theoretical description of fundamental interactions between elementary particles (the standard model) includes the electroweak and strong interactions. In particular, QCD (quantum chromodynamics) describes the strong interactions that occur between quarks and gluons. QCD exhibits the richest range of physical phenomena in a gauge theory including: a non-trivial classical solutions (instantons) and vacuum structure, spontaneous symmetry breaking, quark confinement, and clear evidence of the radiative effects that occur in quantum field theories. This rich range of physical phenomena combined with deep and beautiful principles and the necessity for only a few fundamental input parameters make QCD our most perfect physical theory.

However, because the strongly-interacting particles are bound states of quarks (hadrons), the comparison of QCD predictions with experimental results is an immense theoretical challenge. A major aspect of my research program involves the development and application of theoretical techniques to make QCD predictions that can be compared with experiment. This research currently includes the study of glueballs---mysterious bound states consisting only of gluons. There are several opportunities for new students in this area, particularly in determining the role of instantons in these systems. Another aspect of this work involves the use of Pade approximations and other techniques for increasing the accuracy of QCD perturbative predictions in hadronic processes such as B decays, which are essential for the study of CP violation.

A research effort that is just beginning involves the development and application of a new formulation of quantum electrodynamics (QED). These techniques describe processes through a dressed, gauge invariant physical field. The amazing aspect of this approach is that it apparently resolves the famous infrared problem, an issue that is currently being investigated by a student. Again, there are opportunities for new students to work in this field.

Students working with me develop a broad range of mathematical and computational skills. Some of my former students are currently employing these skills in industry. My students and I enjoy working together as a team, and I try to build an enjoyable, interactive, and productive research environment.

For more information contact me by email at tom.steele@usask.ca, telephone 306-966-6427.

# Experimental Subatomic Physics

## Rob Pywell, Ru Igarashi, Norm Kolb (on leave), Jack Bergstrom (Emeritus)

Very high-energy photons, known as gamma rays, are used to probe the properties of nucleons (neutrons and protons) and the nucleus. In the past, our group performed experiments at the former Saskatchewan Accelerator Laboratory (SAL), now the Canadian Light Source (CLS), and recently at a facility in Mainz, Germany. Future experiments will be performed at a new facility known as the High Intensity Gamma-Ray Source (HIGS) at Duke University in North Carolina, USA (http://www.tunl.duke.edu/web.tunl.2011a.higs.php). This facility generates gamma rays with a completely new technique that will make it possible to extend the ground-breaking experiments performed at SAL and Mainz into new areas, allowing new physics to be measured with unprecedented accuracy. A substantial grant from the Natural Science and Engineering Research Council was awarded to carry out this research.

We have some exciting experimental programs just beginning. We invite students interested in an M.Sc. or Ph.D. degree to work with us. These programs are an extension of the highly successful ones at the former SAL involving studies of fundamental properties of nucleons and light nuclei. These include measurements of near-threshold pion production (which provides stringent tests of Quantum Chromodynamics (QCD) using Chiral Perturbation Theory), measurements of fundamental nucleon properties such as the electric and magnetic polarizabilities, and studies of multi-nucleon dynamics in few-body systems. Our current focus in on a measurement of the double-polarization cross sections (polarized beam and polarized target) for the photodisintegration of the deuteron from threshold (2.2 MeV) up to ~140 MeV. This tests some fundamental assumptions, used in understanding subatomic systems, through what is known as a `sum-rule'.

Our previous students have enjoyed a unique graduate student experience. Working in experimental collaborations that are relatively small, by today's standards, students have been able to get hands-on experience in all areas of experimental physics---beam production, target and detector design and construction, data acquisition electronics (hardware and software), and data analysis techniques (including simulation and modeling software). Very often in large collaborations at larger facilities a student will gain experience in only a few of these areas. We have found that students, upon graduation, have been much sought after and appreciated in the subatomic physics community for the depth of their experience. Students working with us in these new experimental programs will have a very similar educational experience to what past graduate students have enjoyed. For further information, including how to contact us, see our website at http://nucleus.usask.ca