Consolidated Grant 2015

The research of the experimental particle physics group addresses some of the main questions in fundamental physics. One of the most pressing is what is the mechanism that is behind the overwhelming dominance of matter over anti-matter in the Universe? Matter and anti-matter should have been created in equal quantities in the early Universe, yet the gross difference in their natural occurrence is a defining feature of what we observe. Without this asymmetry life could not exist. We believe that neutrinos may hold the key to understanding the matter-antimatter asymmetry. Neutrinos, discovered almost a century ago, are the most evanescent of particles. They have no charge, barely interact with matter and were long thought to be completely massless (like a photon). They can travel through the earth with only the tiniest probability of leaving a trace. To detect neutrinos we have had to build enormous but very sensitive detectors. Our experiments show that neutrinos actually have a very small but significantly non-zero mass. This small non-zero mass allows them to drive the matter-antimatter asymmetry in the early Universe. An important part of our research is to make detailed measurements of the neutrinos, to understand their masses and to calculate if their properties are indeed those required to explain the matter-antimatter dominance. The discovery of the Higgs particle was one of the most important in the last decade. It confirmed the existence of a fundamentally new type of "force" that pervades all of nature and gives mass to elementary particles. Without the Higgs, particles such as electrons could not bind to protons to make hydrogen atoms. Thus normal atomic matter, even stars, could not be formed. We continue to study the Higgs to measure and understand its properties. We are especially interested to see if the Higgs particles provides a window to what we call the "dark universe". It has long been known that there is not enough visible matter in the Universe to explain the speed at which galaxies rotate. There are simply not enough stars. The only explanation appears to be that surrounding us, and in all galaxies, there is a halo of invisible matter which exerts a gravitational influence (hence why the galaxies spin as fast as they do) but which does not interact with light. Calculations suggest there is 5 times more of this dark matter than visible matter in the Universe. The Higgs could interact with dark matter, thus we can use the Higgs to "illuminate" the dark sector for the first time. This is an important part of our research. Observations also suggest that the Universe is inflating, as if there is pressure created by space itself. This process is observed but non-understood. It strongly suggests there is another form of dark energy at work. Altogether the dark universe accounts for 95% of the matter/energy in the Universe with only 5% (that we can observe) being luminous. This makes the study of the dark matter and energy absolutely central to our understanding of the fundamental nature of our world. Thus we have joined experiments whose aim is to try and study and uncover the true nature of dark energy. Our theory of how all particles interact is embodied in what is called the Standard Model. With the exception of neutrinos and their masses it has enormous predictive power and provides a simple framework for elucidating the nature of the universe. Following the scientific method we continue to refine and test the SM at the energy frontier and with dedicated precision experiments. This provides another, and well tested route, to the discovery of new physics.