Experimental Particle Physics Consolidated Grant 2019

Fundamental physics strives to answer the big questions: what is our Universe made of; how did it evolve; what forces govern it and how do they shape the phenomena we observe? In particle physics we build experiments to examine the smallest constituents of the universe, fundamental particles, so that we can address these questions with our findings. Our knowledge of how fundamental particles behave is encapsulated in a theory called the Standard Model. It has enormous predictive power and provides a simple framework to understand the nature of the universe, however, we also know the theory is incomplete. With experiments at the highest energies, we test predictions to determine the limits of our understanding and the validity of our theory. With dedicated precision experiments we probe predictions at incredible levels of accuracy. The faintest trace of any disagreement between theory and data could indicate a discovery of new types of physics, and a step forward in understanding the nature of the Universe. One of the most pressing questions we have concerns why matter should dominate so much over anti-matter in the Universe. Matter and anti-matter should have been created in equal quantities in the early Universe, but very little anti-matter occurs naturally now. The difference in behaviour between matter and anti-matter that caused this is a mystery, and a defining feature of our universe. Without this difference galaxies and planets could not form, and life could not exist. We think neutrinos may hold the key to understanding why it happened. 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 have no mass at all (like a photon). To detect them we have had to build enormous but very sensitive detectors. Our experiments show that neutrinos have a very small mass; it is this that might cause the preponderance of matter over anti-matter. An important part of our research is to make detailed measurements of neutrinos, to understand their masses and if they are responsible for our matter-dominated universe. The discovery of the Higgs particle marks a new era in our understanding. It confirms the existence of a fundamentally new entity that pervades all of nature and gives mass to elementary particles. Without the Higgs electrons could not bind to protons to make hydrogen atoms, and without atoms our universe would be a very different, lifeless place. In our experiments, we study the Higgs to measure and understand its behaviour. We are motivated by the fascinating possibility that the Higgs may help us understand the dark side of the universe; the mysterious dark matter. It has long been known that there are not enough stars visible in galaxies to explain the speed at which stars rotate around them. Our best explanation is that galaxies also contain massive, invisible (dark) matter which supplies the extra gravitational glue necessary to keep stars in their orbits. Calculations suggest this dark matter forms five times as much of the universe as the matter we see. The Higgs could interact with dark matter, giving us a way to illuminate the dark sector of the universe for the first time. Astrophysical observations also suggest that the Universe's expansion is accelerating, as if there is pressure created by space itself. How this happens is not yet understood, although it has been suggested that an unknown (dark) energy permeating the universe could cause the acceleration. Dark energy, together with dark matter, form 95% of the universe. In other words, we have only studied and understood 5% of the cosmos in our existing experiments. It is imperative that we understand more, and we have joined new experiments to investigate dark energy, discover new physics, and ultimately uncover the nature of the Universe.