Sheffield physicists are hunting for the missing 5/6 of the Universe’s matter—the dark matter that holds galaxies together but has never been directly seen. The problem is twofold. First, ordinary matter makes up only a sixth of all matter; the rest is an unknown particle. Second, the Universe is made entirely of matter, not the equal mix of matter and antimatter that lab experiments produce. The team tackles both puzzles. At CERN’s LHC, they search for supersymmetric particles that could be dark matter. In the LZ experiment, they look for dark matter particles colliding with xenon atoms. They also lead a UK project hunting for axions, another dark matter candidate. On the antimatter question, they work on neutrino experiments—T2K, Hyper-K, DUNE—seeking the subtle differences between particles and antiparticles that could explain why matter won out. This is fundamental science. It will not produce a commercial product next year. But past work on particle physics gave us the World Wide Web, medical imaging, and cancer therapy. If the team finds dark matter or measures neutrino CP violation, it would rewrite our understanding of how the Universe works. Meanwhile, their detector technology is already being applied to image nuclear waste, inspect tunnels, and track lightning for the Met Office.
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"What is the Universe made of, and why?" Sheffield's HEP programme aims to address this fundamental question. There are two problems here: about 5/6 of the matter in the Universe seems to be an as yet undiscovered particle (dark matter), and the remaining 1/6 is all matter - not the 50:50 matter-antimatter mix we make in laboratories. We search for the dark matter particle in two ways: at the energy frontier, by seeking to detect new particles created by the high-energy proton-proton collisions of the LHC at CERN, and in direct searches, attempting to observe these particles in the Galaxy itself. The theory of supersymmetry, which predicts a whole set of particles related to, but more massive than, the known particles of the Standard Model (SM), offers a candidate dark matter particle. If supersymmetric particles can be made at the LHC, they should be detected in ATLAS. Our programme searches specifically for new Higgs bosons and for particles related to the SM quarks and gluons. At ATLAS, we also study SM processes involving the force carriers of the weak interaction, probing our understanding of the SM. Looking to the future, we are contributing essential work to the upgrade of the ATLAS experiment required to take full advantage of higher event rates in future running of the LHC. Most of the matter in our Galaxy is dark matter. In the LZ experiment, we search for evidence of dark matter colliding with Xe atoms in the experiment and causing them to recoil. This experiment will be the most sensitive dark matter detector ever constructed. Understanding possible background - non-dark-matter - events is critical to this, and we have world leading expertise in this field. We are also leading a recently funded UK project searching for axions: another possible type of dark matter particle which cannot be detected at the LHC or in standard dark matter experiments. Why is the matter in the Universe all matter, not antimatter? The answer to this question must lie in subtle differences between particles and antiparticles, an effect called CP violation. The CP violating effects so far observed are not nearly large enough to create the Universe we see. The most likely source for more CP violation is in the interactions of neutrinos. A key observation is that neutrinos have mass, and that different types of neutrinos can interchange their identities in flight. The T2K experiment has made measurements of this, and has detected strong, but not yet definitive, evidence of CP violation. We plan to build on this work, both in running experiments (T2K and SBND) and in designing the next generation of neutrino experiments (such as Hyper-K and DUNE) which will have much greater sensitivity. We have developed tools to assist the neutrino community in comparing results and improving our understanding of how neutrinos interact. Our access to Boulby Mine provides an invaluable low-background laboratory for testing materials and detector prototypes. Neutrinos are also important in astrophysics. We play a leading role in efforts to study supernova neutrinos in Super-K and Hyper-K, and have produced an open-source tool to help experimenters in this field implement new theoretical models. This work may help explain how heavy elements are created in supernovae and disseminated through the Universe. We are continuing our R&D work with LAr detectors for future experiments. Finally, we seek to apply HEP technology to industry and to solving global problems. We are designing muon detectors to image nuclear waste, check the integrity of rail and road tunnels, and monitor civil infrastructure. Our expertise in water Cherenkov neutrino detection is being exploited in an experiment designed to monitor compliance with nuclear non-proliferation treaties, and our expertise in data analysis in a project to help the Met Office locate lightning strikes and track severe storms. All of this builds on our STFC core programme to benefit the wider world.
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