Many-body theory of positron interactions with atoms and molecules

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Positrons are the antiparticles of electrons. They are produced in abundance in our Galaxy, and are readily obtained on Earth using accelerators or radioactive isotopes. When positrons come into contact with their matter counterparts, the pair annihilate in a pyrotechnic flash, releasing all their energy as pure light. This emitted light is detectable, and is strongly characterised by the environment the electron was in immediately prior to annihilation, making positrons a unique probe. As such, they have important use in medical imaging in PET (Positron Emission Tomography) scans, diagnostics of industrially important materials, and understanding the distribution of antimatter in the Universe.When low-energy positrons interact with normal matter, such as atoms, they pull strongly on the electrons and may even cause one of the electrons to `dance' around the positron, forming so-called positronium (as the positron and electron may annihilate, this may ultimately be a `dance to the death'). Such effects are known collectively as `correlations'. Correlations have a very strong effect on positron collisions with atoms and molecules. In particular, they can enhance the rate of positron annihilation by many orders of magnitude. They also make the accurate description of the positron-atom system a challenging theoretical problem. Proper interpretation of material science experiments, however, rely heavily on calculations that must fully account for the correlations. For example, to accurately interpret Positron Induced Auger Electron Spectroscopy, a powerful technique used to study defects and corrosion in materials, one requires the exact relative probabilities of annihilation with core electrons of various atoms. Moreover, accurate description of the positron-molecule system is required to help explain the origin of the strong annihilation signal from the galactic centre; to develop new spectroscopic PET-scanning methods for medical imaging, drug development and industrial diagnostics; and to advance antimatter-matter chemistry. Crucial in all cases is the ability of theory to accurately calculate the response of the atomic and molecular structure to the positron.A powerful method of describing the positron-atom or molecule system, which allows for the study and inclusion of correlations in a natural, transparent and systematic way, is many-body theory. In this method, complicated mathematical expressions that describe processes of interest, e.g., positron annihilation with an atomic electron, are replaced by series of relatively simple and intuitive diagrams, each of which represents a distinct correlation process. This programme of research proposes to develop new state-of-the-art diagrammatic many-body theory, and recently emerged revolutionary computational methods for high-precision calculations of positron annihilation with individual electrons in complex atoms and molecules. These computational methods will allow for the summation of millions of diagrams, completely unfeasible using the best existing brute-force methods, providing a powerful framework that can yield precision calculations in addition to keen insight. Moreover, the application of the methods will naturally extend to other important atomic and molecular properties and processes, required for tests of fundamental physics and development of quantum technologies.The unique and unrivalled calculational capability that this programme will develop will enable the most accurate interpretation of industrially important materials science experiments at recently launched international facilities; help provide fundamental insights into antimatter in the Galaxy; explain existing experimental results that remain crying out for theoretical explanation; advance Positron Emission Tomography technology and antimatter-matter chemistry; and overall, illuminate this intricate dance of matter and antimatter.