The positron is the antiparticle of the electron, and the simplest form of antimatter. Their ability to annihilate with atomic electrons makes them a unique probe of matter. As such, they have important use in medical imaging, materials science and astrophysics. They are also at the heart of more complicated antimatter: positronium (Ps, an electron-positron pair) and antihydrogen (a proton-antiproton pair), are under intensive study at international laboratories to test fundamental laws of nature.
When a positron interacts with normal matter (an atom or molecule), it pulls strongly on the atomic/molecular electron cloud, polarising it. The atomic/molecular electron may even temporarily "tunnel" to the positron, forming short-lived "virtual" Ps. These so-called 'correlations' significantly effect positron and Ps interactions with atoms and molecules, e.g. enhancing annihilation rates by many orders of magnitude, modifying the characteristic annihilation signal, and making the accurate description of these systems a challenging many-body problem. Proper interpretation, modelling and development of the antimatter-based fundamental experiments, materials science techniques and antimatter technologies (positron traps and accumulators, PET etc) require calculations that fully account for the correlations.
A powerful framework that can account for the correlations is many-body theory. In many-body theory, processes of interest are represented via relatively simple and intuitive (Feynman) diagrams, enabling identification of the key interactions, efficient computation, and providing keen insight. This project aimed to develop the many-body theory and its state-of-the art computational implementation, towards the accurate description of positron and Ps interactions (binding, scattering and annihilation) with matter, providing fundamental insight required to enable the accurate interpretation of and develop the difficult and costly antimatter experiments, material science techniques and technologies. In particular, it aimed to provide a proof of principle of the feasibility of a computational implementation of the many-body theory for multicentred targets (beyond atoms).
The objectives were successfully achieved, with a many-body theory description of positron multicentred targets developed and implemented in the new "EXCITON+" code; a number of groundbreaking results were obtained, with publications in Nature and multiple Physical Review Letters, PRA Letters, and JCP Emerging Investigator Special Collection etc. A key achievement was the development of the team members, including three Postdoctoral Research Fellows and graduation of multiple PhD students. In addition, the low-energy positron-matter problem provides a rich testbed for the development of other approaches to the many-electron problem, to which our results should provide benchmarks.
