Many-body theory and computation of low-energy antimatter interactions with matter and light

The ability of positrons to annihilate with atomic and molecular electrons forming characteristic gamma rays makes them a unique probe of matter, and gives them important use in medical imaging, astrophysics and materials science. Low-energy positron interactions with atoms and molecules are characterised by strong many-body correlations, including polarization of the electron cloud, screening of the Coulomb interaction, and the unique process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron). They have an overwhelming effect, causing binding, drastically modifying scattering properties, and enhancing annihilation rates by orders of magnitude. They also make the theoretical description of positron interactions with atoms and molecules a very challenging many-body problem. We will radically transform our state-of-the-art and unrivalled many-body theory approach in our EXCITON+ code [Nature, 606, 688 (2022)] to enable the highest-feasible accuracy and efficiency many-body theory description of antimatter-matter interactions, leveraging modern heterogenous high-performance computing to describe systems currently beyond reach of ab initio study. Predictive capability and the most accurate ab initio calculations of positron binding to atoms and molecules, and positron scattering on polyatomic molecules taking proper account of the correlations including virtual-positronium formation will be realized. Systems of interest will include DNA bases and molecules in aqueous solution relevant to positron emission tomography and positherapy, polyaromatic hydrocarbons found in the interstellar medium, and open-vacancy defects in materials. Moreover, the hitherto unexplored processes of positron-annihilation induced interatomic Coulomb decay and positron analogs of interatomic Coulomb electron capture will also be studied. The theory of antimatter-light-matter interactions will be developed.