Many-body theory of positron interactions with atoms and molecules

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.

This programme of research will develop state-of-the-art theoretical and revolutionary computational methods for high-precision calculations of positron (antimatter) interactions and annihilation with atoms and molecules. In addition to their importance for fundamental physics, such calculations are essential to develop industrially important material science techniques and to develop PET (Positron Emission Tomography), important in medical imaging, drug development and diagnostics of flow processes in industrial plants.

Although the immediate focus is on development of fundamental science and knowledge generation and exchange between fundamental researchers, in addition to the numerous academic beneficiaries, my programme of research promises strong positive short and long term impacts on the UK economy and society:

It has been agreed with the host institution that as part of the programme I will co-supervise a PhD student. The student will be exposed to a wide range of important theoretical and computational techniques, whose application span numerous subfields of theoretical physics. This will encourage a new pipeline of researchers in antimatter physics, essential to ensure the UK stays at the international forefront of the field. The student will also gain high-level IT skills, and important experience in problem solving, team-working, data analysis, project planning etc, all desirable and transferable to diverse career pathways and essential to the future economy of the UK.

In the short term, the ability to calculate positron annihilation with core electrons of atoms across the periodic table, which is a key objective of the programme, promises to enable the most accurate interpretation of industrially important material science experiments. The research will benefit those who use such techniques, allowing them to improve their diagnostics and to steer and develop new positron-based material science technologies. Advancing the development of new materials has the potential to positively impact on the UK economy and society, by furthering fundamental science, directly improving the quality of life of UK citizens owing to the exploitation of the new materials, and attracting international investment (for research and through commercialisation) that will contribute to the UK economy.

The ability to perform accurate calculations of positron annihilation on molecules is crucial to develop new methods of PET (Positron Emission Tomography). This imaging technique has important use in medical imaging, drug development and also for diagnostics of flows in industrial plants. By extending current PET technology to avail of the detailed information contained in the light signal emitted in an annihilation event, new spectroscopic PET technologies can be established and developed. These methods could, e.g., enable important non-invasive diagnostics of human tissue, to provide improved pathology studies and clinical management of patients. This, coupled with the advances in matter-antimatter chemistry the project will develop, will also help advance drug development. Developments in such areas have obvious benefits to quality of life, and have the potential to attract international investment that will contribute to the UK economy.

I am passionate about communication of science through outreach and public engagement, and I will undertake a strong and varied programme of such activities throughout the duration of the Fellowship. These activities include developing freely available and accessible podcasts on my research, giving a lay talk at the `Belfast Science Cafe', and developing an antimatter-themed educational installation at the award winning `Belfast W5 - science and discovery centre'. Overall, these activities will increase the general public's understanding and awareness of antimatter, will inspire the next generation of UK scientists, and will reach out to and inform those who wouldn't usually seek out science.