Interference and Resonant Phenomena involving Antimatter

Whilst the apparent imbalance between matter and antimatter in our universe remains a major puzzle in science, valuable insights into the basic interactions between the two have been gained by studying collisions of positrons and positronium with atoms and molecules. The proposed programme is focussed on advancing this knowledge.

Positrons are the antimatter counterpart to electrons with which they eventually annihilate releasing gamma-rays; positronium (Ps) is the atom-like bound state of a positron and an electron, analogous to hydrogen with the positron replacing the proton. In addition to their importance in the fundamental understanding of nature, studies of their interactions with matter allow us, for example, to investigate material properties and to obtain functional imaging of human organs as it is done in positron emission tomography (PET). Positronium formation is so probable that, for example, 80% of gamma-rays detected in PET and 95% of positron annihilations originating from the centre of the Milky Way are from Ps decays.

Lately, we discovered that positronium scatters in a manner similar to that of a bare electron at the same collision velocity (despite Ps being a neutral atom, twice the electron mass). This finding was entirely unexpected and it is guiding theory and experiment towards an accurate description of the interactions of this matter-antimatter hybrid with matter.

Two recent technical attainments are particularly pertinent to our future plans, namely the realization of a positronium beam now tuneable down to energies five times lower than previously obtained, and the development of a new high-resolution positron-beamline apt for inspecting collision dynamics.

These enhanced experimental capabilities have already started to bear fruit with first glimpses of subtle quantum mechanical effects, such as resonances and interference phenomena, previously beyond established observational power. We will probe the mechanisms giving rise to such effects in positron and positronium scattering and thus aid progress towards of a more profound understanding of matter-antimatter physics in general.

Positrons and positronium are fundamental to our understanding of the physical universe. They have also become useful in practical applications ranging from material science to engineering and outstandingly powerful in the biomedical industry.

In this respect, there is currently an international drive towards the development of accurate dosimetry for Positron Emission Tomography (PET) and data for positron/Ps interactions with molecules of biological significance are particularly sought after for simulations of positron tracks in biological media. The clear motivation is that an increased understanding will lead to reduced damage of healthy tissue and, potentially, to efficient targeting (e.g. of cancerous growths). We regularly receive requests to provide cross-section data and we will alert scientists working in this field to our new findings as they emerge.

We will disseminate our experimental results by publishing in high-profile journals and by delivering invited talks and seminars. Additionally, research data will be placed online at UCL Discovery where free access to publications will be available. We will engage with key scientists by pro-actively fostering dialogues through exchange visits and focussed workshops.

Additional benefits will arise for the public/industrial sector through the training of scientists in performing world-class science and in developing a variety of skills (e.g. problem-solving, creative, communication, etc), crucial in innovative endeavours in academia and industry. Mentoring and support for career development will be provided to staff involved in the project throughout its duration, also through the provisions of the UCL Doctoral Skills Development Programme. Recent members of the group have gone on to work at prestigious research labs (e.g. CERN, RIKEN, ETH), in the UK Industry (e.g. AWE, Health Protection Agency, BAE Systems Detica, Thermo Fisher Scientific) and in the financial sector.

Antimatter is a powerful vehicle for stimulating interest and promoting understanding in physics and science by the general public and especially by young people. In the framework of the departmental "Science for the Public" initiatives, we shall strive to generate broader awareness through publications of broader reach and on UCL websites (with the support and advice from the UCL Public Engagement Unit), by arranging school visits to our labs and delivering popular lectures/talks at schools or interested societies.