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.