Positron Collisions with Cold Atoms

The electron and its antiparticle, the positron, can form a hydrogenic bound state known as positronium in collisions with atoms and molecules. Cold atom targets (e.g., magneto-optical traps or MOTs) have become ubiquitous in the investigation of quantum correlations in many-body electronic systems, while the advent of the Surko trap has enabled the production of tailored room temperature positrons beams. These beams have been used to investigate positron binding in molecular systems where manybody affects play an important role. Our current work is focused on cold atomic systems (e.g., Rb) in a well-defined quantum state which can be manipulated before interaction with a room-temperature positron beam. The beam is formed from positrons obtained from nuclear beta decay. Hence, they have helicity due to the non-conservation of the weak interaction. It has been demonstrated that positrons do not suffer significant depolarization as a result of moderation or trapping/cooling collisions and thus, a positron beam obtained from a Surko trap will have a non-zero helicity. Positronium atoms produced, in the ground state or excited state, in collisions with atoms or molecules have either singlet or triplet character and are unstable to annihilation into 2 or 3 gamma rays, respectively. In collisions with a Rb-MOT target, the quantization axis of the spin-polarised Rb atoms can be varied with respect to the helicity of the incident positron beam. We will discuss the prospects to use this system for quantum mechanically complete experiments involving antimatter.