Gaining a deeper understanding of antimatter-matter interactions
Positronium is a relatively little-known atom with a lot of potential. It has no nucleus and is formed from the bond between a negatively charged electron and a positively charged positron – the antiparticle of an electron. This makes positronium the simplest anti-atom: an atom containing antimatter. Atoms of positronium are unstable, which helped scientists first discover it. “Positronium is unstable, meaning it lives for only a finite time, before the electron and positron annihilate each other, turning mass into pure energy in the form of light,” explains Dermot Green, reader in Applied Mathematics and Theoretical Physics at Queen’s University Belfast. “This annihilation signal – bursts of detectable light – was the smoking gun that told us positronium exists.” A deeper understanding of the interactions between positronium and other matter could help with advancements in fields from astrophysics to medicine. “In medicine, positrons are at the heart of positron emission tomography (PET) imaging, for example,” says Green, ANTI-ATOM project coordinator. In the ANTI-ATOM project, which was funded by the European Research Council (ERC), Green and his colleagues used a mixture of theory and computation to devise the theories to describe these complex interactions. “Our work provides fundamental insight on positron interactions with atoms and molecules, and more generally, informs and provides benchmarks for other theoretical and computational approaches to the quantum many-body problem,” adds Green.
Honing in on positron-molecule attachment
“Our work focused on calculating positron attachment to molecules,” says Green. When a positron binds to a molecule, it gives its energy to the molecule, which can start it vibrating, he explains. The positron cannot escape until it gets this energy back. All the while, it is hanging around with lots of electrons in the molecule, so it inevitably annihilates. “Pioneering experimentalists in California, led by the pre-eminent Clifford Surko, designed a positron trap and an energy-tunable beam and used it to measure this enhanced annihilation rate,” notes Green.
Developing a theory of interactions
Describing positron interactions with atoms and molecules is a very challenging problem, due to the strong quantum many-body interactions that characterise these systems. The team therefore turned to sophisticated theoretical methods whose origins are in quantum field theory, but which are adapted to low-energy, finite systems. These methods were translated to state-of-the-art computer codes, with calculations running on supercomputing clusters. “For the computational side, we benefited greatly from collaboration with Charles Patterson at Trinity College Dublin,” remarks Green. “We adapted his EXCITON code, already capable of many-body calculations of electron-matter systems, to include positrons.”
A pioneering positron description
The most important result of the project was the first ab initio description of positron binding to molecules in agreement with experiment, which had remained elusive for decades. “Our results found beautiful agreement with experiment, and provided fundamental insights including on how positron binding can be enhanced,” notes Green. The researchers now plan to expand their theoretical and computational approaches, using them as a solid foundation to consider new problems, including larger molecules, clusters, liquids and other fundamental processes including electron and positron scattering on atoms and molecules. “I have been awarded an ERC Consolidator Grant that will help us to do just that,” adds Green.
Keywords
ANTI-ATOM, positronium, quantum, many-body problem, molecule, attachment, theory, interactions, description
