Positronium interferometry

Smashing elementary particles into each other as in the Large Hadron Collider or, alternatively, shooting beams at materials, helps us understand basic physical interactions. The behavior of antimatter particles is particularly interesting, since the visible Universe is composed of matter, but we still don`t know why it is that way. Thus any differences in the way that matter and antimatter behave could give us new insights into physics. While all particles have antimatter counterparts, positrons (e+), the positively charged antiparticles of electrons (e), are the simplest ones. They are the easiest to experimentally produce and control, have no internal structure, and form the basis of more complex antimatter. However, charged particle beams are difficult for precise experiments. Thus, a type of neutral antimatter beam that is starting to attract attention is positronium, the bound state of a positron and an electron. Due to the quantum nature of matter, elementary particles behave as waves, and thus their interference can be measured. Such experiments continue to provide ever more precise tests of quantum mechanics. The project will use computer simulations to model the interaction of a positronium beam with graphene, guiding experiments by a collaborator in Japan. This may enable the first measurement of neutral antimatter wave interference and yield possible hints of new physics.

Shooting beams at materials is a way to understand basic physical interactions, and the behavior of antimatter particles is particularly interesting. The use positronium, the bound state of a positron and an electron, as a neutral antimatter beam remains one of the most attractive experimental systems. The project was based on a close theoretical and experimental collaboration between researchers at the University of Vienna and the Tokyo University of Science. The Viennese team used computer simulations to model the scattering of a positronium beam from graphene, guiding the experiments planned for the Tokyo laboratory. Several long-term visits to Japan were planned to coordinate the work. Unfortunately, the COVID-19 pandemic severely disrupted the project. No visits to Japan could be made, and entry into the country still remains extremely restricted. Repeated lockdowns and supply-chain shortages further delayed the planned experimental work. Fortunately, all team members and collaborators stayed healthy and safe. Theoretical work by the Viennese team succeeded in modelling positronium scattering from graphene, predicting what the expected signal will be at experimentally available beam energies. Thus, while the first measurement of neutral antimatter wave interference was not yet achieved, the collaboration continues and results can still be expected in the future.