The study of positronium production and antimatter interactions in membranes

All matter has a less known twin, namely antimatter. We have known about the existence of antimatter since 1932, and have been able to produce and control the lightest antiatom - antihydrogen - since 2011. However, antimatter is very difficult to study: the moment matter and antimatter meet, they disappear and are transformed into pure energy. How antimatter reacts to gravity has never been measured, but is of utmost importance to probe Einstein's General Relativity, to try to understand Nature at its fundamental level. This project was set to life at AEgIS - the Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy - at CERN. AEgIS is designed to directly measure the gravitational force on antimatter precisely for the very first time. Gravitational forces are much weaker than electrical forces. Therefore, we can only hope to measure gravity on matter without electrical charge, like atoms. AEgIS will use antihydrogen; an antiproton surrounded by an antielectron (a positron). In AEgIS, antihydrogen will produced by merging antiprotons with positronium; a bound state of electrons and positrons. This project focussed on the production of positronium and aimed at developing alternative production targets. The alternative geometry offered by these targets is very promising for the future, both for increased production of antihydrogen and possibly a more precise measurement of the gravitational force acting on it. Within this project, characterisation of positronium production was carried out and laser excitation of positronium to the third energy level was performed for the first time. This was reported in the scientific journal Physical Review A. Furthermore, thanks to this project, transmission positronium targets were made available to the experiment for the first time: a custom-designed multilayered transmission positronium target was envisioned and successfully produced. The measurements performed were reported in the scientific journal Nuclear Instruments and Methods in Physics Research. Contact with industry was established to develop a device for introducing such fragile transmission targets into the challenging environment of the main experiment. Following this, the company has launched a new product: a cryo-and-vacuum compatible long stroke linear drive piezo actuator. Given the novel nature of the positronium source introduced to AEgIS by this project, the test chamber is not designed for measurements on such targets. A redesign of the experimental setup is underway to facilitate as-complete-as-possible characterisation of transmission targets. This project has thus given the AEgIS experiment a significant push towards the future. A second, utterly different aspect of the project was to study antimatter interactions with thin membranes. This is fundamental science not priorly planned within AEgIS. A secondary beam line in the experimental zone makes a beam of low energy antiprotons available; unique in the world. As part of this project, samples of graphene were installed in the beam line: for the first time, graphene was exposed to a stream of low-energy antiprotons. The result of this exposure was imaged with a scanning electron microscope in the nano lab at the University of Bergen, where some unexpected effects were observed. For future efforts, it would be interesting to optimise the experimental protocol and image exposed samples with a neutral helium microscope, which is non-destructive and highly surface-sensitive. From this we may learn about processes of interaction of antimatter and annihilation fragments in thin membranes. Depending on the interaction of antiprotons with such thin materials - which has never been studied - it may be possible to create the world's smallest sieve, of mono-layered thickness with nano-sized holes, which may have many applications and grand positive implications. This project was highly cross-disciplinary, and touched the fields of particle physics, atomic physics, material science and instrumentation.

Gravitational forces on antimatter has never been measured. This is of utmost importance to probe General Relativity, not making any distinction between matter and antimatter. The AEgIS experiment at CERN aims at measuring for the very first time the gravitational constant of anti-Hydrogen, with an expected resolution of 1 %. The formation of positronium is crucial for the production of anti-Hydrogen. This project proposes to improve the production of positronium, via changes in the experimental setup and production material. Introducing a PET scanner to the experimental setup would improve the characterization of the positronium formation. If the positronium production can be altered to take place on-axis, the transverse movement of the anti-Hydrogen, a main challenge for its downstream study, could be drastically reduced. An improvement here would be ground-breaking in providing a well-behaved beam, improving the gravitational measurement. As a second aspect of the project, we propose to perform fundamental science not currently part of the AEgIS physics programme: the study of antimatter interactions with thin membranes. The scattering of low-energy antiprotons could also be characterized using the PET scanner. A second step is studying masked membranes in the AEgIS environment. The mask could predesign the size and number of holes on a thin membrane, such as graphene. If successful, this could be used as a molecular sieve, which would have grand positive implications. This project is highly cross-disciplinary, by capitalizing on expertise in particle physics, atomic physics, material science and instrumentation. The groups at the University of Oslo has extensive experience with the development and deployment of Positron Emission Tomography scanners. The lithography laboratory at the University of Bergen has the infrastructure and expertise to both produce nanostructured masks and study the membrane surface after exposure to antimatter.