Production and manipulation of Rydberg positronium for a matter-antimatter gravitational free fall measurement

The idea that the universe originated in a primordial cosmic explosion known as the Big Bang is now well established. According to our present understanding, the expansion and cooling that followed the release of energy from this initial singularity should have resulted in the production of equal quantities of matter and antimatter. However, the observed predominance of matter in the universe today contradicts this hypothesis, and is the primary motivation for much of the current research on antimatter. This imbalance may result from a fundamental asymmetry between the properties of matter and antimatter that has not yet been understood, or it may arise from differing gravitational interactions. Despite the great advances made in particle physics and cosmology since antimatter was first discovered in the 1930's, this problem is still unexplained.

There are many different ways in which the properties of antimatter are being studied. For example, several experimental programmes are underway at CERN to create antihydrogen atoms (that is, the bound state between an antiproton and a positron). Precision laser spectroscopy of these antiatoms will permit tests of CPT conservation, the theory that leads us to expect that there is an exact symmetry between matter and antimatter. Other experiments seek to observe the interaction between antimatter and the gravitational field of the Earth. Because gravity is so much weaker than the electromagnetic force, such measurements must be carried out using neutral particles, otherwise experiments tend to become dominated by extremely small stray electric fields (it only takes an electric field of ~ 10-10 V/m to cancel out the force of gravity on an electron or positron).

The experiments that we propose to carry out are directed toward the search for a possible difference between the gravitational interaction of matter and antimatter. We will do this by creating a beam of positronium atoms. In their ground states, these atoms will self-annihilate in less than a micro-second, since they are composed of a particle and its antiparticle. However, to observe the small effect of gravity on Ps we will excite them with lasers and microwave radiation to Rydberg states. This can increase the lifetime to many milliseconds, which will be sufficiently long-lived to permit the observation of the gravitational deflection of a positronium beam. A complimentary experiment is planned at CERN, in which the effects of gravity on antihydrogen atoms will be studied. However, the potential significance of this work on antimatter to our understanding of the universe means that it is essential to perform measurements of a variety of systems in different ways. It is of interest to study Ps as well as antihydrogen since Ps is composed only of leptons. Any measurement involving antimatter and gravity will be of great significance as none has ever been performed before. If, as many expect, matter and antimatter turn out to be identical gravitationally, this will still limit some theoretical possibilities and be a significant result. However, if even a small difference is observed the importance of such a measurement will be very profound indeed.

Moreover, as we will develop to the capability to produce high quality beams of Ps atoms, we will be in a position to conduct many other kinds of experiment. One example is to study the properties of Ps itself with lasers. Since Ps is made from only leptons it is (almost) entirely described by the theory of quantum electrodynamics (QED). Precision measurements are therefore a good test of this theory. Currently there is a small disagreement between QED predictions and the measured value of the Ps hyperfine interval. This discrepancy only amounts to ~ 10 parts per million but usually QED measurements agree extremely accurately with theory. It is important to resolve problems like this in case they are hiding any new physics.

The research described in this proposal is related to the creation and manipulation of positronium atoms, and their use in tests of matter-antimatter gravitational interactions. The direct economic value of this experiment is difficult to quantify, as is often the case in studies of fundamental physics, and in particular for basic research involving antimatter. Nevertheless, there is a considerable societal impact from such work, as evidenced recently by the widespread public interest in the Higgs boson. It is to be expected that a "matter-antimatter-antigravity" experiment will be of interest to scientists and non-scientists alike, and would be reported in diverse media outlets. This has been the case in previous similar work involving antimatter, namely experiments related to antihydrogen production and trapping, as well as the creation of positronium molecules and a spin-polarised positronium gas. Numerous outreach activities will be put in place to take advantage of this. The description of current research in an accessible format invariably leads to an enhanced public understanding and appreciation of science, and a concomitant increase in engagement with science generally.

Furthermore, this project will provide unusually broad training and development opportunities for young researchers; the nature of the research is such that students and postdoctoral researchers will gain hands-on experience in a wide range of topics, including vacuum techniques, nuclear gamma ray spectroscopy, lasers, beam creation, plasma physics and so on. Students will learn not only the skills needed individually to conduct the experiments, but also more general project management and communication methods. Presentation and writing skills will be honed as they prepare reports of their work and present talks and posters. All of these skills are transferable and valued in industry.