Control and Spectroscopy of Excited States of Positronium

Despite the success of the Standard Model (SM) of particle physics there are still some large gaps in our knowledge. Perhaps the most striking of these are the unknown properties of Dark Matter and Energy, and the lack of antimatter in the Universe: the Big Bang should have produced equal amounts of matter and antimatter according to the SM, but we seem to live in a matter-dominated Universe. These mysteries are driving much current research in particle astrophysics and cosmology, but remain unexplained. As these cosmic problems vex us, we can at least take comfort in our mastery of the physics of ordinary matter, electrons and protons and the like, right? Well, perhaps not; there may be some mysteries there as well: using exotic muonic hydrogen atoms accurate measurements of the proton radius have been found to be in serious disagreement with values measured by hydrogen spectroscopy [R. Pohl, et al. (2010). The size of the proton. Nature. 466 (7303): 213-216]. Our experiments may help to shed some light on these seemingly disparate problems. Our goal is to produce atoms composed of electrons and positrons (known as positronium, or Ps, atoms) and perform high resolution spectroscopy on them.

Before we can do this we have to control them, turning a hot gas into a cold collimated beam. We also have to use lasers to put the Ps atoms into highly-excited Rydberg states: this will prevent the positrons and electrons (which are antiparticles of each other) from annihilating. Creating Rydberg states also gives us a way to control the Ps atoms: a pair of separated charges will have a large dipole moment, and that makes it possible to use electric fields to exert a force on these long-lived atoms. We have already shown that we can produce the right long-lived Rydberg states and that we can control them using electrostatic fields. The next step is to refine what we have learned, and to produce higher quality Ps beams, after we have used time-varying fields to slow them down.

Once we have these cold atoms beams we can perform two kinds of experiments: first we will irradiate the atoms with microwaves and observe transitions between states. Because the atoms will be slow and won't annihilate we can probe them for a long time in order to obtain accurate measurements of their energy levels. This lets us test basic QED theory and measure the Rydberg constant, which is needed in the proton radius measurements. This number relates atomic energy levels to the atomic structure, but it is also necessary to know the proton radius to make this connection. One possible reason for the disagreement between the normal hydrogen and muonic hydrogen experiments could be if the Rydberg constant is not known accurately enough. A more exciting reason could be to do with quantum gravity or extra dimensions, but either way we need to understand the problem. Positronium is a lot like hydrogen except it doesn't have any protons, which means that measurements of the Rydberg constant in this system are not complicated by not knowing the proton size. Of course there are other problems to be overcome, but in principle this measurement might help to understand the present discrepancy.

If we can excite our Rydberg Ps atoms with lots of microwaves then we can create special states (called circular states) that have very long lifetimes, of the order of milliseconds. With atoms that live this long we can measure how they fall in the gravitational field of the earth. This will help answer the question: does antimatter fall differently to matter? If the answer is not "no" there will be profound implications for our existing physical theories. There has never been a direct test, but with very long-lived (and cold) Ps we hope to be able to do the measurement.

The research described in this proposal is related to the creation and manipulation of positronium atoms, and their use in precision spectroscopy. The availability of a controlled Ps beam makes it possible to fully evaluate the feasibility of performing a free-fall gravity measurement on this matter-antimatter system, as well as many other experiments, such as studies of Ps-atom/molecule 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 new developments of fundamental antimatter research, for example the recent achievements in CERN where trapped antihydrogen atoms have for the first time been excited with laser light [M. Ahmadi, et al., Observation of the 1S-2S transition in trapped antihydrogen Nature 541, 506-510 (2017)], promising highly accurate CPT tests in the near future.

I fully expect that precision spectroscopy of Ps, and the attendant implications for proton radius measurements and QED tests, will be of interest to scientists and non-scientists alike, and would be reported in diverse media outlets, as indeed the proton radius work itself has been [R. Pohl et al., "Laser spectroscopy of muonic deuterium" Science 353, 669 (2016)]. This has been the case in previous work involving Ps, e.g, the creation of positronium molecules [D. B. Cassidy and A. P. Mills Jr., "The production of molecular positronium", Nature, 449, 195 (2007)], confirming that the public has a general interest in antimatter physics; our research blog [https://antimattergravity.com/] has over 12,000 hits, even though although it is not generally advertised, and mostly contains information of a somewhat technical nature.

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 experimental 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 techniques, including vacuum and ion trapping techniques, nuclear gamma ray spectroscopy, lasers, plasma physics and more. 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 abilities will be honed as they prepare high quality reports of their work and present talks and posters. All of these skills are transferable and valued in industry.