Production of Positronium atoms, ions, and molecules
Lead Research Organisation:
UNIVERSITY COLLEGE LONDON
Department Name: Physics and Astronomy
Abstract
Positronium is an atomic system made from an electron-positron pair, which has many similarities to hydrogen in terms of its atomic properties. Being composed of a particle-antiparticle pair makes the Ps system more complex than hydrogen, however, because the possibility of both self and virtual annihilation processes plays a large role in the fundamental energy level structure and in the practicalities of performing experiments. Nevertheless, recent advances in positron trapping technology have made it possible to produce high-density Ps gases in which Ps-Ps scattering is readily observed, and which can be probed with lasers. Experiments have succeeded in producing Ps2 molecules and exciting them with laser light, observing Ps-Ps scattering, and the subsequent spin polarization of a Ps gas. The shift of Ps energy levels caused by interactions with the internal surfaces of mesoscopic porous films has also been observed.
With an increased beam density it will be possible to generate Ps2 molecules with higher efficiency, and therefore to study their properties in much greater detail. A more robust source of Ps2 molecules also allows for the production of both positive and negative Ps ions, which can be created using lasers to break up a Ps2 molecule. These ions and molecules are stable atomic systems (although they do self-annihilate) and can be studied optically. Because they are composed of three or four bodies of equal mass approximations like the Born-Oppenheimer approach (where electrons and nuclei are treated independently) cannot be used. Thus, Ps ions and molecules present an interesting challenge to theorists. They also possess unique properties that have so far not been widely studied experimentally.
The availability of a source of cold Ps atoms is also a key step in several experimental endeavors, including high resolution spectroscopy and (anti)matter wave interferometry, which can be used to test bound state QED theory and search for new physics. Ps atoms are pure QED systems, as QED is a theory of light and leptons, and so they are especially sensitive to non-QED effects, such as unknown particles or forces. If they can be studied in sufficient detail these simplest of systems may reveal some of the best kept secrets in the universe.
With an increased beam density it will be possible to generate Ps2 molecules with higher efficiency, and therefore to study their properties in much greater detail. A more robust source of Ps2 molecules also allows for the production of both positive and negative Ps ions, which can be created using lasers to break up a Ps2 molecule. These ions and molecules are stable atomic systems (although they do self-annihilate) and can be studied optically. Because they are composed of three or four bodies of equal mass approximations like the Born-Oppenheimer approach (where electrons and nuclei are treated independently) cannot be used. Thus, Ps ions and molecules present an interesting challenge to theorists. They also possess unique properties that have so far not been widely studied experimentally.
The availability of a source of cold Ps atoms is also a key step in several experimental endeavors, including high resolution spectroscopy and (anti)matter wave interferometry, which can be used to test bound state QED theory and search for new physics. Ps atoms are pure QED systems, as QED is a theory of light and leptons, and so they are especially sensitive to non-QED effects, such as unknown particles or forces. If they can be studied in sufficient detail these simplest of systems may reveal some of the best kept secrets in the universe.
Planned Impact
The research described in this proposal is related to the creation and manipulation of positronium atoms that can be produced at high density, leading to the formation of Ps ions and Ps2 molecules. These are fundamentally interesting systems whose properties may differ significantly from that of other atoms and molecules owing to the unique equal-mass nature of the constituents. Testing theory and searching for new physics in simple atomic systems has been a growing area of importance in recent years: new physics is urgently needed but has not been forthcoming from, for example, the LHC measurements or numerous Dark matter searches.
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., Characterization of the 1S-2S transition in antihydrogen [Nature 557, 71 (2018)], promising even more precise CPT tests in the future.
I fully expect that detailed studies of Ps molecules and ions will capture the imagination of scientists and non-scientists alike, and would be reported in diverse media outlets. This has been the case in previous work involving Ps2 [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 15,000 hits.
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.
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., Characterization of the 1S-2S transition in antihydrogen [Nature 557, 71 (2018)], promising even more precise CPT tests in the future.
I fully expect that detailed studies of Ps molecules and ions will capture the imagination of scientists and non-scientists alike, and would be reported in diverse media outlets. This has been the case in previous work involving Ps2 [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 15,000 hits.
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.
People |
ORCID iD |
David Cassidy (Principal Investigator) |
Publications

Babij T
(2022)
Positronium microwave spectroscopy using Ramsey interferometry
in The European Physical Journal D

Cortese E
(2023)
Positronium density measurements using polaritonic effects
in Physical Review A

Cortese E
(2022)
Positronium density measurements using polaritonic effects
Description | I worked with theorists to devise a new way to measure positronium densities using polaritonic methods. This theory work was in part undertaken because the pandemic had a big impact on our abiity to conduct the experiments. As a result we had to rethink what was possible in this project. The theory work is therefore not part of the original plan (which was entirely experimental) but does nevertheless represent an advance in the general area of the project. We found that by placing a high density of positronium atoms in a bragg reflector cavity we could create a polaritonic system that would exhibit spectroscopic properties (namely the Vacuum Rabi shift) that could be used to determine the positronium density. If this can be implemented it will be very useful for optimising high density experiments. |
Exploitation Route | The theory work we have done could be used in other systems and so may prove to be of use, either for general AMO/materials science or for others using positronium. I very much hope to get some data this summer and that may well inform other related experimental work, but as yet there is no data. We are still trying to demonstrate new ways ot make Ps ions and if we are able to do this it could open the door to toher experimental programmes, includeing those of cold chemistry where positrons or electrons can be delivered to other systems via accelerated ions. |
Sectors | Education Other |
Title | New beamline |
Description | We have continued the construction of a new high density positron beam required for thi sproject. The work has been delayed but some prrogress has been made on the design and initial construction. Covid restrictions seriously limited this work but it is back in progress now and we expect to have a working system in the summer of 2022. |
Type Of Material | Improvements to research infrastructure |
Year Produced | 2022 |
Provided To Others? | No |
Impact | None yet |
Description | Southampton Group |
Organisation | University of Southampton |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | I worked with the theory group of Prof S. De Liberato on a new method to measure Ps densities. I was abe to provide information regarding realistic experimental paramaters that could be obtained in high density positronium experiments (based on work I had previously done) |
Collaborator Contribution | The theorists were able to develope a model for the positronium systems that allowed polaritonic methods to be used in a new way that had never been applied to antimatter systems before. We are hoping to realize the experiment in the future. |
Impact | https://doi.org/10.1103/PhysRevA.107.023306 |
Start Year | 2022 |