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

Lead Research Organisation: University College London
Department Name: Physics and Astronomy


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.

Planned Impact

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.


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Alonso A (2016) Single-shot positron annihilation lifetime spectroscopy with LYSO scintillators in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

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Alonso AM (2015) Controlling Positronium Annihilation with Electric Fields. in Physical review letters

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Andersen S (2015) Positronium emission and cooling in reflection and transmission from thin meso-structured silica films in Journal of Physics B: Atomic, Molecular and Optical Physics

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Cooper B (2016) Positronium production in cryogenic environments in Physical Review B

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Cooper BS (2015) A trap-based pulsed positron beam optimised for positronium laser spectroscopy. in The Review of scientific instruments

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Deller A (2016) Measurement of Rydberg positronium fluorescence lifetimes in Physical Review A

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Deller A (2019) SSPALS: A tool for studying positronium in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Description We developed new ways to generate Rydberg Ps atoms, and in the process advanced the areas of laser excitation of matter-antimatter atomic systems. The techniques we developed were thne used for a series of fundamental physics experiments.
Exploitation Route We are already using the methods we developed, and other research groups at CERN have also benefited from our work.
Sectors Other

Description Standard grant
Amount £802,355 (GBP)
Funding ID EP/R006474/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2017 
End 09/2021
Title New detection methodology for Rydberg Ps 
Description We developed new kind of detectors and analysis tools that are now being used by other workers in our field 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact New experiments are made possible bythis methodology thus advancing the field 
Description Mainz group Pohl 
Organisation Johannes Gutenberg University of Mainz
Country Germany 
Sector Academic/University 
PI Contribution Working with Professor Randolf Pohl we have joined a new ERC network (PREN: proton radius European Network). Our contribution was to provide a positronium aspect to the many diverse areas of the new network, which addresses Hadronic physics from an atomic perspective
Collaborator Contribution The PREN network was initiated by Prof. Pohl. They will administer it and use this proto group to develop a larger European network
Impact The collaboration is VERY multidisciplinary but hasn't officially started yet. Hence there are no outcomes yet, academic or otherwise.
Start Year 2017
Description QUB group 
Organisation Queen's University Belfast
Country United Kingdom 
Sector Academic/University 
PI Contribution Calculations of cross sections to compliment experiments
Collaborator Contribution They calculate we do experiments then we publish together
Impact PApers in press
Start Year 2014
Description Saclay group 
Organisation Saclay Nuclear Research Centre
Country France 
Sector Public 
PI Contribution Working with French collaborators has provided new data on Ps forming materials, which we have used for many experiments
Collaborator Contribution they make samples, we test them, and then we also get to use them, saving us a great deal of time and money
Impact several papers (see portfolio, and acknowledgements to Liszkay)
Start Year 2013
Description Serbian group 
Organisation Chinese Academy of Sciences
Department Institute of Biophysics
Country China 
Sector Academic/University 
PI Contribution They run simulations we do validation measurements
Collaborator Contribution Calculations to test different trap designs
Impact Papers in press
Start Year 2014
Description aarhus group 
Organisation Aarhus University
Country Denmark 
Sector Academic/University 
PI Contribution We have shared samples, and had reciprocal student exchange visits to collaborate on new atomic physics experiments.
Collaborator Contribution We have shared samples, and had reciprocal student exchange visits to collaborate on new atomic physics experiments. Time spent in Denmark by postdocs and students was very instructive. Time spent in London by Aarhus PhD student resulted in one paper
Impact Positronium emission and cooling in reflection and transmission from thin meso-structured silica films Søren L Andersen1, David B Cassidy2, Jacques Chevallier1, Ben S Cooper2, Adam Deller2, Thomas E Wall2 and Ulrik I Uggerhøj1 Published 16 September 2015 • © 2015 IOP Publishing Ltd • Journal of Physics B: Atomic, Molecular and Optical Physics, Volume 48, Number 20
Start Year 2014
Description UCL Science Club 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Presentation to 6th formers from London schools
Year(s) Of Engagement Activity 2016