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Spectroscopy of positronium: atom control and gravity measurements

Final Report Summary - SPECTRO-POS (Spectroscopy of positronium: atom control and gravity measurements)

Final Report: Spectropos

The spectro-pos project is aimed at performing advanced spectroscopy using the matter-antimatter system positronium (chemical symbol Ps). This atom is made up from an electron and a positron. Since this is a particle-antiparticle pair this is an unstable system that survives for a short time (on the order of 100 ns) before destroying itself. However, it is possible to prevent this self-destruction by exciting Ps atoms into what are known as Rydberg states. These are highly excited but still bound atomic states. In the Ps system the electron and positron hardly interact with each other when they are in this configuration and the annihilation process becomes negligible. By keeping Ps atoms alive for longer it becomes possible to do many kinds of new experiments with them. Our long-term interests are related to high precision spectroscopy, and studies of how matter-antimatter systems behave in gravitational fields.

The primary goals of the spectro-pos project can be summarized briefly as (1) advances in the creation of a cold positronium (Ps) source, (2) the excitation of Ps atoms into long-lived Rydberg states and (3) subsequent manipulation of the resulting excited Ps atoms via their dipole moments using inhomogeneous electric fields. These tasks are the necessary first steps to producing a long-lived Ps beam which can then be used to measure the gravitational free-fall of a matter-antimatter system. We have succeeded in the goal of creating and manipulating Rydberg Ps atoms (the papers describing how we did this can be found on our website

In order to begin this work it was necessary to establish a completely new laboratory (starting literally with an empty room). In the first year we have succeeded in setting up the new lab, constructing a positron beam and trap, and establishing modified measurement techniques suitable for Ps-laser interactions with our new arrangement. We have subsequently been able to conduct tests of several promising Ps production targets studies of the effects of electromagnetic fields on Ps lifetimes, Rydberg-Stark state production, and measurements of Rydberg Ps fluorescence lifetimes via decay in a Rydberg Ps beam. We have therefore made significant progress.

Activities outlined in the original plan work plan have largely been accomplished. All of the beam and laser installations have been completed. Ps targets have been extensively tested, and Rydberg PS production can now be accomplished with much higher efficiency and control than ever before. PS lifetime shave been directly measured (although we now know that S or D states cannot actually be produced in our system, and we must instead work with Stark states). A suspersonic gas jet has been constructed, but is behind schedule for full testing. Ps atoms optics have been installed and tested (this is ongoing). Attempts to measure black body effects were made, but none were observed. Instead we have started calculations so determine which states will be affected at which temperatures, and this whether we need to cool our environment. This represents most of what was planned for the first 8 quarters.

Some changes have been made to the original plan, based on what we have learned, and on things that did not work as we had expected. Thus, the idea to remoderate the beam to produce smaller spots in a region of zero magnetic field was (at least for now) abandoned. We found that we could do many of the experiments this would have facilitated using the system as it was originally configured. One exception to this was the two-photon direct Rydberg excitation scheme, which requires a larger power density and thus a focussed laser and small Ps source. Attempts to generate Ps with n = 10 in this way failed, almost certainly because the positron beam was too large. However, many other experiments worked, as indicated by our publications. The variety of our work demonstrates that the CIG has been successful in terms of making it possible to engage in collaborative research. Thanks in part to the freedom allowed by the CIG structure I have been able to directly collaborate with several research groups, as indicated below:

(1)Antihydrogen Production and Mesoporous silica films: L. Liszkay CEA Saclay, France

(2) Electron-positron plasma production: T. S. Pedersen, Max Planck Institute for Plasma Physics, Garching, Germany

(3) Positron-Atom bound states confined Ps : G. Gribakin, Queen’s University Belfast, UK

(4) Production and characterization of Zeolite Ps sources: P. Crivelli, ETH, Zurich, Switzerland

(5) Development of alternative trap technology: Z. Petrovic, Institute of Physics, Belgrade, Serbia

(6) EPR/positron studies of silica films: D. Keeble, University of Dundee

(7) Precision Rydberg spectroscopy: R. Pohl, JG University, Mainz, Germany

(8) Interferometry of macroscopic systems: H. Ullbricht, University of Southampton, UK

In addition to visiting collaborators or purchasing small items to facilitate collaborative research efforts, some CIG funds were used to help organize a workshop at UCL: the Workshop on Antimatter and Gravity (WAG) meeting is a fledgling workshop that will grow in future years as research into gravitational interactions of antimatter increase (several CERN experiments are looking at antihydrogen, and experiments with muonium at PSI have been discussed also). It was this very beneficial for us at UCL to host this meeting, whose importance I expect to increase as more experiments com online.

I have now been promoted to Reader and have a very productive research group, currently consisting of two PhD students and two postdocs (funded by the EPSRC and the Leverhulme Trust, UK). The additional CIG funds have been invaluable with facilitating many small experiments, but have mostly been used for the collaborative work mentioned above. The other sources of funding, although substantial, are much more restrictive, so I really appreciate that CIG model and am convinced that it has had a disproportionate benefit to my work. During the curse of the CIG I have been able to obtain significant finding from the UK government EPSRC via two grants: EP/R006474/1 [2017-2021] £802k and EP/K028774/1 [2013-2017] £694k.