Periodic Reporting for period 4 - ANTI-ATOM (Many-body theory of antimatter interactions with atoms, molecules and condensed matter)
Okres sprawozdawczy: 2023-08-01 do 2024-07-31
The positron is the antiparticle of the electron, and the simplest form of antimatter. Their ability to annihilate with atomic electrons makes them a unique probe of matter. As such, they have important use in medical imaging, materials science and astrophysics. They are also at the heart of more complicated antimatter: positronium (Ps, an electron-positron pair) and antihydrogen (a proton-antiproton pair), are under intensive study at international laboratories to test fundamental laws of nature.
When a positron interacts with normal matter (an atom or molecule), it pulls strongly on the atomic/molecular electron cloud, polarising it. The atomic/molecular electron may even temporarily "tunnel" to the positron, forming short-lived "virtual" Ps. These so-called 'correlations' significantly effect positron and Ps interactions with atoms and molecules, e.g. enhancing annihilation rates by many orders of magnitude, modifying the characteristic annihilation signal, and making the accurate description of these systems a challenging many-body problem. Proper interpretation, modelling and development of the antimatter-based fundamental experiments, materials science techniques and antimatter technologies (positron traps and accumulators, PET etc) require calculations that fully account for the correlations.
A powerful framework that can account for the correlations is many-body theory. In many-body theory, processes of interest are represented via relatively simple and intuitive (Feynman) diagrams, enabling identification of the key interactions, efficient computation, and providing keen insight. This project aimed to develop the many-body theory and its state-of-the art computational implementation, towards the accurate description of positron and Ps interactions (binding, scattering and annihilation) with matter, providing fundamental insight required to enable the accurate interpretation of and develop the difficult and costly antimatter experiments, material science techniques and technologies. In particular, it aimed to provide a proof of principle of the feasibility of a computational implementation of the many-body theory for multicentred targets (beyond atoms).
The objectives were successfully achieved, with a many-body theory description of positron multicentred targets developed and implemented in the new "EXCITON+" code; a number of groundbreaking results were obtained, with publications in Nature and multiple Physical Review Letters, PRA Letters, and JCP Emerging Investigator Special Collection etc. A key achievement was the development of the team members, including three Postdoctoral Research Fellows and graduation of multiple PhD students. In addition, the low-energy positron-matter problem provides a rich testbed for the development of other approaches to the many-electron problem, to which our results should provide benchmarks.
When a positron interacts with normal matter (an atom or molecule), it pulls strongly on the atomic/molecular electron cloud, polarising it. The atomic/molecular electron may even temporarily "tunnel" to the positron, forming short-lived "virtual" Ps. These so-called 'correlations' significantly effect positron and Ps interactions with atoms and molecules, e.g. enhancing annihilation rates by many orders of magnitude, modifying the characteristic annihilation signal, and making the accurate description of these systems a challenging many-body problem. Proper interpretation, modelling and development of the antimatter-based fundamental experiments, materials science techniques and antimatter technologies (positron traps and accumulators, PET etc) require calculations that fully account for the correlations.
A powerful framework that can account for the correlations is many-body theory. In many-body theory, processes of interest are represented via relatively simple and intuitive (Feynman) diagrams, enabling identification of the key interactions, efficient computation, and providing keen insight. This project aimed to develop the many-body theory and its state-of-the art computational implementation, towards the accurate description of positron and Ps interactions (binding, scattering and annihilation) with matter, providing fundamental insight required to enable the accurate interpretation of and develop the difficult and costly antimatter experiments, material science techniques and technologies. In particular, it aimed to provide a proof of principle of the feasibility of a computational implementation of the many-body theory for multicentred targets (beyond atoms).
The objectives were successfully achieved, with a many-body theory description of positron multicentred targets developed and implemented in the new "EXCITON+" code; a number of groundbreaking results were obtained, with publications in Nature and multiple Physical Review Letters, PRA Letters, and JCP Emerging Investigator Special Collection etc. A key achievement was the development of the team members, including three Postdoctoral Research Fellows and graduation of multiple PhD students. In addition, the low-energy positron-matter problem provides a rich testbed for the development of other approaches to the many-electron problem, to which our results should provide benchmarks.
The many-body theory of positron interactions with multi-centred targets was successfully developed. It was implemented in our new code "EXCITON+", which was developed from the all-electron code "EXCITON" of Charles Patterson (Trinity College Dublin). Specifically, our approach involves solving the Dyson equation, constructing the positron-molecule interaction diagrammatically including correlations such as polarization of the electron cloud, screening of the electron-positron Coulomb interactions, and the process of virtual-positronium formation (where an atomic/molecular electron temporarily tunnels to the positron).
In [Nature 606, 688 (2022)] we successfully applied the approach to perform the first accurate ab initio description of positron binding in polyatomic molecules. We found excellent agreement with measurements by the pioneering UCSD experimental group of Prof Clifford Surko, solving a long-standing problem in the field. Our work also provided fundamental insight, notably delineating the effects of distinct correlations, the importance of the anisotropic nature of the interactions, and contributions of individual molecular orbitals, and made predictions for nucleobases. In [Phys Rev Lett 130, 263001 (2023)] we extended the capability to calculate positron scattering and annihilation on small molecules, on the same theoretical footing. In [Phys. Rev. A Letter 109, L040801 (2024)] we gave the first accurate ab initio description of positron binding energies in planar molecules, and explained the role of molecular properties in the weakening of positron binding energies with respect to substitution of the halogen atom. This was followed by a joint-theoretical-experimental paper in collaboration with the pioneering UCSD Surko group that demonstrated the predictive power of the methodologies developed via blind comparison and agreement of our new calculations and new measurements. A distinct work [J. Chem. Phys. 160, 084304] applied the many-body theory approach to calculate potential energy curves involving positronic molecular systems, to give new predictions of positronic-bonded molecules (where a positron bonds two anions that would otherwise repel, relating to antimatter-matter chemistry).
A new code was developed and published [Comp. Phys. Commun. 250, 107112 (2020)] that enabled calculation of the electron and positron density in central potentials, relevant to models of condensed matter of positrons in confined electron gas (so-called "jellium" model); this provided much of the content of the PhD thesis of Dr David Waide (graduated 2022) and resulted in a preprint [arXiv:2108.05850].
Insights on positron cooling in molecular gases and comparisons with experiments were given in [Phys Rev Lett 130, 033001 (2023)].
The many-body theory for Ps-atom interactions was developed via comprehensive work in [Phys Rev A 107, 042802 (2023)]; Scattering cross sections and pick-off annihilation rates (where the positron in Ps annihilates on one of the atomic electrons) were calculated on the same footing; the latter calculations the most accurate to date. Ps confined in a cavity was studied using physically motivated potentials (to model Ps in a pore of a microporous material).
Overall, our project has undoubtedly realised a transformative step in understanding of low-energy positron-matter interactions. A number of groundbreaking results were obtained. Dissemination also included ~20 talks at international conferences, including two Plenary talks at flagship international conferences. Our work on positron binding is also due to be included in a new chapter of a proposed new edition of the textbook "Many-Body Theory Exposed! (Wiley)". The project was proposed with a long term vision in mind: the work undertaken in the project is a stepping stone, with much still to do. The work provides a solid foundation in terms of theoretical and computational methodology and the EXCITON+ code that should ensure more groundbreaking results emerge.
In [Nature 606, 688 (2022)] we successfully applied the approach to perform the first accurate ab initio description of positron binding in polyatomic molecules. We found excellent agreement with measurements by the pioneering UCSD experimental group of Prof Clifford Surko, solving a long-standing problem in the field. Our work also provided fundamental insight, notably delineating the effects of distinct correlations, the importance of the anisotropic nature of the interactions, and contributions of individual molecular orbitals, and made predictions for nucleobases. In [Phys Rev Lett 130, 263001 (2023)] we extended the capability to calculate positron scattering and annihilation on small molecules, on the same theoretical footing. In [Phys. Rev. A Letter 109, L040801 (2024)] we gave the first accurate ab initio description of positron binding energies in planar molecules, and explained the role of molecular properties in the weakening of positron binding energies with respect to substitution of the halogen atom. This was followed by a joint-theoretical-experimental paper in collaboration with the pioneering UCSD Surko group that demonstrated the predictive power of the methodologies developed via blind comparison and agreement of our new calculations and new measurements. A distinct work [J. Chem. Phys. 160, 084304] applied the many-body theory approach to calculate potential energy curves involving positronic molecular systems, to give new predictions of positronic-bonded molecules (where a positron bonds two anions that would otherwise repel, relating to antimatter-matter chemistry).
A new code was developed and published [Comp. Phys. Commun. 250, 107112 (2020)] that enabled calculation of the electron and positron density in central potentials, relevant to models of condensed matter of positrons in confined electron gas (so-called "jellium" model); this provided much of the content of the PhD thesis of Dr David Waide (graduated 2022) and resulted in a preprint [arXiv:2108.05850].
Insights on positron cooling in molecular gases and comparisons with experiments were given in [Phys Rev Lett 130, 033001 (2023)].
The many-body theory for Ps-atom interactions was developed via comprehensive work in [Phys Rev A 107, 042802 (2023)]; Scattering cross sections and pick-off annihilation rates (where the positron in Ps annihilates on one of the atomic electrons) were calculated on the same footing; the latter calculations the most accurate to date. Ps confined in a cavity was studied using physically motivated potentials (to model Ps in a pore of a microporous material).
Overall, our project has undoubtedly realised a transformative step in understanding of low-energy positron-matter interactions. A number of groundbreaking results were obtained. Dissemination also included ~20 talks at international conferences, including two Plenary talks at flagship international conferences. Our work on positron binding is also due to be included in a new chapter of a proposed new edition of the textbook "Many-Body Theory Exposed! (Wiley)". The project was proposed with a long term vision in mind: the work undertaken in the project is a stepping stone, with much still to do. The work provides a solid foundation in terms of theoretical and computational methodology and the EXCITON+ code that should ensure more groundbreaking results emerge.
Our new approach implemented in the EXCITON+ code provides a state-of-the-art for ab initio calculations of positron-molecule interactions, currently unrivalled in its ability to treat positron binding, scattering and annihilation accurately and on the same footing. Overall, our project has undoubtedly realised a transformative leap in understanding of low-energy positron-matter interactions.