Utilizing the two-center convergent close-coupling method, we find a several order of magnitude enhancement in the formation of antihydrogen via antiproton scattering with positronium in an excited state over the ground state. The effect is greatest at the lowest energies considered, which encompass those achievable in experiment. This suggests a practical approach to creating neutral antimatter for testing its interaction with gravity and for spectroscopic measurements.

The two-centre convergent close-coupling method is used to calculate antihydrogen (H) formation via positronium (Ps) scattering on antiprotons (p) at near threshold energies. For excited Ps of energy ε, the 1/ε behavior of theH formation cross sections is valid strictly only at the respective threshold, as is the 1/ √ ε behaviour for Ps in the ground state. Simple equations are given for thē H(n ≤ 4) formation cross sections from Ps(n ≤ 3) from zero to around 0.1 eV above threshold. Some of the implications of usingp-Ps collisions to form antihydrogen in beams, and held in traps, are discussed.

The atomic hydrogen target has played a pivotal role in the development of quantum collision theory. The key complexities of computationally managing the countably infinite discrete states and the uncountably infinite continuum were solved by using atomic hydrogen as the prototype atomic target. In the case of positron or proton scattering the extra complexity of charge exchange was also solved using the atomic hydrogen target. Most recently, molecular hydrogen has been used successfully as a prototype molecule for developing the corresponding scattering theory. We concentrate on the convergent close-coupling computational approach to light projectiles, such as electrons and positrons, and heavy projectiles, such as protons and antiprotons, scattering on atomic and molecular hydrogen.

Positron binding to molecules is key to extremely enhanced positron annihilation and positron-based molecular spectroscopy1. Although positron binding energies have been measured for about 90 polyatomic molecules1–6, an accurate ab initio theoretical description of positron–molecule binding has remained elusive. Of the molecules studied experimentally, ab initio calculations exist for only six; these calculations agree with experiments on polar molecules to at best 25 per cent accuracy and fail to predict binding in nonpolar molecules. The theoretical challenge stems from the need to accurately describe the strong many-body correlations including polarization of the electron cloud, screening of the electron–positron Coulomb interaction and the unique process of virtual-positronium formation (in which a molecular electron temporarily tunnels to the positron)1. Here we develop a many-body theory of positron–molecule interactions that achieves excellent agreement with experiment (to within 1 per cent in cases) and predicts binding in formamide and nucleobases. Our framework quantitatively captures the role of many-body correlations and shows their crucial effect on enhancing binding in polar molecules, enabling binding in nonpolar molecules, and increasing annihilation rates by 2 to 3 orders of magnitude. Our many-body approach can be extended to positron scattering and annihilation γ-ray spectra in molecules and condensed matter, to provide the fundamental insight and predictive capability required to improve materials science diagnostics7,8, develop antimatter-based technologies (including positron traps, beams and positron emission tomography)8–10, and understand positrons in the Galaxy11.

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Positrons bind to molecules leading to vibrational excitation and spectacularly enhanced annihilation. 1 Whilst positron binding energies have been measured via resonant annihilation spectra for ∼ 80 molecules in the past two decades, [2][3][4][5][6][7][8][9][10][11][12] an accurate ab initio theoretical description has remained elusive. Of the molecules studied experimentally, calculations exist for only 6, and for these, standard quantum chemistry approaches have proved severely deficient, agreeing with experiment to at best 25% accuracy for polar molecules, and failing to predict binding in non-polar molecules. The theoretical difficulty lies in the need to accurately account for positron-molecule correlations including polarisation of the electron cloud, screening of the positron-molecule Coulomb interaction by molecular electrons, and the unique non-perturbative process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron). Their roles in positron-molecule binding have yet to be elucidated. Here, we develop a diagrammatic many-body description of positron-molecule interactions that takes ab initio account of the correlations, applying it to calculate positron binding energies for the molecules for which both theory and experimental results exist. Delineating the effects of the correlations, we find that in particular, virtual-positronium formation dramatically enhances binding in organic polar molecules, and can be essential to support binding in non-polar molecules. Overall, we find the best agreement with experiment to date (in some cases to within a few percent). The approach can be extended to provide predictive calculations of positron scattering and annihilation γ spectra in molecules and condensed matter. The fundamental insight provided by such capability is required to, e.g., develop antimatter-based technologies including positron traps, beams and positron emission tomography, properly interpret materials science diagnostic techniques, 13,14 and understand positrons in the galaxy. 15 Moreover, the positron-matter problem provides an unforgiving testbed for the development of computational methods to tackle the quantum many-body problem, for which our results can serve as benchmarks.Pioneering technological developments have enabled the trapping, accumulation and delivery 14,16,17 of positrons for study of their fundamental interactions with atoms and molecules, 1, 18 and the formation, exploitation and interrogation of more complicated antimatter, namely positronium (Ps), 19,20 and antihydrogen. 21 The ability of positrons to annihilate with atomic electrons forming characteristic γ rays make them a unique probe over vast length scales, giving them important use in e.g., materials science for ultra-sensitive diagnostics of industrially important materials and surface processes, 13, 14 positron emission tomography (PET) for functional medical imaging, 22 and in astrophysics. 15 Proper interpretation of the difficult and costly antimatter experiments and ma...

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