Table of contents

This volume of Journal of Physics: Conference Series contains the Proceedings of the XV International Workshop on Low Energy Positron and Positronium Physics, which was held at York University, Toronto, Canada, between 29 July and 1 August 2009. This Workshop was one of the official satellites of the XXVI International Conference on Photonic, Electronic and Atomic Collisions, held at Kalamazoo, MI, USA between 22 and 28 July 2009. Our Workshop was run in parallel with the XVI International Conference on Electron-Molecule Collisions and Swarms, enabling delegates to select talks from either meeting. The proceedings of the XVI International Conference on Electron-Molecule Collisions and Swarms are being published in a separate volume of Journal of Physics: Conference Series.

The combined attendance at the two meetings was 110, from 19 countries, with interest being roughly split between the two areas. There were 5 plenary lectures addressed to all participants and on the Positron Workshop side there were 20 invited talks and 40 posters presented in two poster sessions. The subjects covered antihydrogen, positron collisions with atoms and molecules, annihilation, bound and resonant states, positronium and positronium negative ion, plasmas and swarms. The talks and posters highlighted the vigour of the field and underlined the impressive new developments now in progress and which promise to transform the subject.

The next Workshop of this biennial series of meetings will be held in July 2011 at Maynooth University in Ireland.

R I Campeanu J W Darewych A D Stauffer Guest Editors

INTERNATIONAL ADVISORY COMMITTEE FOR POSITRON WORKSHOP

Roberto S Brusa (Italy) Michael Charlton (UK) Arnab S Ghosh (India) Franco A Gianturco (Italy) Gleb F Gribakin (UK) John Humberston (UK) Helge Knudsen (Denmark) Akos Kover (Hungary) Gaetana Laricchia (UK) Marco AP Lima (Brazil) Allen Mills (USA) Yasuyuki Nagashima (Japan) Clifford M Surko (USA) James Walters (UK) Sandra Ward (USA)

LOCAL ORGANIZING COMMITTEE FOR POSITRON WORKSHOP

Radu I Campeanu (York, Chairman) Jurij W Darewych (York) Allan D Stauffer (York) Cody Storry (York)

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Recent positronium collision experiments are reviewed in which the cross-sections for projectile- and/or target-fragmentation processes are determined. In contrast with theory, the likelihood for the latter process appears to be significant for positronium collisions with Xe at only ~3 eV above threshold. Measurements of the total direct ionization cross-section by positron impact, performed to test the reliability of the experimental apparatus and method, are also presented.

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS: about 203 GHz) is sensitive to new physics beyond the Standard Model via a vacuum oscillation between an ortho-Ps and a virtual photon. Previous experimental results of the Ps-HFS show 3.9 σ (15 ppm) discrepancy from the QED calculation. All previous experiments used an indirect method with static magnetic field to cause Zeeman splitting (a few GHz) between triplet states of ortho-Ps, from which the HFS value was derived. One possible systematic error source of the indirect method is the static magnetic field. We are developing a new direct measurement system of the Ps-HFS without static magnetic field. In this measurement we use a gyrotron, a novel sub-THz light source, with a high-finesse Fabry-Perot cavity to obtain enough radiation power at 203 GHz. The present status of the optimization studies and current design of the experiment are described.

Recently, we have observed the efficient emission of positronium negative ions from Cs coated W(100) and polycrystalline Mo and Ta surfaces. The efficient emission enables us to perform new experimental studies on these ions. The investigations of the efficient production of positronium negative ions are reviewed and a measurement of the binding energy of the ions and a photodetachment experiment are proposed.

The ground state hyperfine splitting in positronium, Δ_HFS, is sensitive to high order corrections of QED. A new calculation up to O(α³) has revealed a 3.9 σ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure Δ_HFS employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a preliminary result of Δ_HFS = 203.399±0.029 GHz (143 ppm) has been obtained. A measurement with a precision of O(1) ppm is expected within a few years.

Magnetised positronium is formed by impacting low energy positrons onto a gas covered target immersed in a magnetic field (B ≥ 1T). The resulting weakly bound positronium atoms subsequently travel some distance in an arrangement of Penning-type traps whereupon they can be field ionised. The remnant positrons are accumulated and then detected by forced annihilation on the target. The production efficiency of the magnetised atoms has been measured for different species of gases, gas layer thickness and the strength of the magnetic field. The positronium loss as a function of the distance travelled has been measured and is shown to be caused by the magnetron drift of the positronium atom.

The positronium formation cross-sections for Xe, CO₂ and N₂ have been measured using coincidences between γ-rays from positronium self-annihilation and the resultant ion. In the case of Xe, there is excellent agreement with previous experimental determinations. For CO₂ there is broad agreement in magnitude with previous measurements in contrast with N₂ where good shape agreement at low energies (< 40 eV) is found though the magnitude of the present cross-section is significantly higher.

In this paper, we consider two scattering processes: low-energy positron-hydrogen-molecule and helium-antihydrogen scattering. In the positron-hydrogen-molecule scattering calculations, we use the Kohn variational method to calculate Z_eff, the number of target electrons available to the positron for annihilation. In the helium-antihydrogen scattering calculations, we use the Rayleigh-Ritz variational method to calculate a wave function for the leptons as a function of the distance between the helium and the antihydrogen. This is used, together with the associated nuclear wave function and the wave function for α + Ps⁻, to calculate the cross section for the rearrangement reaction He + → α + Ps⁻, using the T-matrix and a form of the distorted wave approximation. For both processes, positron-electron correlation is taken into account accurately using Hylleraas-type functions.

We have simulated the formation of antihydrogen through three-body recombination using classical trajectories of antiprotons and positrons. The simulations include several effects which are important in current antihydrogen experiments: the full motion of the antiproton repeatedly passing into and out of the positron plasma, the energy loss of antiprotons due to the interaction with the positron plasma, and the field-ionization of antihydrogen en route from the plasma to the detector. We find that whereas the overall simulated rate of formation of antihydrogen has a density dependence close to n²_e, the rate of antihydrogen detection follows a power law less than 2. The difference is due to the effect of density dependent field ionization.

Considerable efforts have been made and are still being made to verify the validity of the principle of the equivalence of gravitational and inertial mass, one of the cornerstones of the classical theory of general relativity. Specific attempts at quantum-mechanical formulations of gravity allow for non-Newtonian contributions which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction is independent of the composition of bodies, this has only been tested for matter systems, but never yet for antimatter systems. By combining techniques from different fields, and relying on recent developments in the production of Positronium and ongoing work to laser-excite Positronium to Rydberg states, such a test with neutral antimatter has become feasible. The primary goal of the AEGIS experiment being built at the Antiproton Decelerator at CERN is to carry out the first direct measurement of the Earth's gravitational acceleration on antihydrogen by means of a classical Moiré deflectometer.

The relativistic calculations of the Zeeman splitting of hyperfine levels in two-fermion systems (hydrogen, anti-hydrogen, muonium, muonic hydrogen) are presented. Our approach is based on the variational equation for bound states derived from quantum electrodynamics [1]. Relativistic corrections to the g-factor are obtained up to O((α)²). Calculations are provided for all quantum states and for arbitrary fermionic mass ratio. The results will be useful for comparison with high-precision measurements.

Calculations have demonstrated that eleven neutral atoms can bind positrons, while many more can bind positronium. This is a short review of recent progress made in understanding some of the underlying mechanisms. The emphasis here being on configuration interaction calculations with excited state configurations. These have demonstrated the existence of a ²P^o excited state of e⁺Ca, which consists predominantly of a positronium cluster orbiting the Ca⁺ ion in the L = 1 partial wave. Preliminary results are presented of excited state positron binding to a model alkali atom, where the excited ¹P^o states are stable over a limited region. Implications for the unnatural parity, ^2,4S^o, states of PsH, LiPs, NaPs and KPs are also discussed. The e⁺Mg, e⁺Cu, e⁺Zn and e⁺Cd systems show a lack of a ²P^o excited state, each instead possessing a low-energy p-wave shape resonance of varying strength.

Experiments show that positron annihilation on molecules frequently occurs via capture into vibrational Feshbach resonances. In these cases, the downshifts in the annihilation spectra from the vibrational mode spectra provide measures of the positron-molecule binding energies. An analysis of these binding energy data is presented in terms of the molecular dipole polarizability, the permanent dipole moment, and the number of π bonds in aromatic molecules. The results of this analysis are in reasonably good agreement with other information about positron-molecule bound states. Predictions for other targets and promising candidate molecules for further investigation are discussed.

Positron annihilation in ammonia is analyzed using the framework of resonant annihilation [G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006)]. In particular, we show that molecular rotations can have a measurable effect on the annihilation rates at room temperatures. Rotation leads to broadening of vibrational Feshbach resonances. Rotations also allow a distinct contribution at low positron energies in the form of a rotational Feshbach resonance. This resonance can enhance the annihilation rate for thermalized room-temperature positrons. Comparison of theory and experiment shows that overtone and combination vibrations, including those due to inversion doubling, likely play an important role.

Relaxed geometries and dipole moments of diatomic molecules interacting with a slow positron are reported as functions of the positron distance to the more electronegative atom. A molecular model for the complex that allows applications to large systems is used. The electron population on the positron is proposed as a weighting function to calculate the average quantities. Results show Self-Consistent-Field quality or better.

Low-energy elastic scattering of electrons from the ground state of positronium atoms embedded in weakly-coupled plasmas has been studied within the framework of an effective range theory. Plasma screening effect has been taken care of by screened Coulomb potential. Highly correlated and variationally determined wave functions for Ps⁻ are used to determine the effective range of the ion states. The results for S-wave singlet phase shifts for the incident electron momentum in the range of [0, 0.5] a.u. are reported, to the best of our knowledge, for the first time in the literature.

Recently a new wave of swarm studies of positrons was initiated based on more complete scattering cross section sets. Initially some interesting and new physics was discovered, most importantly negative differential conductivity (NDC) that occurs only for the bulk drift velocity while it does not exist for the flux property. However the ultimate goal was to develop tools to model positron transport in realistic applications and the work that is progressing along these lines is reviewed here. It includes studies of positron transport in molecular gases, thermalization in generic swarm situations and in realistic gas filled traps and transport of positrons in crossed electric and magnetic fields. Finally we have extended the same technique of simulation (Monte Carlo) to studies of thermalization of positronium molecule. In addition, recently published first steps towards including effects of dense media on positron transport are summarized here.

A parameter-free spherical complex optical potential model is used to calculate the differential and total cross sections for the scattering of positrons by the NH₃ molecules in the energy range 10 eV − 400 eV. The optical potential is constructed from a near-Hartree-Fock one-center expansion of ammonia (NH₃) wave function and it is then treated exactly in a partial-wave analysis involving variable phase approach to extract complex phase shifts. The present theoretically calculated results show good agreement with the available experimental results for total cross sections at positron energy ≥ 30 eV.

Theoretical cross-sections for the ionization of the water molecule by electron and positron impact are presented. The calculations were performed in the framework of the simpler CPE and two distorted-wave models, ES and TS, by employing Gaussian wavefunctions for the description of the target. We found good agreement with the experiment, especially for higher impact energies.

We compare the recent values of positron total cross sections in benzene, cyclohexane and aniline by Zecca et al. with our previous measurements [Karwasz et al. Acta Phys. Pol. A, 107 (2005) 666] on the same apparatus. An apparent discrepancy disappears if Zecca's data are shifted by +0.2 eV; then their data can be approximated by the modified effective range theory, as well. This observation induced us to compare recent Trento data with other experiments and theories for polar targets, H₂O and HCOOH. The evidence points out towards the need of a careful energy calibration in positron experiments.

The results of investigations of ionization of atoms and molecules by positron impact are presented, with particular focus on the electron capture to the continuum phenomenon. The application of recoil ion momentum spectroscopy in positron collisions are also discussed.

Positron-helium collisions have been studied using a two-center close-coupling method. Convergent, pseudoresonance-free cross sections have been obtained by using complete expansions on both the helium and positronium centers. The low energy elastic cross sections, phase-shifts and total cross sections are compared with experimental data and results of other theories.

The possibility of performing kinematically complete measurements of ionization collisions by positron impact brings about the promise of encountering new unexpected phenomena, which otherwise would be impossible to foresee. In this communication, we employ the classical trajectory Monte-Carlo method to explore H₂ ionization fully differential cross section (FDCS), depending on the relative momentum of the electron-positron continuum dipole and the deflection angle of its center of mass. We find a strong structure in the FDCS at non-zero deflection angles. Finally, we discuss some of these new effects, and the possibility of observing them in actual experiments.

Positron scattering is complementary to electron scattering due to the difference in polarity of charge. This makes positrons an alternative tool to study atomic and molecular structure. In this paper, we report our calculations of differential and total cross sections for the elastic scattering of positrons with argon atoms in the energy range from 1 – 10 eV. A spherical optical potential approach is employed. The static potential is included exactly at the Hartree-Fock level, the short-range correlation and long-range polarization effects are target density functional and are free from any adjustable parameter. Our results are in fair agreement between experimental measurements and other theories.

An electrostatic slow positron beam system has been developed for positron scattering studies. The apparatus uses a remoderator in reflection geometry in conjunction with an electrostatic hemispherical energy analyzer for the brightness enhancement. It is possible to determine absolute total scattering cross section values under magnetic field-free conditions down to a few mG. Measurement of the energy distribution of the positrons that are reemitted from the remoderator and total cross sections for positron-He scattering are presented.

The progress toward the construction of a positron reaction microscope is outlined. The design principles of an electrostatic lens system used to focus and transport the positron beam with a ∼ 1 mm diameter spot are briefly discussed. Also presented here are the results obtained from the characterisation of the supersonic gas jet assembly, an increased peaking factor of ∼ 4 has been observed along with a centerline density of ∼ 10¹² cm⁻³.