Showing papers on "Positronium published in 2014"
TL;DR: In this paper, the ground state hyperfine splitting of positronium Δ HFS is sensitive to high order corrections of quantum electrodynamics (QED) in bound state.
67 citations
08 May 2014
TL;DR: In this paper, the authors consider some of the obstacles that will have to be overcome in order to perform a direct measurement of the gravitational free-fall of positronium atoms, and propose a solution to overcome them.
Abstract: We consider some of the obstacles that will have to be overcome in order to perform a direct measurement of the gravitational free-fall of positronium atoms. Foremost among these are the production of positronium atoms in a cryogenic environment, efficient excitation of these atoms to suitably long-lived Rydberg states, and their subsequent control via the interaction of their large electric dipole moments with inhomogeneous electric fields. Recent developments in all of these areas can be directly applied to a positronium free-fall gravity measurement, making such an endeavour both timely and feasible.
64 citations
TL;DR: A review of new developments in the experimental field of the efficient emission of the positronium negative ion, a bound state of one positron and two electrons, from alkali-metal coated surfaces is given in this article.
57 citations
TL;DR: In this paper, the authors studied positron scattering and annihilation on noble-gas atoms using many-body theory methods for positron energies below the positronium formation threshold and provided a near-complete understanding of positron-noble-gas atom system.
Abstract: Positron scattering and annihilation on noble-gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range, the many-body theory yields accurate numerical results and provides a near-complete understanding of the positron--noble-gas atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative effect of virtual positronium formation. These correlations have a large influence on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter ${Z}_{\mathrm{eff}}$ (effective number of electrons participating in annihilation) rise steeply along the sequence of noble-gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.
55 citations
TL;DR: It is shown that because of the intrinsic instability of annihilation decay of BEC, the spontaneously emitted entangled photon pairs are amplified, leading to an exponential buildup of a macroscopic population into end-fire modes at a certain shape of the elongated condensate.
Abstract: A theoretical study suggests a new way to build a gamma-ray laser from a Bose-Einstein condensate of electron-positron pairs.
44 citations
TL;DR: Utilising a high-resolution, trap-based positron beam, both elastic and inelastic scattering of positrons from water vapour are measured, using the Independent Atom Model for positron interactions.
Abstract: Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and distorted wave calculations, and with our own application of the Independent Atom Model for positron interactions.
33 citations
TL;DR: In this article, a comparison of the latest measurements and calculations for low-energy positron collisions with the noble gases and a brief update of the newest studies on other atoms is presented.
Abstract: We review recent cross section results for low-energy positron scattering from atomic targets. A comparison of the latest measurements and calculations for positron collisions with the noble gases and a brief update of the newest studies on other atoms is presented. In particular, we provide an overview of the cross sections for elastic scattering, positronium formation, direct and total ionisation, as well as total scattering, at energies typically between about 0.1 and a few hundred eV. We discuss the differences in the current experimental data sets and compare those results to the available theoretical models. Recommended data sets for the total cross section are also reported for each noble gas. A summary of the recent developments in the scattering from other atoms, such as atomic hydrogen, the alkali and alkaline-earth metals, and two-electron systems is finally provided.
32 citations
TL;DR: A review of experimental methods used in positron spectroscopy and examples of their application to free volume studies are presented in this article, where the authors also provide an overview of the free volume analysis in condensed matter.
Abstract: Positronium (a hydrogen-like bound state of an electron and a positron) is a convenient probe to determine the sizes of subnanometric free volumes (voids) in condensed matter. A review of experimental methods used in positron spectroscopy and examples of their application to the free volume studies are presented.
30 citations
TL;DR: In this article, a discrepancy of 3.9 standard deviations has been found between the measured hyperfine structure (Ps-HFS) and the QED predictions, due to the contribution of unknown new physics or common systematic effects in previous measurements, in all of which the Zeeman effect was used.
Abstract: Positronium is an ideal system for research on Quantum Electrodynamics (QED), especially in a bound state. A discrepancy of 3.9 standard deviations has been found between the measured hyperfine structure (Ps-HFS) and the QED predictions. This may be due to the contribution of unknown new physics or common systematic effects in previous measurements, in all of which the Zeeman effect was used. We propose a new method to directly measure the Ps-HFS using a high power gyrotron. We compare two resonators which have been developed to supply sufficient power to drive the direct transition, a Fabry-Perot resonator and a ring resonator with a diffraction grating. We plan to perform first direct measurement of Ps-HFS within the next six months.
28 citations
TL;DR: The first measurement of the Breit-Wigner resonance of the transition from ortho-position to para-position was reported in this paper, where an optical system was developed to accumulate a power of over 20 kW using a frequency-tunable gyrotron and a Fabry-Perot cavity.
Abstract: We report on the first measurement of the Breit-Wigner resonance of the transition from {\it ortho-}positronium to {\it para-}positronium. We have developed an optical system to accumulate a power of over 20 kW using a frequency-tunable gyrotron and a Fabry-Perot cavity. This system opens a new era of millimeter-wave spectroscopy, and enables us to directly determine both the hyperfine interval and the decay width of {\it p-}Ps.
27 citations
TL;DR: The result for the O(α7me) one photon annihilation contribution to the hyperfine splitting of the ground state energy levels in positronium increases the prediction of quantum electrodynamics by 217±1 kHz.
Abstract: We present the complete result for the O(α7me) one photon annihilation contribution to the hyperfine splitting of the ground state energy levels in positronium. Numerically it increases the prediction of quantum electrodynamics by 217±1 kHz.
TL;DR: In this paper, the results of positronium (Ps) emission into vacuum from self-assembled metal-organic frameworks (MOFs) are presented and discussed in detail, showing that a fraction of the Ps is emitted into vacuum with a distinctly smaller energy than what one would expect for Ps localized in the MOFs' cells.
Abstract: Results of positronium (Ps) emission into vacuum from self-assembled metal-organic frameworks (MOFs) are presented and discussed in detail. Four different MOF crystals are considered, namely, MOF-5, IRMOF-8, ${\mathrm{ZnO}}_{4}$(FMA)${}_{3}$, and IRMOF-20. The measurements reveal that a fraction of the Ps is emitted into vacuum with a distinctly smaller energy than what one would expect for Ps localized in the MOFs' cells. Only calculations considering the Ps delocalized in a Bloch state can reproduce the measured Ps emission energy providing a robust demonstration of wave function delocalization in quantum mechanics. We show how the Bloch state population can be controlled by tuning the initial positron beam energy. Therefore, Ps in MOFs can be used both to simulate the dynamics of delocalized excitations in materials and to probe the MOFs for their advanced characterization.
TL;DR: Analysis and calculations of Ps-Kr and Ps-Ar collisions provide a theoretical explanation of the similarity between the cross sections for positronium scattering and electron scattering for a range of atomic and molecular targets observed by S. Brawley et al.
Abstract: We employ the impulse approximation for a description of positronium-atom scattering. Our analysis and calculations of Ps-Kr and Ps-Ar collisions provide a theoretical explanation of the similarity between the cross sections for positronium scattering and electron scattering for a range of atomic and molecular targets observed by S. J. Brawley et al. [Science 330, 789 (2010)].
TL;DR: In this paper, the positron binding energies to all atoms in the Periodic Table are presented and atoms where positron-atom binding actually exists are identified and the results of these calculations and accurate calculations of other authors (which existed for several atoms only) are used to evaluate recommended values of positron bound energies to the ground states of atoms.
Abstract: Calculations of the positron binding energies to all atoms in the Periodic Table are presented and atoms where the positron-atom binding actually exists are identified. The results of these calculations and accurate calculations of other authors (which existed for several atoms only) are used to evaluate recommended values of the positron binding energies to the ground states of atoms. We also present the recommended energies of the positron excited bound levels and resonances (due to the binding of positron to excited states of atoms) which cannot emit positronium and have relatively narrow widths. Such resonances in positron annihilation and scattering may be used to measure the positron binding energy.
TL;DR: In this paper, a two-step optical transition via $2P$ levels was used to prepare Rydberg positronium (Ps) atoms up to the term limit and the spectra were corrected for the first-order Doppler shift using measured velocities.
Abstract: The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via $2P$ levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\ensuremath{\le}n\ensuremath{\le}31$. The excitation signal amplitude begins to decrease for $ng50$, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.
TL;DR: Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed, using a trap-based beam apparatus, and their measurements are compared to existing experimental results and theoretical calculations.
Abstract: Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180 eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach.
TL;DR: In this article, the Schmidt decomposition method was applied to the partial-wave expanded two-electron wave functions, and from which the one-particle reduced density matrix can be obtained, leading to the quantifications of linear entropy and von Neumann entropy in the H − and Ps − ions.
TL;DR: In this article, a pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated.
Abstract: A pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated. This approach allows us to extend the Ps-atom cross sections, obtained previously in the impulse approximation [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold. Although experimental data are not available in this low-energy region, our results describe well the tendency of the measured cross sections to drop with decreasing velocity at $vl1$ a.u. Our results show that the effect of the Ps-atom van der Waals interaction is weak compared to the polarization interaction in electron-atom and positron-atom scattering. As a result, the Ps scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend minimum is not observed for Ps scattering from these targets. This makes Ps scattering quite different from electron scattering in the low-energy region, in contrast to the intermediate energy range from the Ps ionization threshold up to $v\ensuremath{\sim}2$ a.u., where the two are similar.
TL;DR: Dose dependence of o-Ps lifetime τ3 and intensity I3 at the incident positron energy of 2.15 keV correlates well with the dose dependence of S-parameter and seems to account for the existence of the expected two processes, scission of polymer chains and appearance of free radicals preceding the aggregation of the clusters resulting in the formation of network of conjugated bonds at lower ion doses and carbonization at higher ion doses.
Abstract: Slow positron beam and optical absorption measurements are carried out to study structural defects and positronium formation in 40 keV B(+)-implanted polymethylmethacrylate (B:PMMA) with ion doses from 6.25 × 10(14) to 5.0 × 10(16) ions/cm(2). Detailed depth-selective information on defects in implanted samples was obtained by measuring of Doppler broadening of positron annihilation γ rays as a function of incident positron energy and these experimental results were compared with SRIM (stopping and range of ions in matter) simulation results. Two general processes, appearance of free radicals at lower ion doses ( 10(16) ions/cm(2)), are considered from the Doppler S-E and W-E dependences in the framework of the concept of defects formation during radiation damage of polymer structure. Probabilities of ortho-positronium (o-Ps) formation are analyzed using S-W plot and slow positron annihilation lifetime measurements. Dose dependence of o-Ps lifetime τ3 and intensity I3 at the incident positron energy of 2.15 keV correlates well with the dose dependence of S-parameter and seems to account for the existence of the expected two processes, i.e., scission of polymer chains and appearance of free radicals preceding the aggregation of the clusters resulting in the formation of network of conjugated bonds at lower ion doses and carbonization at higher ion doses. The increase of optical absorption observed with increasing ion implantation dose also suggests a formation of carbonaceous phase in the ion-irradiated PMMA.
TL;DR: In this paper, the authors investigated the possibility to produce a Bose-Einstein condensate made of positronium atoms in a porous silica material containing isolated nanometric cavities.
Abstract: We investigate the possibility to produce a Bose-Einstein condensate made of positronium atoms in a porous silica material containing isolated nanometric cavities. The evolution equation of a weakly interacting positronium system is presented. The model includes the interactions among the atoms in the condensate, the surrounding gas of noncondensed atoms, and the pore surface. The final system is expressed by the Boltzmann evolution equation for noncondensed particles coupled with the Gross-Pitaevskii equation for the condensate. In particular, we focus on the estimation of the time necessary to form a condensate containing a macroscopic fraction of the positronium atoms initially injected in the material. The numerical simulations reveal that the condensation process is compatible with the lifetime of ortho-positronium.
TL;DR: The contribution of positronium hyperfine splitting to the light-by-light scattering process in the exchange of two photons between the electron and positron was calculated in this article.
Abstract: We have calculated the contribution to the positronium hyperfine splitting at order $m \alpha^7$ of the light-by-light scattering process in the exchange of two photons between the electron and positron. Our result is $\Delta E = -0.235355(8) m \alpha^4 \left ( \frac{\alpha}{\pi} \right )^3 = -1.034 kHz$. As a check of our approach we confirm earlier evaluations of the analogous correction for a bound system (such as muonium) with unequal masses.
TL;DR: In this paper, the authors studied the spinor dynamics and annihilation processes in the p-Ps/o-Ps system using both solutions of the time-dependent Gross-Pitaevskii equations and a semiclassical rate-equation approach.
Abstract: Bose-Einstein condensates (BECs) of positronium (Ps) have been of experimental and theoretical interest due to their potential application as the gain medium of a γ-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho-positronium (o-Ps) and para-positronium (p-Ps), whose annihilation lifetimes differ by three orders of magnitude. In this paper, we study the spinor dynamics and annihilation processes in the p-Ps/o-Ps system using both solutions of the time-dependent Gross-Pitaevskii equations and a semiclassical rate-equation approach. The spinor interactions have an O(4) symmetry which is broken to SO(3) by an internal energy difference between o-Ps and p-Ps. For an initially unpolarized condensate, there is a threshold density of ≈ 10 19 cm 3 at which spin mixing between o-Ps and p-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability.
TL;DR: In this paper, a light-by-light scattering correction to the real decay of parapositronium into two photons was proposed. But the result was not as precise as the one given in this paper.
Abstract: Recent and ongoing experimental work on the positronium spectrum motivates new efforts to calculate positronium energy levels at the level of three loop corrections. We have obtained results for one set of such corrections involving light-by-light scattering of the photons produced in a two-photon virtual annihilation process. Our result is an energy shift $1.58377(8) m \alpha^7/\pi^3$ for the n=1 singlet state, correcting the ground state hyperfine splitting by -6.95 kHz. We also obtained a new and more precise result for the light-by-light scattering correction to the real decay of parapositronium into two photons.
TL;DR: In this article, the authors reported the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events, and measured the o-Ps formation fraction and its lifetime.
Abstract: The Double Chooz experiment measures the neutrino mixing angle $\theta_{13}$ by detecting reactor $\bar{
u}_e$ via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the ortho-positronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44$\%$ $\pm$ 12$\%$ (sys.) $\pm$ 5$\%$ (stat.) and $3.68$ns $\pm$ 0.17ns (sys.) $\pm$ 0.15ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.
TL;DR: In this paper, the authors reported the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events and measured the o-Ps formation fraction and its lifetime.
Abstract: The Double Chooz experiment measures the neutrino mixing angle θ
13 by detecting reactor $$ {\overline{
u}}_e $$
via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the ortho-positronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44 % ±12 % (sys.) ±5 % (stat.) and 3.68 ns ±0.17 ns (sys.) ±0.15 ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.
TL;DR: In this article, mesoporous silica films with various porosities were prepared via a sol-gel method using a nonionic amphiphilic triblock copolymer F127 as the structure-directing agent.
Abstract: Porous silica films with various porosities were prepared via a sol-gel method using a nonionic amphiphilic triblock copolymer F127 as the structure-directing agent. Doppler broadening of positron annihilation radiation (DBAR) spectra were collected for the prepared films using a variable energy slow positron beam. Different linear relationships between positron annihilation line shape parameters S and W are found for the as-deposited films and calcined ones, indicative of the decomposition of the copolymer porogen in the as-deposited films upon calcination. This also reveals the variation of positron annihilation sites as a function of F127 loading or porosity. Strong correlations between positronium 3γ annihilation fraction, S parameter and porosity of the mesoporous silica films with isolated pores are obtained, which may provide a complementary method to determine closed porosities of mesoporous silica films by DBAR.
TL;DR: In this paper, a simple and useful formalism for hadrons spectroscopy in QCD was proposed, based on Relativistic Hamiltonians derived from the path integrals.
Abstract: Relativistic Hamiltonians, derived from the path integrals, are known to provide a simple and useful formalism for hadrons spectroscopy in QCD. The accuracy of this approach is tested using the QED systems, and the calculated spectrum is shown to reproduce exactly that of the Dirac hydrogen atom, while the Breit-Fermi nonrelativistic expansion is obtained using Foldy-Wouthuizen transformation. Calculated positronium spectrum, including spin-dependent terms, coincides with the standard QED perturbation theory to the considered order O(alpha^s).
TL;DR: In this paper, the authors defined single-particle potentials for a positron and an electron-positron correlation potential for light atoms interacting with light atoms (H, He, Li, and Be) by inverting a singleparticle Schrodinger equation.
Abstract: In this work, we define single-particle potentials for a positron and a positronium atom interacting with light atoms (H, He, Li, and Be) by inverting a single-particle Schr\"odinger equation. For this purpose, we use accurate energies and positron densities obtained from the many-body wave function of the corresponding positronic systems. The introduced potentials describe the exact correlations for the calculated systems including the formation of a positronium atom. We show that the scattering lengths and the low-energy $s$-wave phase shifts from accurate many-body calculations are well accounted for by the introduced potential. We also calculate self-consistent two-component density-functional-theory positron potentials and densities for the bound positronic systems within the local-density approximation. They are in a very good agreement with the many-body results, provided that the finite-positron-density electron-positron correlation potential is used, and they can also describe systems comprising a positronium atom. We argue that the introduced single-particle positron potentials defined for single molecules are transferable to the condensed phase when the intermolecular interactions are weak. When this condition is fulfilled, the total positron potential can be constructed in a good approximation as the superposition of the molecular potentials.
TL;DR: In this paper, the Aarhus positron beam line was used to measure the positronium formation fraction from porous silica thin films created by the Glancing Angle Deposition technique.
Abstract: The Aarhus positron beam line is utilised to measure the positronium formation fraction from porous silica thin films created by the Glancing Angle Deposition technique. The highest formation fraction found from these studies in a backscattering geometry is (57.7 ± 1.0)% in good agreement with other measurements. In transmission mode, the maximum positronium output is found to be (12.5 ± 0.5)%. These are the first measurements of positronium formation in transmission of a porous silica thin film, a starting point for future attempts to optimise the positronium formation in transmission.
TL;DR: In this article, a review summarizes experimental studies with proton beams at fixed target and colliding beam accelerators, covering a center-of-mass energy range from 39 to 7000 GeV for nucleon and antiproton targets.
Abstract: Both charm and bottom quarks form nonrelativistic bound states analogous to positronium. The J/psi and psi(2S) charmonium states and the first three Upsilon(nS) bottomonium states, all spin-triplet S-wave quarkonium states below open-heavy-flavor thresholds, have relatively large branching ratios to e- e+ or mu- mu+ pairs. In hadron collisions, experiments measuring lepton pairs can determine polarization by using angular correlation techniques. The polarization, in turn, can be related theoretically to the production mechanism for the bound state. This review summarizes experimental studies with proton beams at fixed target and colliding beam accelerators, covering a center-of-mass energy range from 39 to 7000 GeV for nucleon and antiproton targets. Analyses using various polarization frames and spin-quantization axes are described and results compared. A pattern emerges that connects experimental results over the whole energy span. The theoretical implications of the pattern are presented and a set of new measurements is proposed.