Showing papers on "Positronium published in 2016"

Gamma-ray spectroscopy of positron annihilation in the Milky Way

Abstract: T. Siegert, et al., “Gamma-ray spectroscopy of positron annihilation in the Milky Way”, Astronomy & Astrophysics, Vol. 586, January 2016, https://doi.org/10.1051/0004-6361/201527510. Reproduced with permission from Astronomy & Astrophysics, © ESO”.

Laser excitation of the n=3 level of positronium for antihydrogen production

Abstract: S. Aghion,1,2 C. Amsler,3 A. Ariga,3 T. Ariga,3 G. Bonomi,4,5 P. Bräunig,6 J. Bremer,7 R. S. Brusa,8,9 L. Cabaret,10 M. Caccia,2,11 R. Caravita,12,13 F. Castelli,2,14 G. Cerchiari,15 K. Chlouba,16 S. Cialdi,2,14 D. Comparat,10 G. Consolati,1,2 A. Demetrio,6 L. Di Noto,12,13 M. Doser,7 A. Dudarev,7 A. Ereditato,3 C. Evans,1,2 R. Ferragut,1,2 J. Fesel,7 A. Fontana,5 O. K. Forslund,7 S. Gerber,7 M. Giammarchi,2,3 A. Gligorova,17 S. Gninenko,18 F. Guatieri,8,9 S. Haider,7 H. Holmestad,19 T. Huse,19 I. L. Jernelv,7 E. Jordan,15 A. Kellerbauer,15 M. Kimura,3 T. Koettig,7 D. Krasnicky,12,13 V. Lagomarsino,12,13 P. Lansonneur,23 P. Lebrun,23 S. Lehner,20 J. Liberadzka,7 C. Malbrunot,7,20 S. Mariazzi,20,* L. Marx,7 V. Matveev,18,21 Z. Mazzotta,2,14 G. Nebbia,22 P. Nedelec,23 M. Oberthaler,6 N. Pacifico,17 D. Pagano,4,5 L. Penasa,8,9 V. Petracek,16 C. Pistillo,3 F. Prelz,2 M. Prevedelli,24 L. Ravelli,8,9 L. Resch,7 B. Rienäcker,7 O. M. Røhne,19 A. Rotondi,5,25 M. Sacerdoti,2,14 H. Sandaker,19 R. Santoro,2,11 P. Scampoli,3,26 L. Smestad,7,27 F. Sorrentino,12,13 M. Spacek,16 J. Storey,3 I. M. Strojek,16 G. Testera,13 I. Tietje,7 S. Vamosi,20 E. Widmann,20 P. Yzombard,10 J. Zmeskal,20 and N. Zurlo5,28 (AEgIS Collaboration) 1Politecnico of Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy, 2Istituto Nazionale di Fisica Nucleare Milano, via Celoria 16, 20133 Milano, Italy 3Laboratory for High Energy Physics, Albert Einstein Center for Fundamental Physics,

Potential of the J-PET detector for studies of discrete symmetries in decays of positronium atom - a purely leptonic system

Abstract: The Jagiellonian Positron Emission Tomograph (J-PET) was constructed as a prototype of the cost-effective scanner for the simultaneous metabolic imaging of the whole human body. Being optimized for the detection of photons from the electron-positron annihilation with high time- and high angular-resolution, it constitutes a multi-purpose detector providing new opportunities for studying the decays of positronium atoms. Positronium is the lightest purely leptonic object decaying into photons. As an atom bound by a central potential it is a parity eigenstate, and as an atom built out of an electron and an anti-electron it is an eigenstate of the charge conjugation operator. Therefore, the positronium is a unique laboratory to study discrete symmetries whose precision is limited in principle by the effects due to the weak interactions expected at the level of (~10$^{-14}$) and photon-photon interactions expected at the level of (~10$^{-9}$). The J-PET detector enables to perform tests of discrete symmetries in the leptonic sector via the determination of the expectation values of the discrete-symmetries-odd operators, which may be constructed from the spin of ortho-positronium atom and the momenta and polarization vectors of photons originating from its annihilation. In this article we present the potential of the J-PET detector to test the C, CP, T and CPT symmetries in the decays of positronium atoms.

Recent progress in the description of positron scattering from atoms using the Convergent Close-Coupling Theory

Abstract: Much progress in the theory of positron scattering on atoms has been made in the ten years since the review of Surko, Gribakin and Buckman [J. Phys. B 38, R57 (2005)]. We review this progress for few-electron targets with a particular emphasis on the two-centre convergent close-coupling and other theories which explicitly treat positronium (Ps) formation. While substantial progress has been made for Ps formation in positron scattering on few-electron targets, considerable theoretical development is still required for multielectron atomic and molecular targets.

Recent progress in the description of positron scattering from atoms using the convergent close-coupling theory

Abstract: Much progress in the theory of positron scattering on atoms has been made in the ten years since the review of Surko, Gribakin and Buckman [J. Phys. B 38, R57 (2005)]. We review this progress for few-electron targets with a particular emphasis on the two-centre convergent close-coupling and other theories which explicitly treat positronium (Ps) formation. While substantial progress has been made for Ps formation in positron scattering on few-electron targets, considerable theoretical development is still required for multielectron atomic and molecular targets.

Scattering data for modelling positron tracks in gaseous and liquid water

Abstract: We present in this study a self-consistent set of scattering cross sections for positron collisions with water molecules, in the energy range 0.1-10 000 eV, with the prime motivation being to provide data for modelling purposes. The structure of the database is based on a new model potential calculation, including interference terms, which provides differential and integral elastic as well as integral inelastic positron scattering cross sections for water molecules over the whole energy range considered here. Experimental and theoretical data available in the literature have been integrated into the database after a careful analysis of their uncertainties and their self-consistency. These data have been used as input parameters for a step-by-step Monte Carlo simulation procedure, providing valuable information on energy deposition, positron range, and the relative percentages of specific interactions (e.g. positronium formation, direct ionisation, electronic, vibrational and rotational excitations) in gaseous and liquid water.

Potential of the J-PET detector for studies of discrete symmetries in decays of positronium atom - a purely leptonic system

Abstract: The Jagiellonian Positron Emission Tomograph (J-PET) was constructed as a prototype of the cost-effective scanner for the simultaneous metabolic imaging of the whole human body. Being optimized for the detection of photons from the electron-positron annihilation with high time- and high angular-resolution, it constitutes a multi-purpose detector providing new opportunities for studying the decays of positronium atoms. Positronium is the lightest purely leptonic object decaying into photons. As an atom bound by a central potential it is a parity eigenstate, and as an atom built out of an electron and an anti-electron it is an eigenstate of the charge conjugation operator. Therefore, the positronium is a unique laboratory to study discrete symmetries whose precision is limited in principle by the effects due to the weak interactions expected at the level of (~10$^{-14}$) and photon-photon interactions expected at the level of (~10$^{-9}$). The J-PET detector enables to perform tests of discrete symmetries in the leptonic sector via the determination of the expectation values of the discrete-symmetries-odd operators, which may be constructed from the spin of ortho-positronium atom and the momenta and polarization vectors of photons originating from its annihilation. In this article we present the potential of the J-PET detector to test the C, CP, T and CPT symmetries in the decays of positronium atoms.

Observation of a shape resonance of the positronium negative ion

Abstract: The Positronium negative ion is formed by two electrons bound to a positron, and experimental investigations of its states and energy levels are difficult due to its short lifetime. Here, the authors report on laser spectroscopy of positronium using a source of efficiently produced ions.

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

Abstract: Predicted 20 years ago, positron binding to neutral atoms has not yet been observed experimentally. A scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for charge transfer in Ps collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.

Electrostatically Guided Rydberg Positronium.

Abstract: We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670 V cm^{-1} electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance.

Determination of the $3\gamma $ Fraction from Positron Annihilation in Mesoporous Materials for Symmetry Violation Experiment with J-PET Scanner

Abstract: Various mesoporous materials were investigated to choose the best material for experiments requiring high yield of long-lived positronium. We found that the fraction of 3\gamma annihilation determined using \gamma-ray energy spectra and positron annihilation lifetime spectra (PAL) changed from 20% to 25%. The 3gamma fraction and o-Ps formation probability in the polymer XAD-4 is found to be the largest. Elemental analysis performed using scanning electron microscop (SEM) equipped with energy-dispersive X-ray spectroscop EDS show high purity of the investigated materials.

Positronium formation in collisions between positrons and alkali-metal atoms (Li, Na, K, Rb and Cs) in Debye plasma environments

Abstract: Positronium (Ps) formation processes in collisions between positrons and alkali atoms (Li, Na, K, Rb and Cs) in Debye plasma environments have been investigated using the classical trajectory Monte Carlo (CTMC) method for the energy range 1–500 keV. Interactions between the active electron in an alkali atom with the positron have been described using a model potential along with the Debye–Huckel potential. In such a process, positronium formation cross sections have been calculated in unscreened, as well as in Debye plasma, conditions. The results show that the cross sections for positronium formation are significantly affected by Debye screening lengths. Also, positronium formation depends on collision energy and plasma conditions. The effect of Debye plasma environments on such a formation process is explained in terms of the classical trajectory framework.

Theoretical Formalism To Estimate the Positron Scattering Cross Section

Abstract: A theoretical formalism is introduced in this article to calculate the total cross sections for positron scattering. This method incorporates positron–target interaction in the spherical complex optical potential formalism. The study of positron collision has been quite subtle until now. However, recently, it has emerged as an interesting area due to its role in atomic and molecular structure physics, astrophysics, and medicine. With the present method, the total cross sections for simple atoms C, N, and O and their diatomic molecules C2, N2, and O2 are obtained and compared with existing data. The total cross section obtained in the present work gives a more consistent shape and magnitude than existing theories. The characteristic dip below 10 eV is identified due to the positronium formation. The deviation of the present cross section with measurements at energies below 10 eV is attributed to the neglect of forward angle-discrimination effects in experiments, the inefficiency of additivity rule for mole...

Regularities in positronium formation for atoms and molecules

Abstract: In an effort to aid the modelling of positron and positronium (Ps) transport in biological media we have compiled recent experimental results for the total Ps formation in positron scattering from atoms and molecules. A simple function was found to adequately describe the total Ps formation cross section for both atoms and molecules. The parameters of this function describe the magnitude and shape of the Ps formation cross section and are compared to physical characteristics of the target atoms and molecules. A general trend in the magnitude of the total Ps formation cross section is observed as a function of the target atom/molecule dipole polarisability. The functional form may enable quick estimation of the Ps cross section for molecules for which experimental measurements or theoretical estimates do not exist.

Experimental and theoretical cross sections for positron scattering from the pentane isomers.

Abstract: Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present.

Near-threshold behavior of positronium-antiproton scattering

Abstract: Using the convergent close-coupling theory we study the threshold behavior of cross sections for positronium (Ps) of energy $E$ scattering on antiprotons. In the case of $\mathrm{Ps}(1s$) elastic scattering, simple power laws are observed for all partial waves studied. The partial-wave summed cross section is nearly constant, and dominates the antihydrogen formation cross section at all considered energies, even though the latter is exothermic and behaves as $1/{E}^{1/2}$. For $\mathrm{Ps}(2s$), oscillations spanning orders of magnitude on top of the $1/E$ behavior are found in the elastic and quasielastic cross sections. The antihydrogen formation is influenced by dipole-supported resonances below the threshold of inelastic processes. Resonance energies form a geometric progression relative to the threshold. The exothermic antihydrogen formation cross sections behave as $1/E$ at low energies, but are oscillation free. We demonstrate that all these rich features are reproduced by the threshold theory developed by Gailitis [J. Phys. B: At. Mol. Phys. 15, 3423 (1982)].

Study on cooling of positronium for Bose–Einstein condensation

Abstract: A proposed new method of cooling positronium is to realize the Bose–Einstein condensation (BEC) of positronium. We perform detailed studies of three processes: (1) thermalization processes between positronium and the silica walls of a cavity, (2) Ps–Ps scattering and (3) laser cooling. The thermalization process is shown to be not sufficient for BEC. Ps–Ps collision is shown to have a big effect on the cooling performance. We combine both methods and establish an efficient cooling process for BEC. We also propose a new optical laser system for the cooling.

Positronium collisions with rare-gas atoms

Abstract: We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method (Fabrikant and Gribakin 2014 Phys. Rev. A 90 052717) and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (break-up). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps–Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.

Heating due to momentum transfer in low-energy positronium-antiproton scattering

Abstract: We investigate the consequences of unexpectedly large elastic cross sections for the scattering of low-energy antiprotons from $n\ensuremath{\le}3$ positronium (Ps) on the experimental implementation of antihydrogen formation via Ps-antiproton collisions. The integrated elastic cross sections, obtained using the two-center convergent close-coupling theory, can be up to three orders of magnitude greater than their counterparts for antihydrogen formation. The differential momentum transfer cross sections, which suppress the large cross sections at forward scattering angles, show remarkably rich behavior across all scattering angles. We discuss the implications of these findings for the heating, via momentum transfer, of clouds of trapped antiprotons that are typically used for the creation of antihydrogen.

Large numbers of cold positronium atoms created in laser-selected Rydberg states using resonant charge exchange

Abstract: Lasers are used to control the production of highly excited positronium atoms (Ps*). The laser light excites Cs atoms to Rydberg states that have a large cross section for resonant charge-exchange collisions with cold trapped positrons. For each trial with 30 million trapped positrons, more than 700 000 of the created Ps* have trajectories near the axis of the apparatus, and are detected using Stark ionization. This number of Ps* is 500 times higher than realized in an earlier proof-of-principle demonstration (2004 Phys. Lett. B 597 257). A second charge exchange of these near-axis Ps* with trapped antiprotons could be used to produce cold antihydrogen, and this antihydrogen production is expected to be increased by a similar factor.

On the Stability of Classical Orbits of the Hydrogen Ground State in Stochastic Electrodynamics

Abstract: De la Pena 1980 and Puthoff 1987 show that circular orbits in the hydrogen problem of Stochastic Electrodynamics connect to a stable situation, where the electron neither collapses onto the nucleus nor gets expelled from the atom. Although the Cole-Zou 2003 simulations support the stability, our recent numerics always lead to self-ionisation. Here the de la Pena-Puthoff argument is extended to elliptic orbits. For very eccentric orbits with energy close to zero and angular momentum below some not-small value, there is on the average a net gain in energy for each revolution, which explains the self-ionisation. Next, an 1 / r 2 potential is added, which could stem from a dipolar deformation of the nuclear charge by the electron at its moving position. This shape retains the analytical solvability. When it is enough repulsive, the ground state of this modified hydrogen problem is predicted to be stable. The same conclusions hold for positronium.

Internal consistency in the close-coupling approach to positron collisions with atoms

Abstract: The positron-atom scattering problem contains the rearrangement channel of positronium (Ps) formation. While this makes the problem particularly difficult to calculate, it has the unusual benefit of validation via consideration of the internal consistency of the vastly different one- and two-centre close-coupling approaches. For example, the ionisation cross section in the former must be the same as the sum of breakup and Ps formation cross sections in the latter. This places a severe test on both approaches, which we review here for positron scattering on hydrogen and helium atoms.

Rotational excitation of H 2 by positron impact in adiabatic rotational approximation

Abstract: We present rotational excitation cross sections for low energy scattering of positrons by H2, below positronium formation threshold (≈8.2 eV). The cross sections were computed from fixed-nuclei scattering amplitudes generated with the Schwinger multichannel method using the adiabatic rotational approximation. Comparison with other similar theoretical results shows that the magnitude of the cross section associated to the transition J i = 0 → J f = 2 is strongly affected by the methodology used to model the correlation-polarization effects.

Do positrons measure atomic and molecular diameters

Abstract: We report on density functional calculations (DFT) of elastic integral scattering cross-sections for positron collisions with argon, krypton, nitrogen and methane. The long-range asymptotic polarization potential is described using higher-order terms going much beyond an induced dipole potential (−α / r 4) while the short-range interaction is modeled by two different forms of electron – positron correlation potential (Boronski-Nieminen and Quantum Monte Carlo potentials). The results of both approaches agree quite well with the recent theoretical and measured values. Based on the present and previous theoretical and experimental data we discuss some systematics observed in integral scattering cross-sections below the positronium formation threshold. In particular we point out on the correlation between the values of scattering cross-sections and atomic dimensions.

Positronium energy levels at order m α 7 : Product contributions in the two-photon-annihilation channel

Abstract: Ongoing improvements in the measurement of positronium transition intervals motivate the calculation of the $O(m{\ensuremath{\alpha}}^{7})$ corrections to these intervals. In this work we focus on corrections to the spin-singlet parapositronium energies involving virtual annihilation to two photons in an intermediate state. We have evaluated all contributions to the positronium $S$-state energy levels that can be written as the product of a one-loop correction on one side of the annihilation event and another one-loop correction on the other side. These effects contribute $\mathrm{\ensuremath{\Delta}}E=\ensuremath{-}0.561971(25)m{\ensuremath{\alpha}}^{7}/{\ensuremath{\pi}}^{3}$ to the parapositronium ground-state energy.

Polarization dependence of n=2 positronium transition rates to Stark-split n=30 levels via crossed-beam spectroscopy

Abstract: We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F, the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F, the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001) obtained with n = 11 and will be useful in the preparation of high-n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.

P-Wave Positron-Hydrogen Scattering, Annihilation, and Positronium Formation

Abstract: In a previous paper (Bhatia A.K. 2016), a hybrid theory for the scattering of positrons from hydrogen atoms was applied to calculate S-wave phase shifts, annihilation, and positronium formation cross sections. This approach is now being applied to calculate P-wave positron-hydrogen scattering. The present results, obtained using short-range correlation functions along with long-range correlations in the Schrodinger equation at the same time, agree very well with the results obtained in an earlier calculation by Bhatia et al. (1974), using the Feshbach projection operator formalism. In these earlier calculations, the correction due to the long-range correlations was applied to the variational results. In spite of the fact that this ad hoc correction destroyed the variational bound, the final results have been considered accurate. Annihilation cross-sections, positronium formation cross-sections, calculated in the distorted-wave approximation, are also presented.