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The nonlinear theory of amplitude modulation of electrostatic wave envelopes in a collisionless electron-positron (EP) pair plasma is studied by using a set of Vlasov-Poisson equations in the context of Tsallis' q-nonextensive statistics. In particular, the previous linear theory of Langmuir oscillations in EP plasmas [Saberian and Esfandyari-Kalejahi, Phys. Rev. E 87, 053112 (2013)] is rectified and modified. Applying the multiple scale technique (MST), it is shown that the evolution of electrostatic wave envelopes is governed by a nonlinear Schrödinger (NLS) equation with a nonlocal nonlinear term ∝P∫|ϕ(ξ′,τ)|²dξ′ϕ/(ξ−ξ′) [where P denotes the Cauchy principal value, ϕ is the small-amplitude electrostatic (complex) potential, and ξ and τ are the stretched coordinates in MST], which appears due to the wave-particle resonance. It is found that a subregion 1/3

Positronium spectroscopy (n = 1 hyperfine splitting, n = 2 fine structure, and the 25-15 interval) has reached a precision of order 1 MHz. Vigorous ongoing efforts to improve the experimental results motivate the calculation of the positronium energy levels at order mα7. In this Letter, we present the result for a complete class of such contributions--those involving virtual annihilation of positronium to three photons in an intermediate state. We find an energy shift of 2.6216(1l) mα7/(nπ)³ = 11.5/n³ kHz for the spin-triplet 5 state with principal quantum number n. The corresponding energy shift for true muonium (the μ+μ- bound state) is 2.38/n³ MHz with an additional -5.33/n³ MHz coming from electronic vacuum polarization. [ABSTRACT FROM AUTHOR]

We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to n = 2 sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., 13S → 23P → nS/nD), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a 23S1 component represents an efficient route to producing long-lived pure 23S1 atoms via single-photon excitation. [ABSTRACT FROM AUTHOR]

Ultrarelativistic-energy electron ring structures have been observed from laser-wakefield acceleration experiments in the blowout regime. These electron rings had 170-280 MeVenergies with 5%-25% energy spread and ~ 10 pC of charge and were observed over a range of plasma densities and compositions. Threedimensional particle-in-cell simulations show that laser intensity enhancement in the wake leads to sheath splitting and the formation of a hollow toroidal pocket in the electron density around the wake behind the first wake period. If the laser propagates over a distance greater than the ideal dephasing length, some of the dephasing electrons in the second period can become trapped within the pocket and form an ultrarelativistic electron ring that propagates in free space over a meter-scale distance upon exiting the plasma. Such a structure acts as a relativistic potential well, which has applications for accelerating positively charged particles such as positrons. [ABSTRACT FROM AUTHOR]

The first absolute experimental determinations of the differential cross sections for the formation of ground-state positronium are presented for He, Ar, H2, and CO2 near 0°. Results are compared with available theories. The ratio of the differential and integrated cross sections for the targets exposes the higher propensity for forward emission of positronium formed from He and H2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s → 2p → nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n = 17 and 18 while the latter is performed for n = 11 in a homogeneous electric field of 1.9 kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n = 2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms. [ABSTRACT FROM AUTHOR]

Two oppositely charged particles created within a microscopic space-time region can be separated, accelerated over a much larger distance, and brought to a recollision by a laser field. Consequently, new reactions become feasible, where the energy absorbed by the particles is efficiently released. By investigating the laser-dressed polarization operator, we identify a new contribution describing high-energy recollisions experienced by an electron-positron pair generated by pure light when a gamma photon impinges on an intense, linearly polarized laser pulse. The energy absorbed in the recollision process over the macroscopic laser wavelength corresponds to a large number of laser photons and can be exploited to prime high-energy reactions. Thus, the recollision contribution to the polarization operator differs qualitatively and quantitatively from the well-known one, describing the annihilation of an electronpositron pair within the microscopic formation region. [ABSTRACT FROM AUTHOR]

Giant cross-magnetic-field steps can occur as a result of positron-electron collisions. Within a constant magnetic field (e.g., 1 T), a collision between a positron and an electron can result in a correlated drift across the magnetic field for a continuous range of impact parameters. Within this range, drift distances orders of magnitude larger than that associated with like-charge collisions were observed by computer simulation. Outside of this range, the collisional behavior is similar to that for collisions between particles with the same charge. A theoretical analysis of the phenomenon using center-of-mass and relative coordinates provides insights regarding the occurrence of giant cross-magnetic-field steps. [ABSTRACT FROM AUTHOR]

When Dirac electrons on the surface of a topological insulator are gapped, the resulting quantum anomalous Hall effect leads to universal magneto-optical Faraday and Kerr effects in the low-frequency limit. However, at higher frequencies different excitations can leave their own fingerprints on the magneto-optics and can therefore be probed. In particular, we investigate the role of localized in-gap states-which inevitably appear in the presence of charged impurities-on these higher-frequency magneto-optical effects. We have shown that these states resonantly contribute to the Hall conductivity and are magneto-optically active. These in-gap states lead to peculiar resonant signatures in the frequency dependence of the Faraday and Kerr angles, distinct in character to the contribution of in-gap excitonic states, and they can be probed in ellipsometry measurements. [ABSTRACT FROM AUTHOR]

Single molecules that exhibit narrow optical transitions at cryogenic temperatures can be used as local electricfield sensors. We derive the single-charge sensitivity of aromatic organic dye molecules, based on quantum mechanical considerations. Through numerical modeling, we demonstrate that by using currently available technologies it is possible to optically detect charging events in a granular network with a sensitivity better than 10-5e/√Hz and track positions of multiple electrons, simultaneously, with nanometer spatial resolution. Our results pave the way for minimally invasive optical inspection of electronic and spintronic nanodevices and building hybrid optoelectronic interfaces that function at both single-photon and single-electron levels. [ABSTRACT FROM AUTHOR]

The impact of a primary positron onto a surface may lead to the emission of a correlated positronelectron pair. By means of a lab-based positron beam we studied this pair emission from various surfaces. We analyzed the energy spectra in a symmetric emission geometry. We found that the available energy is shared in an unequal manner among the partners. On average the positron carries a larger fraction of the available energy. The unequal energy sharing is a consequence of positron and electron being distinguishable particles. We provide a model which explains the experimental findings. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO3 consists of nondegenerate t2g states with different dimensional characters. While the dxy bands have quasi-2D dispersions with weak tz dependence, the hfted dtxz/tyz bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the dtxz/tyz Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO3 in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O Is core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of neai-EF electrons in the surface region. [ABSTRACT FROM AUTHOR]

We propose methods for extracting limits on the strength of P-odd interactions of pseudoscalar and pseudovector cosmic fields with electrons, protons, and neutrons, by exploiting the static and dynamic parity-nonconserving amplitudes and electric dipole moments they induce in atoms. Candidates for such fields are dark matter (including axions) and dark energy, as well as several more exotic sources described by Lorentz-violating standard model extensions. Atomic calculations are performed for H, Li, Na, K, Rb, Cs, Ba+, Tl, Dy, Fr, and Ra+ . From these calculations and existing measurements in Dy, Cs, and Tl, we constrain the interaction strengths of the parity-violating static pseudovector cosmic field to be 7 x 10-15 GeV with an electron, and 3 x 10-8 GeV with a proton. [ABSTRACT FROM AUTHOR]

A scheme of an intense coherent gamma-ray source based on the spontaneous radiation of positronium atoms in a Bose-Einstein condensate (BEC) due to two-photon collective annihilation decay is investigated analytically arising from the second quantized formalism. 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. The considered scheme may also be applied to a BEC of atoms or quasiparticles as a laser mechanism with double coherence to create entangled photonic beams with a macroscopic number of photons. [ABSTRACT FROM AUTHOR]

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)]. [ABSTRACT FROM AUTHOR]

A massive, but light, Abelian U(1) gauge boson is a well-motivated possible signature of physics beyond the standard model of particle physics. In this Letter, the search for the signal of such a U(1) gauge boson in electron-positron pair production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron is described. Exclusion limits in the mass range of 40 MeV/c² to 300 MeV/c², with a sensitivity in the squared mixing parameter of as little as ε²=8×10-7 are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Electron-positron pair production in oscillating electric fields is investigated in the nonperturbative threshold regime. Accurate numerical solutions of quantum kinetic theory for corresponding observables are presented and analyzed in terms of a proposed model for an effective mass of electrons and positrons acquired within the given strong electric field. Although this effective mass cannot provide an exact description of the collective interaction of a charged particle with the strong field, physical observables are identified which carry direct and sensitive signatures of the effective mass. [ABSTRACT FROM AUTHOR]

The correlated motion of a positron surrounded by electrons is a fundamental many-body problem. We approach this by modeling the momentum density of annihilating electron-positron pairs using the framework of reduced density matrices, natural orbitals, and natural geminals (electron-positron pair wave functions) of the quantum theory of many-particle systems. We find that an expression based on the natural geminals provides an exact, unique, and compact expression for the momentum density. The natural geminals can be used to define and to determine enhancement factors for enhancement models going beyond the independent-particle model for a better understanding of the results of positron annihilation experiments. [ABSTRACT FROM AUTHOR]

Analytic calculations are presented of the contribution to the anomalous magnetic moment of a muon from the channels of annihilation of an electron-positron pair to a pair of charged pi mesons, with radiative correction connected with the final state, as well as corrections to the lowest-order kernel. The result with pointlike pi-meson assumptions is apoint=apoint (1)+∆apoint, apoint(1)=7.0866×10-9, ?apoint=-2.4×10-10. Taking into account the pion form factor in the frames of the Nambu-Jona-Lasinio (NJL) approach leads to aNJL=aNJL(1)+∆aNJL, aNJL(1)=5.48×10-8, ∆aNJL=-3.43×10-9. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime spectroscopy (PALS) measurements were performed between 93 and 293 K in order to study the supercooled smectic-£ (Sm-£) phase of 4-n-butyl-4'-isothiocyanato-1,1 '-biphenyl (4TCB), the ordered molecular crystal of 4TCB, and the phase transition between the Sm-£ phase and the ordered molecular crystal of 4TCB. The phase transition was well reflected in the abrupt increase of the o/t/io-positronium (o-Ps) lifetime and intensity. The value of the o-Ps lifetime in the Sm-£ liquid crystalline phase of 4TCB, i.e., 2.21 ns at room temperature, was explained by the formation of bubbles induced by Ps atoms, which are created due to a liquidlike state of the butyl chains of 4TCB molecules in the Sm-£ phase. The temperature dependence of the o-Ps intensity for the supercooled Sm-E phase can be explained by thermal generation of sites where bubbles are formed; an activation energy equal to 0.30 ± 0.02 eV was estimated. This value was compared with the activation energies of molecular motions. The o-Ps lifetime in the ordered molecular crystal was interpreted as originating from the annihilation of o-Ps confined in molecular vacancy-type imperfections in the crystal lattice. The value of the o-Ps pickoff annihilation between 1.8 and 1.9 ns is in accordance with the size of the molecular vacancy for the 4TCB crystal lattice. Its intensity is lower than 5%. The isothermal crystallization of the 4TCB Sm-£ phase was observed by PALS. The low-dimensional crystal growth was concluded from the Avrami equation fitted to the time dependence of the o-Ps intensity, which resulted in an Avrami exponent equal to 1.73 [ABSTRACT FROM AUTHOR]

Positronium (Ps) is shown to exist in a delocalized state in self-assembled metalorganic crystals that have large 1.3-1.5 nm cell sizes. Belonging to a class of materials with record high accessible specific surface areas, these highly porous crystals are the first to allow direct probing with simple annihilation lifetime techniques of the transport properties of long-lived triplet Ps in what is hypothesized to be a Bloch state. Delocalized Ps has unprecedented (high) Ps mobility driven primarily by weak phonon scattering with unusual and profound consequences on how Ps probes the lattice. [ABSTRACT FROM AUTHOR]

The present work studied the role of the polar group unconjugated oxygen on the inhibition of positronium (Ps) formation in two binary blends made from a set of chosen constituent polymers with polar and weakly polar groups (nonpolar). The polymer blend samples of PVC-EVA and PVC-SAN were investigated by coincidence Doppler broadening and positron lifetime techniques. The strong polar acetate group in the EVA contributed to positron annihilation with electrons of unconjugated oxygen (-C+=O-) as revealed by the momentum distribution curves peaking around 17 PL (10-3 m0c). The ortho-Ps intensity indicated the unconjugated oxygen shows about a 28% Ps reduction even in the presence of a strong Ps inhibiting halogen (Cl-). In contrast, this effect was not seen in the PVC-SAN blends since SAN contains a weakly polar (nonpolar) acrylonitrile group (C=N). Our results indicate the chlorine of PVC in the blends is a major contributor to Ps inhibition through the formation of a (Cl~-e+) bound state but the unconjugated oxygen in EVA of the PVC-EVA blend also plays a similar, but lesser, role. [ABSTRACT FROM AUTHOR]

We present a model of electron-positron pair production in pion-nucleon collisions in the exclusive reaction pN --> Ne+e~. The model is based on an effective field theory approach, incorporating 16 baryon resonances below 2 GeV. Parameters of the model are fitted to pion photoproduction data. We present the resulting dilepton invariant mass spectra for p ~ p collisions up to √s = 1.9 GeV center-of-mass collision energy. These results are meant to give predictions for the planned experiments at the HADES spectrometer in GSI, Darmstadt. [ABSTRACT FROM AUTHOR]

Recently it was found that energetic positronium (Ps) emission from both n- and p-type Si(100) occurs primarily via an exciton-like surface state PsX [D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr., Phys. Rev. B 84, 195312 (2011)]. The underlying Ps production mechanism was deduced from the observed Ps emission yield and kinetic energy, using laser and thermally generated electrons to populate the PsX state, but not all of the observed effects have been explained. In this brief report we show that excitonic Ps emission also occurs from n-Si(l 11 ), but with characteristics that more closely resemble p-Si(100) than n-Si(lOO). Considering both ( 100) and (111) crystal orientations may help in the formulation of a more complete theoretical description of the excitonic Ps production mechanism. [ABSTRACT FROM AUTHOR]

By utilizing a unique characteristic of the affinitively trapped positrons, here we demonstrate a powerful method to estimate the sizes of the embedded Cu nanoprecipitates (and subnanoprecipitates) (ECNPs) in Fe. The positron-electron momentum density distributions in the ECNPs with various sizes in a thermally aged Fe-Cu dilute alloy were measured by using the two-dimensional angular correlation of positron annihilation radiation technique and the momentum smearing around the Fermi cutoff was clearly observed. It is clarified that this smearing effect is proportional to the inverse of the average radii of the ECNPs and thus it can be employed to estimate the ECNPs' sizes. Moreover, the ECNPs' sizes obtained by the present method are compared with those obtained by the three-dimensional atom probe technique and good agreement is observed, justifying the size estimation using the smearing effect. It is noteworthy that the present technique can detect the smearing due to the ECNPs with their radii even below about 0.5 nm and thus gives an important trend in the smearing reflecting the average size of the ECNPs in this range. [ABSTRACT FROM AUTHOR]

We report Doppler-free measurements of the positronium (Ps) Lyman-&agr; transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength &lgr;L = 243.0218 ± 0.0005 nm, we also observed a crossover resonance at &lgr;C = 243.0035 ± 0.0005 nm, arising from the excitation of 13S1, atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1 1S0 ground state. Since (&lgr;L-&lgr;C) is related to the Ps hyperfine interval Ehfs, this observation constitutes the first optical measurement of this quantity and yields Ehfs = 198.4 ±4.2 GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states. [ABSTRACT FROM AUTHOR]

We report the first direct measurement of the hyperfine transition of the ground state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 10 kW power using a gyrotron, a mode converter, and a Fabry-Pérot cavity. The hyperfine transition has been observed with a significance of 5.4 standard deviations. The transition probability is measured to be A = 3.l+1.6-1.2 × 10-8 agreement with the theoretical value of 3.37 × 10-8 s-1. [ABSTRACT FROM AUTHOR]

A method is investigated to manipulate the annihilation dynamics of a dense gas of positronium atoms employing superradiance and subradiance regimes of the cooperative spontaneous emission of the system. The corresponding annihilation dynamics is explored in two setups with regard to its fundamental novel properties and controlled by the gas density and by the intensity of a driving strong resonant laser field. In particular, the method allows us to increase the annihilation lifetime of an ensemble of positronium atoms by trapping the atoms in the excited state via collective radiative effects in the resonant laser field. In the second setup, the effect is enhanced by employing a cavity field. The maximum lifetime increase is by a factor of about 200 for para-positronium and by a factor of about 100 for ortho-positronium. [ABSTRACT FROM AUTHOR]

The propagation of circularly polarized electromagnetic waves along a constant background magnetic field in an electron-positron plasma is calculated by means of both a fluid and a kinetic theory treatment. In the fluid theory, relativistic effects are included in the particle motion, the wave field, and in the thermal motion by means of a function, which depends only on the plasma temperature. In this work we analyze the consistency of the fluid results with those obtained from a kinetic treatment, based on the relativistic Vlasov equation. The corresponding kinetic dispersion relation is numerically studied for various temperatures, and results are compared with the fluid treatment. Analytic expressions for the Alfven velocity are obtained for the fluid and kinetic models, and it is shown that, in the kinetic treatment, the Alfven branch is suppressed for large temperatures. [ABSTRACT FROM AUTHOR]

We report optical spectroscopic measurements of molecular positronium (Ps2), performed via a previously unobserved L = 1 excited state. Ps2 molecules created in a porous silica film, and also in vacuum from an Al(lll) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state. Future experiments making use of the photoionized vacuum L = 1 Ps2 could provide a source of Ps+ ions, as well as other multipositronic systems, such as Ps2H- or Ps20. [ABSTRACT FROM AUTHOR]

The article discusses a study on the efficient production of Rydberg positronium (Ps) atoms using a two-step process, comprising incoherent laser excitation, first to the 2 (sup) 3 (/sup)P state and then to states with principal quantum numbers ranging from 10 to 25. The study found that the overall efficiency of the production is determined by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (sup) 3 (/sup) S-2 (sup) 3 (/sup)P transition.