[en] The production of two or more jets of hadrons in photoproduction events at the HERA e⁺p collider has been studied using the ZEUS detector. By tagging the final state positron, two samples of event have been isolated where the photon exchanged between proton and positron is quasi-real (of virtuality P² << 0.02 GeV²) and virtual (0.1 < P² < 0.55 GeV²) respectively. It is shown that photons in both P² ranges show resolved structure. The P² evolution of the structure of the photon is studied by measuring the relative contribution of direct and resolved photon processes to the cross-section for the production of two or more jets. Events have been classified as direct or resolved based upon the value of the final state observable X_γ^obs. The data suggest that the contribution from resolved photon processes is suppressed relative to that from direct photon processes as P² rises and are in general agreement with leading order calculations. (author)

[en] The multiplicity distribution for P-barP collisions at √s = 540 GeV is written as the sum of Poisson distributions at different impact parameter b. An energy independent relation between the variable z-bar and b is suggested. With this relation, the multiplicity distributions at √s = 200 and 900 GeV are described well. The distribution at √s = 1600 GeV is predicted

[en] The cross sections for Compton scattering from positronium are calculated in the range from 1 to 100 keV incident photon energy. The calculations are based on the A² term of the photon-electron or photon-positron interaction. Unlike in hydrogen, the scattering occurs from two centers and the interference effect plays an important role for energies below 8 keV. Because of the interference, the criterion for validity of the impulse approximation for positronium is more restrictive compared to that for hydrogen.

[en] We have investigated the recently reported discrepancy between theory and a new experiment [Palathingal et al., Phys. Rev. A 51, 2122 (1995)] for single-quantum annihilation of positrons with bound electrons. Fully relativistic calculations of total cross sections for the production and the annihilation of positrons involving an electron bound in the K or L shells of atoms have been performed in screened atomic potentials, for a number of elements ranging from germanium (Z=32) to thorium (Z=90) and for positron kinetic energies up to 4 MeV above threshold. Very good agreement with earlier calculations, that had been performed over a more restricted range of elements and energies, is obtained, also confirming that the Z and shell dependence previously seen continues at higher energies. The results presented here extend theory to energies where recent experiments have been performed, at forward angle, which were assumed to characterize the Z and shell dependence of the corresponding total cross sections. Our new theoretical predictions are not consistent with the strong dependence of the K- and L-shell bound-pair-annihilation cross sections on the nuclear charge of the target reported in those experiments. An explanation of these discrepancies is proposed, namely, that it is not correct to assume the same ratios for total cross sections as for forward distributions. This is demonstrated using existing data for the related photoeffect process, and it is seen that such forward data are indeed consistent with the pair-annihilation experiments

[en] Rearrangement in antiproton (p-bar) and positronium (Ps) collisions, p-bar+Ps→H-bar+e^-, is a promising process to produce large amount of antihydrogen atom (H-bar). The formation cross section is calculated by using a time-dependent coupled channel (TDCC) method. Numerical accuracy of the TDCC method is demonstrated in a calculation of Ps-formation cross sections in positron and hydrogen collisions. The present result shows a dominant peak of the cross section around a center of mass collision energy of 10 eV

[en] It is possible to transform electron (positron) beams of the future e⁺e^- colliders into γ beams with approximately the same energies and luminosities. With the help of the CompHEP system the author calculates the total cross sections in the tree approximation for the production of different three-particle final states in e⁺e^-, γe^±, and γγ collisions in the energy range 0.1-2 TeV. These calculations provide the basis for comparison of different experimental possibilities with electron and gamma beams. In particular, the author is interested in the possibility of observing various final states at different energies and in a comparison of the numbers of events for different colliding beams. 20 refs., 4 figs., 1 tab

[en] Using a pulsed dye laser in conjunction with a pulsed e⁺ source, the two photon, first order Doppler-free 1₃S¹→2₃S¹ transition is induced in positronium (Ps) with approximately 20% probability, the 2₃S¹ Ps is photoionized, and the e⁺ fragment is collected. An approximately 1.5 GHz wide resonance (20:1 signal to noise) is observed within 0.5 GHz of the calculated frequency. The resonance signal has an amplitude proportional to the square of the laser intensity and Stark shifts to higher frequency under an applied electric field as expected. (Auth.)

[en] This chapter examines methods of forming positronium, positronium formation as a probe of matter, and fundamental properties of the positronium atom. Topics considered include positronium formation in gases, positronium formation in powders, sources of cryogenic positronium, positronium spin state selection, formation of the positronium negative ion, galactic positronium, surface defect studies, surface magnetism studies, studies of optically active molecules, density fluctuations in gases, positronium decay rates, and positronium spectroscopy. It is concluded that there has been steady progress in measuring and calculating the fundamental properties of positronium, as shown by two-photon optical excitation of positronium

[en] Results of neutral detector experiments in Compton scattering of quasi-real photons on electrons and positrons are given over the range of total energy of the beams of 2E = 1000 to 1048 MeV. The total cross section of the process and the angular distributions are in full agreement with predictions made on the basis of quantum electrodynamics (QED). The ratio of the observed total cross section of quasi-real photons on electrons to that on positrons is equal to 0.99 +- 0.02. The quantitative limits of the cross section for production of a heavy electron (HE) are established

[en] Currently throughout the world there has been a rapid increase in the number of ultra-high power petawatt-class laser facilities. These lasers have been shown via the interaction of plasmas to produce high energy electrons, protons and gamma rays. As the laser power increases the potential to produce ultra-high flux polarized gamma ray sources has been shown both theoretically and numerically via particle-in-cell simulations. One potential use of such gamma ray sources is the study of quantum electrodynamics (QED), which describes the interaction of photons with electrons. Although QED has been tested experimentally and found to agree with experiments to high precision to lowest order, higher order measurements are needed to test QED and determine whether there could be new as yet undiscovered phenomena. Delbrueck scattering is a higher order process, which involves the scattering of photons off the Coulomb fields of nuclei, and is the only one which has been measured due to its large cross section. We have found recently that polarized gamma ray sources can achieve high precision measurements of Delbrueck scattering. Therefore, these ultra-high power facilities offer the possibility of measuring Delbrueck scattering to high precision. Along with an introduction and conclusion we will review the physics of Delbrueck scattering, high flux polarized gamma-ray sources from laser-plasma and laser-electron beam interactions, and the numerical techniques to calculate the Delbrueck scattering cross section. (author)