Higher-order QED corrections to single-W production in electron–positron collisions

Four-fermion processes with a particle lost in the beam pipe are studied at LEP to perform precision tests of the electroweak theory. Leading higher-order QED corrections to such processes are analyzed within the framework of the structure function (SF) approach. The energy scale entering the QED SF is determined by inspection of the soft and collinear limit of the radiative corrections to the four-fermion final states, paying particular attention to the process of single-W production. Numerical predictions are shown in realistic situations for LEP experiments and compared with existing results. A Monte Carlo event generator, including exact tree-level matrix elements, vacuum polarization, higher-order leading QED corrections and anomalous trilinear gauge couplings, is presented. Received: 5 October 2000 / Published online: 11 May 2001     

Jet algorithms in electron–positron annihilation: perturbative higher order predictions

This article gives results on several jet algorithms in electron–positron annihilation: Considered are the exclusive sequential recombination algorithms Durham, Geneva, Jade-E0 and Cambridge, which are typically used in electron–positron annihilation. In addition also inclusive jet algorithms are studied. Results are provided for the inclusive sequential recombination algorithms Durham, Aachen and anti-k _t , as well as the infrared-safe cone algorithm SISCone. The results are obtained in perturbative QCD and are N³LO for the two-jet rates, NNLO for the three-jet rates, NLO for the four-jet rates and LO for the five-jet rates.     

Statistical models of hadron production—simple models for complicated processes

Thermal statistical models are a simple and effective tool to describe particle production in high energy heavy ion collision. It is shown that for higher moments finite volume corrections become important observable quantities. They make it possible to differentiate between different statistical ensembles even in the thermodynamic limit. The text was submitted by the author in English.     

Recovery of electron irradiated α-Ti single crystals-A positron annihilation study

Single crystals of α-Ti were irradiated at 100 K with 6 MeV electrons to a fluence of 2 × 10²⁰ electrons/m². The irradiation damage and its thermally-activated recovery, in the range 100-800 K, was studied by positron annihilation spectroscopy using the Doppler-broadening technique. Notable features of the recovery spectrum are (1) a large drop in the vacancy-associated defect signal between 110 and 135 K and (2) an enhancement of the defect signal between 250 and 300 K. The first feature is associated with vacancy annihilation by migrating self-interstitials and the second with clustering by freely migrating vacancies.     

Charmed baryons in electron–positron collisions

Charmed baryons are classified, and the experimental data on charmed-baryon states are reviewed paying special attention to those obtained at the electron–positron colliders.     

Study of strange charmed baryons in electron–positron collisions

A modern classification of charmed baryons is presented, and the quark model is briefly described for the ground states and energy levels of excited states. In addition, a survey of experimentally observed states of strange charmed baryons is given.     

Study of ageing in Al–Mg–Si alloys by positron annihilation spectroscopy

In many common Al–Mg–Si alloys (6000 series) intermediate storage at or near ‘room temperature’ after solutionising leads to pronounced changes of the precipitation kinetics during the ensuing artificial ageing step at ≈180 °C. This is not only an annoyance in production, but also a challenge for researchers. We studied the kinetics of natural ‘room temperature’ ageing (NA) in Al–Mg–Si alloys by means of various different techniques, namely electrical resistivity and hardness measurement, thermoanalysis and positron lifetime and Doppler broadening (DB) spectroscopy to identify the stages in which the negative effect of NA on artificial ageing might appear. Positron lifetime measurements were carried out in a fast mode, allowing us to measure average lifetimes in below 1 min. DB measurements were carried out with a single detector and a ⁶⁸Ge positron source by employing high momentum analysis. The various measurements show that NA is much more complex than anticipated and at least four different stages can be distinguished. The nature of these stages cannot be given with certainty, but a possible sequence includes vacancy diffusion to individual solute atoms, nucleation of solute clusters, Mg agglomeration to clusters and coarsening or ordering of such clusters. Positron lifetime measurements after more complex ageing treatments involving storage at 0 °C, 20 °C and 180 °C have also been carried out and help to understand the mechanisms involved.     

The influence of electron–phonon interaction on the OSL decay curve shape

The significance of the electron–phonon interaction for optically stimulated luminescence (OSL) process in quartz is demonstrated. OSL variation with temperature has been investigated for four samples of natural quartz. Changes of the OSL decay rate have been observed for all components of the OSL signal. The scale and tendency of these changes are comparable with outcomes of computer simulations carried out for the model composed of two deep electron traps, one shallow trap and one recombination centre, taking into account the electron–phonon interactions.     

Electron–positron pair production from vacuum in the field of high-intensity laser radiation

The works dealing with the theory of e⁺e^– pair production from vacuum under the action of highintensity laser radiation are reviewed. The following problems are discussed: pair production in a constant electric field E and time-variable homogeneous field E(t); the dependence of the number of produced pairs \({N_{{e^ + }{e^ - }}}\) on the shape of a laser pulse (dynamic Schwinger effect); and a realistic three-dimensional model of a focused laser pulse, which is based on exact solution of Maxwell’s equations and contains parameters such as focal spot radius R, diffraction length L, focusing parameter Δ, pulse duration τ, and pulse shape. This model is used to calculate \({N_{{e^ + }{e^ - }}}\) for both a single laser pulse (n = 1) and several (n ≥ 2) coherent pulses with a fixed total energy that simultaneously “collide” in a laser focus. It is shown that, at n ? 1, the number of pairs increases by several orders of magnitude as compared to the case of a single pulse. The screening of a laser field by the vapors that are generated in vacuum, its “depletion,” and the limiting fields to be achieved in laser experiments are considered. The relation between pair production, the problem of a quantum frequency-variable oscillator, and the theory of groups SU(1, 1) and SU(2) is discussed. The relativistic version of the imaginary time method is used in calculations. In terms of this version, a relativistic theory of tunneling is developed and the Keldysh theory is generalized to the case of ionization of relativistic bound systems, namely, atoms and ions. The ionization rate of a hydrogen-like ion with a charge 1 ≤ Z ≤ 92 is calculated as a function of laser radiation intensity (F and ellipticity ρ.     

CP-violation effect in neutralino pair production in e+ −e− annihilation and the electric dipole moment of the electron

It is shown that in the supersymmetric extensions of the standard electroweak theory, the threshold behaviour of the cross section of production of two different neutralinos in e⁺ −e⁻ annihilation may be sensitive to the violation of CP invariance by the neutral gaugino-higgsino mass term. The corresponding CP violation effect may be substantial quite far from the threshold. A relation (although rather indirect) between the form of the behaviour of this section in the threshold region and the value of the electric dipole moment of the electron may exist.     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.     

Enhanced electron–positron pair production by frequency chirping in one-and two-color laser pulse fields

Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment t = 0 leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.     

Parametrization of Bose–Einstein correlations and reconstruction of the space–time evolution of pion production in ee annihilation

A parametrization of the Bose–Einstein correlation function of pairs of identical pions produced in hadronic e⁺e⁻ annihilation is proposed within the framework of a model (the τ-model) in which space–time and momentum space are very strongly correlated. Using information from the Bose–Einstein correlations as well as from single-pion spectra, it is then possible to reconstruct the space–time evolution of pion production.     

Nonlinear propagation of weakly relativistic ion-acoustic waves in electron–positron–ion plasma

This work presents theoretical and numerical discussion on the dynamics of ion-acoustic solitary wave for weakly relativistic regime in unmagnetized plasma comprising non-extensive electrons, Boltzmann positrons and relativistic ions. In order to analyse the nonlinear propagation phenomena, the Korteweg–de Vries (KdV) equation is derived using the well-known reductive perturbation method. The integration of the derived equation is carried out using the ansatz method and the generalized Riccati equation mapping method. The influence of plasma parameters on the amplitude and width of the soliton and the electrostatic nonlinear propagation of weakly relativistic ion-acoustic solitary waves are described. The obtained results of the nonlinear low-frequency waves in such plasmas may be helpful to understand various phenomena in astrophysical compact object and space physics.     

The influence of electron donors and acceptors on the isomer distribution in the photolysis of β-ionone

The application of the CIDNP technique has shown that geometrical cis-trans photoisomerization of β-ionone and its isomers in the presence of pyrene in polar media occurs via radical ion pairs. Based on the combined analysis of CIDNP results and kinetic data on the formation and decay of isomers, it has been demonstrated that this channel exerts a significant influence on the isomer distribution in this system and on the reaction with other electron donors and acceptors.     

Behaviour of vacancies in dilute Fe–Re alloys: a positron annihilation study

Positron annihilation lifetime spectroscopy was used in a room temperature study of the influence of heat treatment on behaviour of vacancies in Fe_0.97Re_0.03 and Fe_0.94Re_0.06 alloys. In this experiment, the vacancies were created during the formation and further mechanical processing of the iron systems under consideration so the lifetime spectra of positrons were collected at least twice. The first samples were taken just after the melting process in an arc furnace, and the second ones were taken for the specimens annealed at 1,270 K and then cold-rolled at room temperature. After that, the spectra were measured for all studied samples after annealing at some temperatures gradually increasing from 300 to 1,270 K. It was found that vacancy-Re pairs are the dominant type of structural defects in alloys just after the melting process. In the case of alloys after a cold rolling process, the dominant type of structural defects is vacancies associated with edge dislocations. Moreover, for cold-rolled samples annealed at 473–573 K, the growth of the vacancy clusters associated with edge dislocations is observed by an increase in the mean positron lifetime. Finally, at temperatures above 573 K, vacancy clusters associated with edge dislocations as well as vacancy-Re pairs become unstable, and freely migrating vacancies sink at grain boundaries.     

Radiative production of the lightest neutralino in e + e − annihilation

The production of the lightest stable neutralino in the process including general mixing of gauginos and Higgsinos is studied. General formulas for the differential cross section are presented. The dependence of the differential cross section on the photon energy spectrum and also the dependence of the total cross section on the beam energies for three different mixing scenarios are illustrated. The text was submitted by the author in English.