Resonance effects with the photoproduction of electron-positron pairs in the field of a plane electromagnetic wave

The differential cross section is obtained for the formation of electron-positron pairs by a photon at a Coulomb center in the field of a plane electromagnetic wave whose intensity η = eA′/m? 1 and frequency Ω′/m? 1. Expressions are given for the resonance frequencies of the photon for which the emission of a virtual electron (positron) at a mass shell occurs. It is shown that resonance occurs only for electron or positron energies E > m²/Ω′. The resonance differential cross section under interference conditions is obtained. It is shown that the resonance cross section can exceed by several orders of magnitude the ordinary cross section for the photoproduction of pairs without a field.     

On the theory of surface impedance of metals placed in a magnetic field

The smooth non-monotonous dependence of the metal surface impedance upon the magnetic field H is investigated theoretically for the cases of diffuse and specular reflection of electrons from the specimen boundary. The type of the electron-surface interaction has been found to have very little effect on the magnitude of the impedance Z_α(H) in the range of weak magnetic fields [equation (1)]. In a strong field [equation (2)] the surface impedance behaves differently for diffuse and specular reflection. The form of the Z_α(H) function depends essentially on the ratio of the electromagnetic wave frequency ω and the collision frequency of electrons ν. This provides a possibility of establishing experimentally the frequency of electron collisions with volume scatterers.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.     

Correlations between emerging alpha-particles and heavy ions in 32S+197 Au collisions

In-plane angular correlations between α-particles and projectile residue nuclei (Z=5?22) emerging from ³²S+¹⁹⁷Au collisions have been measured at 373 MeV incident energy. In the combined Coulomb field of the projectile residue and the target nucleus coincident α-particles are strongly focused into a narrow cone. Model calculations indicate that α-emission originates from the region of the projectile which is facing the target nucleus during the collision.     

High Z effects in accounting for radiative component of the electron magnetic moment in hydrogen-like atoms

The behavior of electron energy levels in hydrogen-like atoms is studied while taking into account the nonperturbative interaction between the radiative component of the magnetic moment of a free electron Δg _free and the Coulomb field of an atomic nucleus with charge Z, including those with Z > 137. It is shown that for Zα ? 1 the energy-level shift is rather effectively determined through the matrix elements of the corresponding Dirac-Pauli operator with relativistic Coulomb wave functions. At the same time, for superheavy nuclei with Z ～ 170, this shift, generated by Δg _free, is genuinely nonperturbative, behaves like ～Z ⁵ near the threshold of negative continuum, exceeds all the estimates of radiative corrections coming from vacuum polarization and electron self-energy known so far, and turns out to be at least of the same order as the effects of nuclear charge screening by filled electron shells.     

Photon production in Au+Au collisions at \sqrt {s_{NN} } = 130 GeV

We report raw photon, neutral pion and eta measurements at RHIC. Photons in the energy range from 100MeV ? 4GeV were detected by reconstructing e ⁺ e ^? pair production, γ+Z→e ⁺+e ^?+Z, with the STAR Time Projection Chamber (TPC). Along with the photon detection technique we discuss the purity of the photon candidates and measurements of hadronic decays via their electromagnetic decay channels. The π⁰→γ and η→γγ decay channels are addressed.     

Dissociative recombination of electrons and O 2 + molecular ions in the field of intense visible laser radiation

We analyze the low-temperature dissociative recombination reaction e ^?+O ₂ ⁺ → O(¹ D)+O(³ P) in the field of visible monochromatic laser radiation. The analysis is performed in terms of the multichannel quantum defect theory using the stationary formalism of the radiative collision matrix. We calculate the dependences of the reaction cross section on the incident electron energy, the external electromagnetic field strength and frequency, and also the angle between the directions of the electron beam and the electric vector for linearly polarized radiation. The cross section is shown to increase by several orders of magnitude for a certain choice of these parameters, suggesting the possible laser stimulation of this reaction.     

Simultaneous solution of Kompaneets equation and radiative transfer equation in the photon energy range 1-125 keV

Radiative transfer equation in plane parallel geometry and Kompaneets equation is solved simultaneously to obtain theoretical spectrum of 1-125 keV photon energy range. Diffuse radiation field are calculated using time-independent radiative transfer equation in plane parallel geometry, which is developed using discrete space theory (DST) of radiative transfer in a homogeneous medium for different optical depths. We assumed free-free emission and absorption and emission due to electron gas to be operating in the medium. The three terms n, n² and (∂n/∂x_k) where n is photon phase density and x_k=(hν/kT_e), in Kompaneets equation and those due to free-free emission are utilized to calculate the change in the photon phase density in a hot electron gas. Two types of incident radiation are considered: (1) isotropic radiation with the modified black body radiation I^MB[1] and (2) anisotropic radiation which is angle dependent. The emergent radiation at τ=0 and reflected radiation τ=τ_max are calculated by using the diffuse radiation from the medium. The emergent and reflected radiation contain the free-free emission and emission from the hot electron gas. Kompaneets equation gives the changes in photon phase densities in different types of media. Although the initial spectrum is angle dependent, the Kompaneets equation gives a spectrum which is angle independent after several Compton scattering times.     

Large contribution of virtual Delbrück scattering to the emission of photons by relativistic nuclei in nucleus–nucleus and electron–nucleus collisions

Delbrück scattering is the elastic scattering of a photon in the Coulomb field of a nucleus via a virtual electron loop. The contribution of this virtual subprocess to the emission of a photon in the collision of ultra-relativistic nuclei, Z₁Z₂→Z₁Z₂γ, is considered. We identify the incoming virtual photon as being generated by one of the relativistic nuclei involved in the binary collision and the scattered photon as being emitted in the process. The energy and angular distributions of the photons are calculated. The discussed process has no infrared divergence. The total cross section obtained is 14 barn for Au–Au collisions at the RHIC collider and 50 barn for Pb–Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary tree-level nuclear bremsstrahlung in the considered photon energy range, m_e⪡E_γ⪡m_eγ, where γ is the Lorentz factor of the nucleus. Finally, photon emission in electron–nucleus collisions, eZ→eZγ, is discussed in the context of the eRHIC option.     

Radiative quenching and excitation of metastable states upon differential scattering of atoms: I. Uniform quasi-classical theory

A theory of radiative-collisional transitions between the ground and metastable ¹S states of a colliding atom is proposed. The theory uses the uniform quasi-classical approximation generalized to the case of spherically asymmetric interactions. The theory takes into account the angular momentum of an emitted (absorbed) photon and allows one to calculate the total and differential scattering cross sections in a wide range of radiation frequencies including both wings and the center of the line of a forbidden atomic transition. The range of admissible collision energies and intensities of an external radiation field is restricted by the use of the adiabatic approximation, as well as the approximation of a weak coupling between the ground and excited states of a quasi-molecule, the potentials of which are assumed to be monotonically repulsive.     

Positron lines from subcritical heavy ion-atom collisions

Positron lines were observed in heavy ion-atom collisions at bombarding energies close to the Coulomb barrier in subcritical systems with the sum of the atomic numbers of the colliding nucleiZ _u =Z ₁+Z ₂ being smaller thanZ _u =172. Each collision system, studied,²⁰⁸Pb +²⁰⁸Pb(Z_u=164),²³⁸U+¹⁸¹Ta(Z_u=165), and²³⁸Au(Z_u=171), exhibits the emission of two positron lines withZ _u -independent c.m. energies of ～ 258 keV and ～ 340 keV, and with widths of about 30 keV, superimposed on continuous positron spectra from nuclear pair decay and pair emission induced by the time changing Coulomb field of the collision. The production cross section of thee ⁺-lines rises with a high power ofZ _u (ocZ _u ²²), which is comparable to theZ _u ²⁰-dependence for the collision induced positrons.     

On the phenomenological three-graviton vertex

In General Relativity, the graviton interacts in three-graviton vertex with a tensor that is not the energy-momentum tensor of the gravitational field. We consider the possibility that the graviton interacts with the definite gravitational energy-momentum tensor that we previously found in the G ² approximation. This tensor in a gauge, where nonphysical degrees of freedom do not contribute, is remarkable, because it gives positive gravitational energy density for the Newtonian center in the same manner as the electromagnetic energy-momentum tensor does for the Coulomb center. We show that the assumed three-graviton vertex does not lead to contradiction with the precession of Mercury’s perihelion. In the S-matrix approach used here, the external gravitational field has only a subsidiary role, similar to the external field in quantum electrodynamics. This approach with the assumed vertex leads to the gravitational field that cannot be obtained from a consistent gravity equation.     

Auto-ionization of positrons in heavy ion collisions

Autoionization of positrons occurs as a fundamentally new process of quantum electrodynamics, if empty 1s- or 2p _1/2? etc. electronic shells obtain binding energies larger than 2m _e c ². This effect should be experimentally observable in the scattering of very heavy ions (Z≧80) on each other since in such collisions superheavy electronic molecules are formed (superheavy quasi-molecules). The scattering mechanism and the distribution of autoionization positrons are discussed. The adiabaticity of the heavy ion collision is studied and the electron-positron pair production background to the ionization problem is estimated. Analytic solutions are obtained for 1/r-potentials for the caseZα≧1. The phase shifts of negative energy solutions in the case of cutoff Coulomb potentials reveal the accuracy of the autoionization formalism.     

Annihilation of a relativistic positron and K-electron to yield a photon and second K-electron

We study the annihilation of a fast positron and a K-electron resulting in the emission of a photon and a second K-electron. It is assumed that all electrons and positrons move in the Coulomb field of the nucleus and that the Coulomb parameter αZ is much less than unity (α=1/137 is the fine-structure constant and Z is the atomic number). The electron-electron interaction, which is responsible for the ejection of the electron by the atom, is taken into account in the first order of perturbation theory. We calculate the differential and total cross sections of the process and construct the ratio of the cross sections of double and single ionization as a function of the energy of the incident positron. Finally, we establish the high-energy limit of this ratio, equal to 0.34/Z ². Zh. éksp. Teor. Fiz. 113, 786–804 (March 1998)     

Photon echo generated at the transition 0–1 in ytterbium vapor

Photon echo generated at the inter-combination transition (6s ²) ¹ S ₀ ? (6s6p) ³ P ₁ of ¹⁷⁴Yb was investigated for pure ytterbium vapor and for its mixtures with atomic buffers. In pure ytterbium vapor, the polarization of photon echoes at this 0?C1 transition coincides with the polarization of the second exciting pulse for all combinations of linear and circular polarizations of exciting radiation pulses. Photon echo does not appear either for linear orthogonal or for opposite circular polarizations of exciting pulses in pure ytterbium. In mixtures of ytterbium with atomic buffers (Kr, Xe), collision induced photon echo arises only for exciting pulses of linear orthogonal polarizations, its power is essentially less than that of the ordinary echo generated by pulses with parallel polarizations in the same mixture. Polarization of collision induced echo is linear, and it coincides with polarization of the first exciting pulse. Experimental results agree with calculations, and they confirm that the collision induced photon echo at this transition arises exclusively due to anisotropy of depolarizing collisions.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Soliton model of atom

The Einstein-de Broglie soliton concept is applied to simulate stationary states of an electron in a hydrogen atom. According to this concept, the electron is described by the localized regular solutions to some nonlinear equations. It is shown that the electron-solilon center travels along some stationary orbit around the Coulomb center. The electromagnetic radiation is absent as the Poynting vector has non-wave asymptote O(r ^–3)after averaging over angles.     

Measurement of the cross-section of Zγ and limits on ADD models at the CMS with 7?TeV Large Hadron Collider data

The measurement of the inclusive cross-section for Z?? production at LHC with 7?TeV proton?Cproton collision is presented. The electron and muon decay modes are used to reconstruct the Z boson. The total cross-section is measured for photon transverse energy greater than 10?GeV and with photon and charged lepton separation in the pseudorapidity-azimuthal angle plane greater than 0.7. This study is extended by a measurement of $Z(\nu\bar{\nu})\gamma $ cross-section. A search is performed for extra dimensions in the Arkani-Hamed, Dimopoulos, Dvali framework using the final state of a graviton and photon. The limits are extended with MD?>?1.25?C1.31?TeV for n?=?2?C6. The measurement is found to be in agreement with the Standard Model prediction.     

Coulomb breakup of light nuclei in the field of a heavy ion at relativistic collision energies

A simple method for calculating the amplitude and the cross section for the Coulomb breakup of a light nucleus into two fragments in the field of a heavy ion at relativistic collision energies is proposed on the basis of time-dependent perturbation theory. It is shown that the resulting amplitude for the process in question has a correct nonrelativistic limit. The contribution of the longitudinal component of the Coulomb field of a heavy ion tends to zero in the ultrarelativistic limit. A specific implementation of the method is demonstrated by taking the example of the Coulomb breakup reaction ²⁰⁸Pb(⁸B, ⁷Bep)²⁰⁸Pb at various collision energies. The results are found to be in agreement with experimental data.