Angular distribution of lepton pairs in Drell-Yan-like processes

We compute the angular distribution of lepton pairs at fixed transverse momentum using soft-gluon resummation. Theoretical predictions are compared with the recent NA10 experimental data. We also give predictions in the case ofZ ⁰ production.     

Single bremsstrahlung processes in gauge theories

We find that, for QED and for SU(N) gauge theories, single hard bremsstrahlung cross sections are remarkably simple in the ultrarelativistic limit. They can be written as a product of two factors: one factor is connected with the lowest order, elastic process, and the second one is related to the infrared factor which describes soft quantum emission. We present explicit formulae for various processes.     

An evaluation of the bremsstrahlung contribution to specimen damage in XPS using Auger peak intensities

The contribution of the bremsstrahlung radiation from a conventional X-ray source to specimen damage is related to the total amount of X-ray energy absorbed by the specimen. For organic compounds, which contain light elements such as C, O, and N, absorption of X-rays leads primarily to 1s photoemission. In this study the Auger-to-photoelectron peak (KLL/1s) intensity ratio was measured using both conventional and monochromatic AlKα X-ray sources for a series of compounds containing light elements from C to Mg. These ratios show a systematic 15–70% increase with conventional source relative to the monochromatic source, as the atomic number decreases for all elements studied except Mg. Calculations of 1s hole production, based on characteristic line and bremsstrahlung intensities measured with an energy-dispersive X-ray analyzer, showed the same trend, but smaller percentage increases. The differences between the experimental and theoretical values are attributed mainly to an overestimate of the characteristic/bremsstrahlung ratio using the energy-dispersive X-ray data. The bremsstrahlung contribution to specimen damage for carbon was estimated to be between ≈ 4% (calculated from X-ray data) and ≈ 44% (calculated from relative Auger intensities).     

Inverse problems of aggregation processes

The coagulation frequency is the key ingredient in the population balance (Smoluchowski) equation of coagulation kinetics. An inverse problem is formulated to extract the coagulation frequency from transient size distributions when these distributions are self-similar. Two numerical examples illustrate the procedure. The first demonstrates the inverse problem for the recovery of singular coagulation frequencies, while the second shows the procedure when self-similarity is approximate. Transient droplet coagulation experiments in a turbulent flow field have been performed. The resulting size distributions are observed to be self-similar. The inverse problem is used to determine the drop coagulation frequency. This frequency shows significant deviation from the coagulation frequencies derived from simple models of drop-drop interactions in a turbulent flow field.     

Tunnelling contribution in non-radiative processes

A semiclassical formalism, already introduced to treat non-radiative processes in multidimensional spaces, has been extended to compute total non-radiative transition rates. A simple approximate analysis results in simple formulae which discriminate between quantum-tunnelling contributions and classical ones. Applications to diffusion processes of vacancies in solids are considered.     

Approximations for high-energy photon-photon scattering in two-photon processes and double bremsstrahlung

Presented are some approximation formulae for the processese ⁺ e ^? →e ⁺ e ^?γ^*γ^* →e ⁺ e ^?γγ (e ⁺ e ^?) scattering with intermediate γγ scattering) ande ⁺ e ^?→e ⁺ e ^?γγ (double bremsstrahlung) at high energies for forwarde ⁺ e ^? scattering, using not only the usual equivalent-photon approximation method but also direct approximations under certain kinematic conditions. In both cases the outgoing photons can be polarized. Their numerical check is examined by comparing these results with those of the exact calculations obtained in a recent paper. The agreement of both results is quite satisfactory if the required conditions are satisfied.     

Radiative corrections to pion-nucleus bremsstrahlung

We calculate the one-photon loop radiative corrections to virtual pion Compton scattering → , that subprocess which determines in the one-photon exchange approximation the pion-nucleus bremsstrahlung reaction Z → Z . Ultraviolet and infrared divergencies of the loop integrals are both treated by dimensional regularization. Analytical expressions for the O() corrections to the virtual Compton scattering amplitudes, A(s, u, Q) and B(s, u, Q) , are derived with their full dependence on the (small) photon virtuality Q from 9 classes of contributing one-loop diagrams. Infrared finiteness of these virtual radiative corrections is achieved (in the standard way) by including soft photon radiation below an energy cut-off . In the region of low center-of-mass energies, where the pion-nucleus bremsstrahlung process is used to extract the pion electric and magnetic polarizabilities, we find radiative corrections up to about -3% for = 5 MeV. Furthermore, we extend our calculation of the radiative corrections to virtual pion Compton scattering → by including the leading pion-structure effect in the form of the polarizability difference - . Our analytical results are particularly relevant for analyzing the data of the COMPASS experiment at CERN which aims at measuring the pion electric and magnetic polarizabilities with high statistics using the Primakoff effect.     

Two-photon bremsstrahlung processes in atoms: Polarization effects and analytic results for the Coulomb potential

The partial wave analysis of two-photon free-free (bremsstrahlung) electron transition cross sections during scattering by a static potential U(r), as well as by an atom with a nonzero angular momentum, is carried out. The dipole interaction with radiation is taken into account in the second order of perturbation theory for the general case of elliptic polarization of photons. The polarization and angular dependences of the two-photon potential scattering amplitude is presented as a combination of the scalar product of electron momenta and photon polarization vectors and five atomic parameters containing Legendre polynomials of the scattering angle as well as radial matrix elements depending on the initial (E) and final (E′) electron energies. The results are applicable both for spontaneous double bremsstrahlung at nonrelativistic energies and for induced absorption and emission in the field of a light wave. Specific polarization effects (circular and elliptic dichroism) are analyzed for two-photon bremsstrahlung processes associated with the interference of the Hermite and anti-Hermite parts of the amplitude and depending on the sign of photon helicity. The limiting cases of high and low photon frequencies are investigated analytically, and the asymptotic forms of radial matrix elements and amplitudes for the general form of the U(r) potential are determined. Closed analytic expressions are derived for the radial matrix elements of the Coulomb potential in the form of integrals of hypergeometric function, and singularities are singled out in explicit form for E′ → E. The methods of approximate calculation of the radial matrix elements are discussed, and the results of their exact numerical calculation, as well as angular distributions and the cross sections of induced one-and two-photon emission and absorption, are given for the case of the Coulomb potential. The numerical results show that dichroism effects are quite accessible for experimental observations.     

Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes

The rates of axion emission by nucleon–nucleon bremsstrahlung are calculated with the inclusion of the full momentum contribution from a nuclear one pion exchange (OPE) potential. The contributions of the neutron–neutron (nn), proton–proton ( pp) and neutron–proton (np) processes in both the non-degenerate and degenerate limits are explicitly given. We find that the finite-momentum corrections to the emissivities are quantitatively significant for the non-degenerate regime and temperature-dependent, and should affect the existing axion mass bounds. The trend of these nuclear effects is to diminish the emissivities.