New calculations on e+-H ionization

We report new distorted-wave calculations of total ionization cross sections for positron scattering from hydrogen atoms. Comparisons with existing experimental results are made and we also analyze the effects of the 30 acceptance angle of one of the experimental measurements.     

Results of the Bielefeld-Brookhaven e+-H experiment

Positronium formation (Ps) cross sections for positrons impinging on atomic hydrogen were measured in the impact energy range from 13 to 255 eV at the High Intensity Positron (HIP) beam at Brookhaven National Laboratory (BNL). The Ps-formation cross section was found to rise rapidly from the threshold at 6.8 eV to a maximum value of (2.98±0.18) ×10^–16 cm² for 15 eV positrons. By 75 eV it drops below the detection limit of 0.17×10^–16 cm² which is the present level of statistical uncertainty. A more elaborate data analysis was performed leading to somewhat smaller cross sections than originally published [1]. During the course of this experiment further relative data on impact-ionization cross sections were accumulated.     

Surface composition analysis by low-energy ion scattering

Low-energy ion scattering (LEIS) is an analytical tool that provides information on the atomic composition of the outer surface, when noble gas ions are used as projectiles. In fact, quantitative composition analysis is currently done on a huge variety of materials, including catalysts and organic materials. The information on the surface composition is contained in the signal of backscattered ions (typically 1–3 keV He⁺,Ne⁺). In order to translate the LEIS signal to an elemental surface concentration all factors determining the LEIS signal must be known. These are in particular the scattering cross section and the ion fraction of the backscattered particles. The scattering cross section, which is due to the screened electrostatic potential between target atom and projectile, is well-known for the prevailing conditions of LEIS. It is an intriguing fact that, despite the large quantity of successful applications, the charge exchange processes in LEIS are not yet fully understood. It is e.g. not known why in LEIS for a given atomic species on the surface the signal usually does not depend on which other species are present (absence of matrix effects). Significant progress has recently been made in the understanding of the underlying charge exchange processes.Therefore, the aim of this review is twofold: on the one hand, to summarize the present understanding of the factors that determine the ion fraction of the scattered projectiles in LEIS, i.e. charge exchange processes. On the other hand, to summarize how quantitative surface composition analysis can be accomplished.In addition, we critically review publications that deal with surface composition analysis by LEIS, and analyze in which cases and by what means this was achieved and where and why it was successful or failed. After reading this review the reader will be able to deal with the pitfalls encountered in LEIS and to choose preferred experimental conditions for quantitative surface composition analysis.     

Probing MeV dark matter at low-energy e+e- colliders

It has been suggested that the pair annihilation of dark matter particles chi with mass between 0.5 and 20 MeV into e+e- pairs could be responsible for the excess flux (detected by the INTEGRAL satellite) of 511 keV photons coming from the central region of our Galaxy. The simplest way to achieve the required cross section while respecting existing constraints is to introduce a new vector boson U with mass M(U) below a few hundred MeV. We point out that over most of the allowed parameter space, the process e+e--->U(gamma), followed by the decay of U into either an e+e- pair or an invisible (nu(-)nu or chi(-)chi) channel, should lead to signals detectable by current B-factory experiments. A smaller, but still substantial, region of parameter space can also be probed at the Phi factory DAPhiNE.     

Spin-dependent low-energy 4He+ ion scattering from nonmagnetic surfaces

We investigated electron-spin-polarized (4)He(+) ion scattering on various nonmagnetic surfaces at kinetic energies below 2 keV. It was observed that the scattered He(+) ion yield depends on the He(+) ion spin. We interpret this spin-dependent scattering in terms of the spin-orbit coupling that acts transiently on the He(+)1s electron spin in the He(+)-target binary collision. This interpretation qualitatively explains the relationship between the spin-dependent scattering and the scattering geometry, incident velocity, and magnetic field arrangement. This is the first study to report spin-orbit coupling caused by projectile electron spin in ion scattering.     

Phenomenology of low-energy ππ scattering

Exact representations of the low-energy ππ amplitudes by (for the S-waves) generalized effective range formulas and by (for the P-wave) a resonance plus background expression are derived from inverse-amplitude partial-wave dispersion relations, and applied to the phenomenological analysis of a typical set of phase shifts deduced from peripheral pion production. With the aid of rigorous crossing conditions it is found that the simplest satisfactory account of the S-waves is provided by a current algebra type solution with (Adler) zeros and small scattering lengths. Solutions without zeros (S-wave dominant) are completely ruled out. Inelasticity is important in describing the rise of the isoscalar phase shift above the σ resonance, but can be neglected below 1 GeV in other channels. The P-wave is ?-dominated, but its width is sensitive to background terms. The phenomenological importance of rigorous results is stressed.     

Theory of low-energy ππ scattering

A four-parameter family of solutions to the ππ partial-wave dispersion relations is obtained using the inverse amplitude method. Choosing the parameters consistent with phenomenological values and so as to maximize agreement with rigorous sum rules and inequalities leads to a firm prediction of the phase shifts for S and P wave scattering and for the amplitudes below threshold. Neglecting inelasticity, the I = 0S-wave resonates near $\text{s}\text{= 540}\text{MeV}$, the I = 2 S-wave falls to about ?12° near 1 GeV, and the P-wave is ?-dominated. The S-waves have subthreshold zeroes consistent with the Adler condition. Inelastic effects are estimated and found to be small below the rho meson mass.     

e−-H(2S) elastic scattering in the two-potential eikonal approximation

The differential scattering cross-sections fore ^??H (2S) elastic scattering are calculated at intermediate energies by using the two-potential eikonal approximation. The results are compared with the recent theoretical data and the conventional Glauber cross-sections.     

Systematics of low-energy πN scattering

A simple current algebra model is shown to account for a wide range of low-energy πN experimental data including twenty-eight subthreshold expansion coefficients, six scattering lengths, and the scattering phase shifts below T_π = 300 MeV. A sigma term of σ_NN = 62 ± 10 MeV is obtained.