Theoretical study of antiproton and position collisions related to antimatter physics

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Complex structures of cross sections are sometimes induced by resonance phenomena in positron scatterings by atoms. Comparison with the results for electron scatterings give detailed information on the electronic properties interactions of atoms. We investigated the basic physics of the resonance scatterings of positrons in terms of large-scale hypersherical close-coupling calculations. The present method remedies the defects of existing old theories which use insufficient number of atomic orbitals for the expansion.Most of theoretical works on antiproton collisions with hydrogen atoms are based on the one-center atomic orbital expansion. However, we can expect that the electronic clouds are distorted considerably in the neighborhood of the antiproton owing to the repulsive Coulomb interaction. A large-scale two-center expansion was employed to study this effect and it was found that the distribution of the electron is really evacuated near the antiproton. Traditional one-center expansions can not represent such local distribution properly.At low energies where the average velocity of the electron is much larger than the incident velocity of the antiproton, the electron flistribution spreads up to several hundred atomic units when the collision has finished. Such diffuse electronic states can not be adequately represented by bound-type basis functions such as the Slater or the Gaussian orbitals. In order to solve this difficulty we introduced B-spline expansion to the low-energy scatterings. The B-splines are piecewise polynomials which are continuous up to some degree of derivatives at the borders. The B-spline expansion gives 20 % larger ionization cross sections than the bound-type expansions.