Orthopositronium-orthopositronium scattering at medium energies

The scattering of ortho-positronium (o-Ps) atoms has been investigated using the first Born approximation (FBA) and Born-Oppenheimer (B-O) model at medium energies. The angle integrated total elastic cross sections of o-Ps-o-Ps scattering have been reported. Apart from this, the inelastic cross sections for discrete excitations (principal quantum number, n <= 3) of the single and both the Ps atoms as well as single and double Ps atom ionization have also been obtained. We have also predicted the total cross section of o-Ps-o-Ps scattering by adding all these partial cross sections. The first Born and B-O scattering amplitudes have also been evaluated using partial wave technique. It has been noticed that odd values of the partial wave B-O amplitude vanishes for even parity transitions and vice versa. To the best of our knowledge, this feature has not been noticed for other systems.