Positronium scattering off a hydrogen target has been studied employing a three-state positronium model close-coupling approximation (CCA) with and without electron exchange. Elastic, excitation and quenching cross sections are reported at low and medium energies. The effect of electron exchange is found to be significant at low energies. The ratio of quenching to the total cross section (the conversion ratio) approaches the value of 0.25 with increase of energy, as expected.

Scattering of orthopositronium off helium target has been investigated using close-coupling method in the energy range 0-110 eV. Two basis sets, ͑a͒ Ps(1s)ϩHe(1s 2 , 1s2 1 s, 1s2 1 p) and ͑b͒ Ps(1s,2p) ϩHe(1s 2 , 1s2 1 s, 1s2 1 p), have been employed to find the scattering parameters. Low-order phase shifts, scattering length, and elastic and excitation cross section up to nϭ2 are reported using close-coupling approximation. Total cross section is also given by adding other partial cross section and compared with available measured data and existing theoretical predictions. Our total cross section at zero energy is very close to the theoretical prediction of Drachman and Houston ͓J. Phys B 3, 1657 ͑1970͔͒ and measured data of Canter et al. ͓Phys. Rev. A 12, 375 ͑1975͔͒. Present total cross section is in qualitative agreement with measured data of the University College London group in the energy range considered. In particular, present predictions are in good agreement with the UCL group in the energy range 20-30 eV. It has been found that elastic cross section is dramatically reduced, at zero or near zero energies, with the inclusion of target excitation in the expansion scheme. Moreover, ionization cross section of the Ps atom is found to be a major contributor to the total cross section above the ionization threshold. INTRODUCTIONWe consider orthopositronium (o-Ps)-helium-atom scattering at low and intermediate energies. The importance of (o-Ps)-atom scattering was first realized by Massey and Mohr ͓1͔ who evaluated the first Born amplitude ͑FBA͒ using only the electron exchange interaction as direct FBA vanishes for the processes in which the initial and final states of the Ps atom have the same parity. Even for the simplest system, i.e., (o-Ps)-H, theoretical calculation is much more difficult than the corresponding electron ͑positron͒-atom scattering. Added difficulty is due to the fact that both the atoms ͑Ps and H͒ have internal degrees of freedom and one has to encounter multiparticle and multicenter integrals. In the case of helium the situation is even worse.Theoretically, study of (o-Ps)-He scattering has been initiated by Fraser ͓2,3͔ and Fraser and Kraidy ͓4͔ using the static exchange model, in which both the atoms are retained in their respective ground states while the effect of electron exchange is properly taken care of. Barker and Bransden ͓5,6͔ have investigated the same problem with and without the adiabatic part of van der Waals' potential. This lowestorder long-range potential is found to modify the scattering parameters. Drachman and Houston ͓7͔ have investigated (o-Ps)-He scattering and obtained the values of Z eff and scattering length using a local exchange model potential. In this investigation they have included the effect of correlation by taking a close channel wave function having eighty-four independent terms and solved the problem by Kohn variational method. This paper is expected to provide meaningful results. The Belfast group ͓8͔ has applied the target-elastic pseudostate clos...

Positronium-hydrogen elastic scattering is investigated using the six-state full close coupling approximation employing the basis set Ps(1s, 2s, 2p) + H(1s, 2s, 2p). S-wave singlet and triplet scattering and also the corresponding scattering length and effective range are compared with the corresponding predictions of Drachman and Houston and the Belfast group. We also report phase shifts up to l = 2 and the integrated elastic cross sections. Present results indicate that the effect of target excitation as well as the effect of excitations of both the atoms are significant in the energy region considered. We also report the angle integrated Ps(1s) + H(1s) → Ps(2p) + H(1s) and Ps(1s) + H(1s) → Ps(1s) + H(2p) excitation cross sections.

Hydrogen-anti-hydrogen scattering has been studied at low energies using quantal method. We employ the close-coupling method using different atomic basis sets. The same method has also been used to investigate hydrogen-hydrogen scattering. s-wave phase shifts and integrated elastic cross-sections are reported for both the systems. There is a structure in the elastic cross-section for hydrogen-hydrogen scattering.European physicists were successful in preparing anti-hydrogen (H) at CERN [1] by using a low-energy anti-proton (p) ring in late December 1995, and more recently at Fermilab [2]. These atoms are moving with a speed close to that of light. Their future plan is to obtain H at thermal velocities and to trap H. Impressive progresses have been made in this direction [3][4][5].Availability of H at sub-milli-Kelvin energies will make it possible to compare directly matter and anti-matter properties with high precision [6][7][8]. It has been pointed out that the main cause of loss of H trapped in a magnetic gradient trap is due to H-H 2 and H-He collision processes. The scattering cross-sections for these processes will help to fabricate a technical know-how in slowing down and trapping H. Therefore, theoretical investigations of these processes are of great importance.To the best of our knowledge, no positive-energy calculation has so far been carried out for matter-anti-matter systems with anti-matter signature. On the other hand, different workers have studied atom-atom scattering. Bates and Griffing [9] have initiated the study on the H-H system using the first Born approximation. Shingal et al. [10] have investigated H-H scattering using a coupled-channel semi-classical impact parameter method with rectilinear trajectories and reported the excitation and ionization cross-sections in the energy range 1 to 100 keV. They have used a 22-state basis for each state of symmetry. The semi-classical impact parameter method is not expected to be valid at low energies. In heavy-particle collision a large number of partial waves unlike light particle scattering are required for convergent results. This requirement has restricted a full quantal calculation to investigate heavy-particle

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