Hidden crossing theory

The most accurate values of the s-wave positronium formation cross section calculated by Humberston (1982) and Humberston et al. (1997) are shown in Figure 4.1. (The latter results are more accurate but there is no difference between the two sets of results on the scale of this figure.) This cross section is much smaller than the s-wave elastic scattering cross section and also, as we shall see, much smaller than other contributions to <7ps of low orbital angular momentum. It has recently been shown by Ward, Macek and Ovchinnikov (1998), using hidden crossing theory, that the small magnitude of the s-wave contribution to [Pg.156]

Ashley, Moxom and Laricchia (1996) measured the positron impact-ionization cross section in helium and found that its energy dependence up to 10 eV beyond the threshold was quite accurately represented by a power law, as in equation (5.8), but with the exponent having the value 2.27 rather than Klar s value of 2.651. This discrepancy prompted Ihra et al. (1997) to extend the Wannier theory to energies slightly above the ionization threshold using hidden crossing theory. They derived a modified threshold law of the form... 

Janev, R.K. and Solov ev, E.A. (1998). Application of hidden crossing theory to positron-hydrogen collisions. In Photonic, Electronic and Atomic Collisions, eds. F. Aumayr and H. Winter (World Scientific) pp. 393-398. 

Ward, S.J., Macek, J.H. and Ovchinnikov, S.Yu. (1998). Hidden crossing theory applied to positronium formation. Nuc. Inst. Meth. B 143 175-183. 

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