UNUSUAL NATURE OF POSITRONIUM-PROTON ELASTIC-SCATTERING

The Lippmann-Schwinger equation for positronium-proton elastic scatter ing lacks a first-order term because the center of mass and center of charge are coincident. A consequence of this is that the high-energy b ehavior of the phase shift is governed by the second-Born approximatio n. Asymptotic analysis indicates that the high-energy form of the phas e shift is delta(L) similar to O(1/k(3)), where k is the incident posi tronium momentum. The asymptotic analysis has been confirmed by an exp licit close-coupling calculation including the Ps(1s), Ps(2s), Ps(2p), H(1s), H(2s), and H(2p) states.