Abstract
We present calculated elastic differential cross sections for
positron-acetylene scattering, obtained by using the Schwinger
multichannel method. Our results are in very good agreement with
quasielastic experimental data of Kauppila Nucl. Instrum. Meth. Phys.
Res. B 192, 162 (2002). We also discuss the existence of a virtual
state (zero-energy resonance) in e(+)-C2H2 collisions, based on the
behavior of the integral cross section and of the s-wave phase shift. As
expected the fixed-nuclei cross section and annihilation parameter
(Z(eff)) present the same energy dependence at very low impact energies.
As the virtual state energy approaches zero, the magnitude of both cross
section and Z(eff) are extremely enhanced (at zero impact energy). The
possibility of shifting from a low-lying virtual state to a shallow
bound state is not expected to significantly affect room-temperature
annihilation rates.
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