Resonance phenomena and threshold features in positron–helium scattering

Investigations of resonances and threshold behaviors in positron--helium scattering have been made using the momentum-space coupled-channels optical method. The positronium formation channels are considered via an equivalent-local complex potential. The s-wave resonances and the Wigner cusp feature at the positronium (n=1) formation threshold are compared with the previous reports. The p-and the d-wave resonances and a Wigner cusp feature at the positronium (n=2) formation threshold are reported for the first time.