Unnatural parity resonance states in positron-excited hydrogen scattering

The coupled-channels optical method for positron scattering has been applied to investigate resonance states with unnatural parities in positron-excited hydrogen system. The positronium formation channels and continuum channel are included via a complex equivalent local potential. Resonance states with angular momenta L=1 to L=2 and parities (-1)^L+1 are calculated. Resonance energies and widths are reported and compared with other theoretical calculations. We found that the opening positronium formation channels play an important role in forming nondipole Feshbach resonances.