Inversion doublets of 3N+N cluster structure in excited states of 4He

Excited states of He-4 are studied in four-body calculations with explicitly correlated Gaussian bases. All the levels below E-x = 26 MeV are reproduced reasonably well using realistic potentials. An analysis is made to show how the 0(2)(+) state becomes a resonance but those having almost the same structure as this state in different spin-isospin channels are not observed as resonances. The role of tensor force is stressed with a particular attention to the level spacing between the two 0(-) states. The calculation of spectroscopic amplitudes, nucleon decay widths, and spin-dipole transition strengths demonstrates that the 0(2)(+) state and the three lowest-lying negative-parity states with 0(-) and 2(-) have H-3 + p and He-3 + n cluster configurations, leading to the interpretation that these negative-parity states are the inversion-doublet partners of the 0(2)(+) state.