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

Excited states of ⁴He 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 0₂⁺ 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 0₂⁺ state and the three lowest-lying negative-parity states with 0^- and 2^- have ³H+p and ³He+n cluster configurations, leading to the interpretation that these negative-parity states are the inversion-doublet partners of the 0₂⁺ state.