New insight into the low-energy $^9$He spectrum

The spectrum of $^{9}\mathrm{He}$ was studied by means of the $^{2}\mathrm{H}$($^{8}\mathrm{He}$,$p$)$^{9}\mathrm{He}$ reaction at a laboratory energy of 25 MeV/nucleon and small center-of-mass angles. Energy and angular correlations were obtained for the $^{9}\mathrm{He}$ decay products by complete kinematical reconstruction. The lowest resonant state of $^{9}\mathrm{He}$ is found at 2.0 \ifmmode\pm\else\textpm\fi{} 0.2 MeV with a width of $~2$ MeV and is identified as $1/{2}^{\ensuremath{-}}$. The observed angular correlation pattern is uniquely explained by the interference of the $1/{2}^{\ensuremath{-}}$ resonance with a virtual $1/{2}^{+}$ state (a limit $ag\ensuremath{-}20$ fm is obtained for the scattering length), and with a $5/{2}^{+}$ resonance at energy $\ensuremath{\geqslant}4.2$ MeV.