LAUSR

Background: Theoretical calculations suggest the presence of low-lying excited states in $^{25}\mathrm{O}$. Previous experimental searches by means of proton knockout on $^{26}\mathrm{F}$ produced no evidence for such excitations.Purpose: We search for excited states in $^{25}\mathrm{O}$ using the $^{24}\mathrm{O}(d,p)^{25}\mathrm{O}$ reaction. The theoretical analysis of excited states in unbound $^{25,27}\mathrm{O}$ is based on the configuration interaction approach that accounts for couplings to the scattering continuum.Method: We use invariant-mass spectroscopy to measure neutron-unbound states in $^{25}\mathrm{O}$. For the theoretical approach, we use the complex-energy Gamow Shell Model and Density Matrix Renormalization Group method with a finite-range two-body interaction optimized to the bound states and resonances of $^{23--26}\mathrm{O}$, assuming a core of $^{22}\mathrm{O}$. We predict energies, decay widths, and asymptotic normalization coefficients.Results: Our calculations in a large $spdf$ space predict several low-lying excited states in $^{25}\mathrm{O}$ of positive and negative parity, and we obtain an experimental limit on the relative cross section of a possible ${J}^{\ensuremath{\pi}}={1/2}^{+}$ state with respect to the ground state of $^{25}\mathrm{O}$ at ${\ensuremath{\sigma}}_{1/2+}/{\ensuremath{\sigma}}_{g.s.}=0.{25}_{\ensuremath{-}0.25}^{+1.0}$. We also discuss how the observation of negative parity states in $^{25}\mathrm{O}$ could guide the search for the low-lying negative parity states in $^{27}\mathrm{O}$.Conclusion: Previous experiments based on the proton knockout of $^{26}\mathrm{F}$ suffered from the low cross sections for the population of excited states in $^{25}\mathrm{O}$ because of low spectroscopic factors. In this respect, neutron transfer reactions carry more promise.