Search for excited states in ${}^{25}$O

Theoretical calculations suggest the presence of low-lying excited states in ${}^{25}$O. Previous experimental searches by means of proton knockout on ${}^{26}$F produced no evidence for such excitations. We search for excited states in ${}^{25}$O using the ${ {}^{24}$\text{O}$ (d,p) {}^{25}$\text{O}$ }$ reaction. The theoretical analysis of excited states in unbound ${}^{25,27}$O is based on the configuration interaction approach that accounts for couplings to the scattering continuum. We use invariant-mass spectroscopy to measure neutron-unbound states in ${}^{25}$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 ${}^{23-26}$O, assuming a core of ${}^{22}$O. We predict energies, decay widths, and asymptotic normalization coefficients. Our calculations in a large $spdf$ space predict several low-lying excited states in ${}^{25}$O of positive and negative parity, and we obtain an experimental limit on the relative cross section of a possible ${ {J}^{\pi} = {1/2}^{+} }$statewithrespecttotheground-stateof$^{25}$Oat$\sigma_{1/2+}/\sigma_{g.s.} = 0.25_{-0.25}^{+1.0}$. We also discuss how the observation of negative parity states in ${}^{25}$O could guide the search for the low-lying negative parity states in ${}^{27}$O. Previous experiments based on the proton knockout of ${}^{26}$F suffered from the low cross sections for the population of excited states in ${}^{25}$O because of low spectroscopic factors. In this respect, neutron transfer reactions carry more promise.