Search for excited states in O 25

Background: Theoretical calculations suggest the presence of low-lying excited states in O25. Previous experimental searches by means of proton knockout on F26 produced no evidence for such excitations. Purpose: We search for excited states in O25 using the O24(d,p)O25 reaction. The theoretical analysis of excited states in unbound O25,27 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 O25. 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 O23-26, assuming a core of O22. We predict energies, decay widths, and asymptotic normalization coefficients. Results: Our calculations in a large spdf space predict several low-lying excited states in O25 of positive and negative parity, and we obtain an experimental limit on the relative cross section of a possible Jπ=1/2+ state with respect to the ground state of O25 at σ1/2+/σg.s.=0.25-0.25+1.0. We also discuss how the observation of negative parity states in O25 could guide the search for the low-lying negative parity states in O27. Conclusion: Previous experiments based on the proton knockout of F26 suffered from the low cross sections for the population of excited states in O25 because of low spectroscopic factors. In this respect, neutron transfer reactions carry more promise.