Identifying structures in the continuum: Application to $^{16}$Be

Abstract

The population and decay of two-nucleon resonances offer exciting new opportunities to explore dripline phenomena. The understanding of these systems requires a solid description of the three-body (core+N+N) continuum. The identification of a state with resonant character from the background of non-resonant continuum states in the same energy range poses a theoretical challenge. It is the purpose of this work to establish a robust theoretical framework to identify and characterize three-body resonances in a discrete basis. A resonance operator is proposed, which describes the sensitivity to changes in the potential. Resonances are then identified from the lowest eigenstates of the resonance operator. The operator is diagonalized in a basis of Hamiltonian pseudostates, built within the hyperspherical harmonics formalism using the analytical THO basis. The energy and width of the resonance are determined from its time dependence. The method is applied to 16Be in a 14Be+n+n model. An effective core+n potential, fitted to the available information on the subsystem 15Be, is employed. The 0+ ground state resonance of 16Be presents a strong dineutron configuration, which favors the picture of a correlated two-neutron emission. Fitting the three body interaction to the experimental two-neutron separation energy |S2n|=1.35(10) MeV, the computed width is Gamma(0+)=0.16 MeV. From the same Hamiltonian, a 2+ resonance is also predicted with E_r(2+)=2.42 MeV and Gamma(2+)=0.40 MeV. The dineutron configuration and the computed 0+ width are consistent with previous R-matrix calculations for the true three-body continuum. The extracted values of the resonance energy and width converge with the size of the pseudostate basis and are robust under changes in the basis parameters. This supports the reliability of the method in describing the properties of unbound core+N+N systems in a discrete basis.