Identifying structures in the continuum: Application to Be 16

Background: The population and decay of two-nucleon resonances offer exciting new opportunities to explore dripline phenomena. A proper 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 nonresonant continuum states in the same energy range poses a theoretical challenge. Purpose: Establish a robust theoretical framework to identify and characterize three-body resonances in a discrete basis, and apply the method to the two-neutron unbound system Be16. Method: A resonance operator is proposed, which describes the sensitivity to changes in the potential. Resonances, understood as normalizable states describing localized continuum structures, are identified from the eigenstates of the resonance operator with large negative eigenvalues. For this purpose, the resonance operator is diagonalized in a basis of Hamiltonian pseudostates, which in the present work are built within the hyperspherical harmonics formalism using the analytical transformed harmonic oscillator basis. The energy and width of the resonance are determined from its time dependence. Results: The method is applied to Be16 in a Be14+n+n model. An effective core+n potential, fitted to the available experimental information on the binary subsystem Be15, is employed. The 0+ ground state resonance of Be16 presents a strong dineutron configuration. This 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 Γ(0+)=0.16 MeV. From the same Hamiltonian, a 2+ resonance is also predicted with r(2+)=2.42 MeV and Γ(2+)=0.40 MeV. Conclusions: 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.This work has been partially supported by the Spanish Ministerio de Ciencia, Innovación y Universidades, and the European Regional Development Fund (FEDER), under Projects No. FIS2017-88410-P, No. FPA2016-77689-C2-1-R, and No. FIS2014-51941-P and by the European Union’s Horizon 2020 research and innovation program under Grant No. 654002.Peer reviewe