Identifying structures in the continuum: Application to 16 Be

Abstract

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 16Be. Method: A resonance operator is proposed, which describes the sensitivity to changes in the potential. Resonances,understoodasnormalizablestatesdescribinglocalizedcontinuumstructures,areidentifiedfromthe eigenstates of the resonance operator with large negative eigenvalues. For this purpose, the resonance operator is diagonalizedinabasisofHamiltonianpseudostates,whichinthepresentworkarebuiltwithinthehyperspherical 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 16Be in a 14Be+n+n model. An effective core+n potential, fitted to the available experimental information on the binary subsystem 15Be, is employed. The 0+ ground state resonance of 16Be 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 (0+)=0.16 MeV. From the same Hamiltonian, a 2+ resonance is also predicted with εr(2+)=2.42 MeV and (2+)=0.40 MeV. Conclusions: The dineutron configuration and the computed 0+ width are consistent with previous R-matrix calculationsforthetruethree-bodycontinuum.Theextractedvaluesoftheresonanceenergyandwidthconverge 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.