Identifying structures in the continuum: Application to Be16

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 16 Be.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 16 Be in a 14 Be + n + n model. An effective core + n potential, fitted to the available experimental information on the binary subsystem 15 Be, is employed. The 0 + ground state resonance of 16 Be 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 εr(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.

Description of continuum structures in a discrete basis: Three-body resonances and two-nucleon decays

“…Recently, in Ref. [32] we proposed an alternative procedure to identify and characterize few-body resonances, based on the diagonalization of a resonance operator in a basis of Hamiltonian pseudostates,…”

“…In Refs. [17,32], we used the 14 Be + n potential employed in [9] to construct 16 Be wave functions in a three-body model. With this potential, the 15 Be ground state is a d 5/2 resonance at 1.8 MeV above one-neutron emission.…”

Two-Nucleon Emitters Within a Pseudostate Approach

“…This state represents the ground-state resonance of 16 Be, which shows a fast convergence with respect to the size of the model space, as shown in the right panel. As discussed in Ref [7,8], the corresponding wave function presents a dominant dineutron component, which favors the picture of a correlated two-neutron emission from the ground state of 16 Be.…”

Report on scipost_201910_00035v1

“…Even this surprisingly complex and exotic system, 19 B, can play the role of a single body if we keep adding neutrons to the system. In fact, three 1n resonances have been measured in 20 B, in which the exotic 19 B structure acts like a core. By adding an extra neutron, a 2n resonance has been observed in 21 B, again on top of a 19 B core (Fig.…”

“…2). Moreover, this resonance does not decay through the available 20 B ones, but seems to emit the two neutrons directly in its decay towards 19 B, making of the 21 B ground state a good candidate for the exotic 2n emission. This confirms the boron isotopes as an ideal ground for the study not only of exotic structures, but also of exotic decays.…”