We formulate the Gamow shell model (GSM) in coupled-channel (CC) representation for the description of proton/neutron radiative capture reactions and present the first application of this new formalism for the calculation of cross-sections in mirror reactions 7 Be(p, γ) 8 B and 7 Li(n, γ) 8 Li. The GSM-CC formalism is applied to a translationally-invariant Hamiltonian with an effective finiterange two-body interaction. Reactions channels are built by GSM wave functions for the ground state 3 2 − and the first excited state 1 2 − of 7 Be/ 7 Li and the proton/neutron wave function expanded in different partial waves.PACS numbers: 03.65. Nk, 33.15.Ry

The search for a resonant four-neutron system has been revived thanks to the recent experimental hints reported in Phys. Rev. Lett. 116, 052501 (2016) [1]. The existence of such a system would deeply impact our understanding of nuclear matter and requires a critical investigation. In this work, we study the existence of a four-neutron resonance in the quasi-stationary formalism using ab initio techniques with various two-body chiral interactions. We employ the No-Core Gamow Shell Model and the Density Matrix Renormalization Group method, both supplemented by the use of natural orbitals and a new identification technique for broad resonances. We demonstrate that while the energy of the four-neutron system may be compatible with the experimental value, its width must be larger than the reported upper limit, supporting the interpretation of the experimental observation as a reaction process too short to form a nucleus.

The binding-energy pattern along the neutron-rich oxygen chain, governed by an interplay between shell effects and many-body correlations impacted by strong couplings to one- and two-neutron continuum, make these isotopes a unique testing ground for nuclear models. In this work, we investigate ground states and low-lying excited states of $^{23-28}$O using 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 $^{23-26}$O, assuming a core of $^{22}$O. Our results suggest that the ground-state of $^{28}$O has a threshold character, i.e., is very weakly bound or slightly unbound. We also predict narrow excited resonances in $^{25}$O and $^{27}$O. The inclusion of the large continuum space significantly impacts predicted binding energies of $^{26-28}$O. This implies that the careful treatment of neutron continuum is necessary prior to assessing the spectroscopic quality of effective interactions in this region

Background: Atomic nuclei often exhibit collective rotational-like behavior in highly excited states, well above the particle emission threshold. What determines the existence of collective motion in the continuum region, is not fully understood.Purpose: In this work, by studying the collective rotation of the positive-parity deformed configurations of the one-neutron halo nucleus 11 Be, we assess different mechanisms that stabilize collective behavior beyond the limits of particle stability.Method: To solve a particle-plus-core problem, we employ a non-adiabatic coupled-channel formalism and the Berggren single-particle ensemble, which explicitly contains bound states, narrow resonances, and the scattering continuum. We study the valence-neutron density in the intrinsic rotor frame to assess the validity of the adiabatic approach as the excitation energy increases.Results: We demonstrate that collective rotation of the ground band of 11 Be is stabilized by (i) the fact that the = 0 one-neutron decay channel is closed, and (ii) the angular momentum alignment, which increases the parentage of high-components at high spins; both effects act in concert to decrease decay widths of ground-state band members. This is not the case for higher-lying states of 11 Be, where the = 0 neutron-decay channel is open and often dominates. Conclusion:We demonstrate that long-lived collective states can exist at high excitation energy in weakly bound neutron drip-line nuclei such as 11 Be. Introduction. -Studies of exotic nuclei far from the valley of beta-stability reveal novel features, such as the formation of halo structures [1, 2], near-threshold clustering effects [3][4][5][6], and presence of new types of correlations [7]. In all these cases, the atomic nucleus exhibits properties characteristic of an open quantum system, whose properties are dramatically altered by the coupling to scattering and reaction channels [8].

Background: According to standard stellar evolution, lithium abundance is believed to be a useful indicator of the stellar age. However, many evolved stars like red giants show huge fluctuations around expected theoretical abundances that are not yet fully understood. The better knowledge of nuclear reactions that contribute to the creation and destruction of lithium can help to solve this puzzle. Purpose: In this work we apply the Gamow shell model (GSM) formulated in the coupled-channel representation (GSM-CC) to investigate the mirror radiative capture reactions 6 Li(p, γ) 7 Be and 6 Li(n, γ) 7 Li. Method: GSM offers the most general treatment of couplings between discrete resonant states and the non-resonant continuum. The cross-sections are calculated using a translationally invariant Hamiltonian with the finite-range interaction which is adjusted to reproduce spectra, binding energies and one-nucleon separation energies in 6−7 Li, 7 Be. The reaction channels are built by coupling the wave functions of ground state 1 + 1 and excited states 3 + 1 , 0 + 1 , 2 + 1 of 6 Li with the projectile wave function in different partial waves. Results: We include all relevant E1, M 1, and E2 transitions from the initial continuum states to the final bound states J = 3 2 − 1 and J = 1 2 − of 7 Li and 7 Be. Our microscopic astrophysical factor for the 6 Li(p,γ) 7 Be reaction follows the average trend of the experimental value as a function of the center of mass energy. For 6 Li(n, γ) 7 Li, the calculated cross section agrees well with the data from the direct measurement of this reaction at stellar energies. Conclusion: We demonstrate that the s-wave radiative capture of proton (neutron) to the first excited state J π = 1 2 + 1 of 7 Be (7 Li) is crucial and increases the total astrophysical S-factor by about 40 %.

Bound states of dipole-bound negative anions are studied by using a non-adiabatic pseudopotential method and the Berggren expansion involving bound states, decaying resonant states, and nonresonant scattering continuum. The method is benchmarked by using the traditional technique of direct integration of coupled channel equations. A good agreement between the two methods has been found for well-bound states. For weakly-bound subthreshold states with binding energies comparable with rotational energies of the anion, the direct integration approach breaks down and the Berggren expansion method becomes the tool of choice.PACS numbers: 03.65. Nk, 33.15.Ry

Bound and resonance states of the dipole-bound anion of hydrogen cyanide HCN − are studied using a non-adiabatic pseudopotential method and the Berggren expansion technique involving bound states, decaying resonant states, and non-resonant scattering continuum. We devise an algorithm to identify the resonant states in the complex energy plane. To characterize spatial distributions of electronic wave functions, we introduce the body-fixed density and use it to assign families of resonant states into collective rotational bands. We find that the non-adiabatic coupling of electronic motion to molecular rotation results in a transition from the strong-coupling to weakcoupling regime. In the strong coupling limit, the electron moving in a subthreshold, spatially extended halo state follows the rotational motion of the molecule. Above the ionization threshold, electron's motion in a resonance state becomes largely decoupled from molecular rotation. Widths of resonance-band members depend primarily on the electron orbital angular momentum.

Background: Deformed neutron-rich magnesium isotopes constitute a fascinating territory where the interplay between collective rotation and single-particle motion is strongly affected by the neutron continuum. The unbound f p-shell nucleus 39 Mg is an ideal candidate to study this interplay.Purpose: In this work, we predict the properties of low-lying resonant states of 39 Mg, using a suite of realistic theoretical approaches rooted in the open quantum system framework. − ground-state candidate exhibits a resonant structure reminiscent of that of its one-neutron halo neighbor 37 Mg, which is dominated by the f 7 2 partial wave at short distances and a p 3 2 component at large distances. A J Conclusion:We demonstrate that the subtle interplay between deformation, shell structure, and continuum coupling can result in a variety of excitations in an unbound nucleus just outside the neutron drip line.