Exceptional points in the scattering continuum

The manifestation of exceptional points in the scattering continuum of atomic nucleus is studied using the real-energy continuum shell model. It is shown that low-energy exceptional points appear for realistic values of coupling to the continuum and, hence, could be accessible experimentally. Experimental signatures are proposed which include the jump by $2\pi$ of the elastic scattering phase shift and a salient energy dependence of cross-sections in the vicinity of the exceptional point.Comment: 7 pages, 9 figure

Near-threshold correlations of neutrons

The appearance of charged-particle clustering in near-threshold configuration is a phenomenon that can be explained in the Open Quantum System description of the atomic nucleus. In this work we apply the realistic Shell Model Embedded in the Continuum to elucidate the emergence of neutron correlations in near-threshold many-body states coupled to l=1,2 neutron decay channels. Spectral consequences of such continuum coupling are briefly discussed together with the emergence of complex multi-neutron correlations.Comment: Invited talk at XXXIII Mazurian Lakes Conference on Physic

Bound states of dipolar molecules studied with the Berggren expansion method

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 non-resonant 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.Comment: 12 pages, 10 figure

Description of the proton and neutron radiative capture reactions in the Gamow shell model

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 7Be(p,gamma)8B and 7Li(n,gamma)8Li. The GSM-CC formalism is applied to a translationally-invariant Hamiltonian with an effective finite-range 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 7Be/7Li and the proton/neutron wave function expanded in different partial waves

Nuclear rotation in the continuum

${\textbf{Background:}}$ 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. ${\textbf{Purpose:}}$ 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. ${\textbf{Method:}}$ 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. ${\textbf{Results:}}$ We demonstrate that collective rotation of the ground band of $^{11}$Be is stabilized by (i) the fact that the $\ell=0$ one-neutron decay channel is closed, and (ii) the angular momentum alignment, which increases the parentage of high-$\ell$ 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 $\ell=0$ neutron-decay channel is open and often dominates. ${\textbf{Conclusion:}}$ We demonstrate that long-lived collective states can exist at high excitation energy in weakly bound neutron drip-line nuclei such as $^{11}$Be

Ab-initio No-Core Gamow Shell Model calculations with realistic interactions

No-Core Gamow Shell Model (NCGSM) is applied for the first time to study selected well-bound and unbound states of helium isotopes. This model is formulated on the complex energy plane and, by using a complete Berggren ensemble, treats bound, resonant, and scattering states on equal footing. We use the Density Matrix Renormalization Group method to solve the many-body Schr\"{o}dinger equation. To test the validity of our approach, we benchmarked the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for $^3$H and $^4$He nuclei. We also performed {\textit ab initio} NCGSM calculations for the unstable nucleus $^5$He and determined the ground state energy and decay width, starting from a realistic N$^3$LO chiral interaction.Comment: 17 pages, 14 figures. Revised version. Discussion on microscopic overlap functions, SFs and ANCs is added. Added references. Accepted for publication at PR

Description of the proton-decaying 02+^+_2 resonance of the α\alpha particle

The recent precise experimental determination of the monopole transition form factor from the ground state of $^4$He to its $0^+_2$ resonance via electron scattering has reinvigorated discussions about the nature of this first excited state of the $\alpha$ particle. The $0^+_2$ state has been traditionally interpreted in the literature as the isoscalar monopole resonance (breathing mode) or, alternatively, as a particle-hole shell-model excitation. To better understand the nature of this state, which lies only $\sim$ 410 keV above the proton emission threshold, we employ the coupled-channel representation of the no-core Gamow shell model. By considering the $[^3$H$+ p]$, $[^3$He$+ n]$, and $[^2$H+$^2$H] reaction channels, we explain the excitation energy and monopole form-factor of the $0^+_2$ state. We argue that the continuum coupling strongly impacts the nature of this state, which carries characteristics of the proton decay threshold.Comment: 5 pages, 2 figure

Near-threshold resonances in 11C and the 10B(p,{\alpha})7Be aneutronic reaction cross section

The nucleus 11C plays an important role in the boron-proton fusion reactor environment as a catalyzer of the 10B(p,{\alpha})7Be reaction which, by producing a long-lived isotope of 7Be, poisons the aneutronic fusion process 11B(p,2{\alpha})4He. The low-energy cross section of 10B(p,{\alpha})7Be depends on the near-threshold states 7/2+1 , 5/2+2 , 5/2+3 in 11C whose properties are primarily known from the indirect measurements. We investigate the continuum-coupling induced collectivization of these resonances in the shell model embedded in the continuum. We predict a significant enhancement of the 10B(p,{\alpha})7Be cross section at energies accessible to the laser-driven hot plasma facilities.Comment: 4 pages, 2 figure

β−p\beta^-{\rm p} and β−α\beta^-\alpha decay of the 11^{11}Be neutron halo ground state

Beta-delayed proton emission from the neutron halo ground state of $^{11}$Be raised much attention due to the unusually high decay rate. It was argued that this may be due to the existence of a resonance just above the proton decay threshold. In this Letter, we use the lenses of real-energy continuum shell model to describe several observables including the Gamow-Teller rates for the $\beta^-$-delayed $\alpha$ and proton decays, and argue that, within our model, the large $\beta^-{\rm p}$ branching ratio cannot be reconciled with other data.Comment: 5 pages, 3 figure

Gamow shell model description of radiative capture reactions 6^6Li(p,γ)(p,\gamma)7^7Be and 6^6Li(n,γ)(n,\gamma)7^7Li

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. 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,\gamma)$$^7$Be and $^6$Li$(n,\gamma)$$^7$Li. 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. All relevant $E1$, $M1$, 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 are included. We demonstrate that the $s$-wave radiative capture of proton (neutron) to the first excited state $J^{\pi}=1/2_1^+$ of $^7$Be ($^7$Li) is crucial and increases the total astrophysical $S$-factor by about 40 \%.Comment: arXiv admin note: text overlap with arXiv:1502.0163