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A. M. Shirokov

Publications and source records attributed to A. M. Shirokov.

At least 19 recordsLinked to original sources
arXiv2026

Finite-spectrum Lorentz integral transform calculation of the $^{4}$He photoabsorption cross section in the no-core shell model

We develop and validate a finite-spectrum implementation of the Lorentz integral transform (LIT) within the \textit{ab initio} no-core shell model (NCSM) for calculating the photoabsorption cross section of $^4$He. A large set of $1^-$ eigenstates is explicitly calculated in the NCSM, and the LIT is constructed from their excitation energies and the corresponding $E1$ transition strengths. This finite-spectrum approach is complementary to conventional inhomogeneous-equation and Lanczos-based implementations of the LIT method for photoabsorption cross sections. Using the Daejeon16 interaction, we extract the photoabsorption cross section and examine its stability with respect to the model-space truncation, excitation-energy cutoff, and LIT parameters. The reliability of the finite-spectrum extraction is assessed by comparing the $E1$ polarizability and bremsstrahlung sum rule obtained from the discrete NCSM spectrum with the same quantities obtained by integrating the extracted cross section. The extracted cross section captures the principal features of the available $^4$He photonuclear data in the giant-dipole-resonance region and is consistent, in the low-energy rise and main-peak region, with earlier chiral-interaction NCSM-LIT results obtained from Lanczos-based evaluations, while the present calculation with the Daejeon16 interaction exhibits a more pronounced high-energy shoulder. The present work provides a controlled finite-spectrum NCSM-LIT route from explicitly calculated many-body eigenstates and transition strengths to photoabsorption cross sections.

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arXiv2025

Alpha-Particle Monopole Form Factors with Ab Initio No-Core Shell Model

The state-of-the-art ab initio nuclear many-body approaches with modern nuclear forces are challenged by the recent experimental measurement of the monopole form factor of the $0^+_1\rightarrow 0^+_2$ transition in the $α$ particle [Kegel et al., Phys. Rev. Lett. 130, 152502 (2023)]. We investigate the elastic and inelastic $0^+_1\rightarrow 0^+_2$ transition form factors using the ab initio no-core shell model (NCSM). We observe a good convergence of both form factors with respect to the basis size employing the Daejeon16 nucleon-nucleon ($NN$) interaction. Our NCSM results are very close to the effective interaction hyperspherical harmonic calculations using $NN$ plus three-nucleon interactions based on the chiral effective field theory which take into account the continuum effects via the Lorentz integral transform. The significant difference between the ab initio results with various modern nuclear interactions and of some of them with the recent experimental data provides motivations for deeper investigation of this observable.

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arXiv2025

Applications of the Modified Hulthén-Kohn Method for Bound and Scattering States

We apply the Hulthèn-Kohn method suggested by V. D. Efros [Phys. Rev. C 99, 034620 (2019)] for calculating various observables in the continuum and discrete spectrum using two-body interactions in single- and coupled-channel systems. This method is promising for many-body applications and ab initio description of nuclear reactions. We explore the convergence of phase shifts and wave functions as well as the location of S-matrix poles which enables obtaining both resonance and bound state parameters. We find that adopting wave functions from approximate bound-state solutions for the short-range components of basis wave functions leads to good convergence.

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arXiv2025

Machine Learning for Extrapolating No-Core Shell Model Results to Infinite Basis

We utilize the machine learning to extrapolate to the infinite model space the no-core shell model (NCSM) results for the energies and rms radii of the 6He ground state and 6Li lowest states. The extrapolated energies and rms radii converge as the NCSM results from larger model spaces are included in the training dataset for ensemble of artificial neural networks thus enabling an accurate predictions for these observables.

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arXiv2025

Ab initio study of Z(N) = 6 magicity

The existence of magic numbers of protons and neutrons in nuclei is essential for understanding nuclear structure and fundamental nuclear forces. Over decades, researchers have conducted theoretical and experimental studies on the new magic number Z(N) = 6, focusing on observables such as radii, binding energy, electromagnetic transition, and nucleon separation energies. We perform the ab initio no-core shell model calculations for the occupation numbers of the lowest single particle states in the ground states of Z(N) = 6 and Z(N) = 8 isotopes (isotones). Our calculations do not support Z(N) = 6 as a magic number over a span of atomic numbers. However, 14C and 14O exhibit the characteristics of double-magic nuclei.

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arXiv2025

Quadrupole dynamics of carbon isotopes and 10Be

Electric quadrupole (E2) moments and transitions provide measures of nuclear deformation and related collective structure. However, matrix elements of the E2 operator are sensitive to the nuclear wave function at large distances and are poorly convergent within the ab initio no-core shell model approach. We demonstrate for the first time that the ratio of neutron to proton quadrupole transition matrix elements, Mn/Mp, is well-converged in the ab initio no-core shell model and provides a new and robust tool for comparing with experimental results. We find that our parameter-free results for Mn/Mp for the carbon isotopes and 10Be compare well with experiment, where available, and offer new insight into the quadrupole dynamics of nuclear response.

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arXiv2024

Trineutron resonances in the SS-HORSE-NCSM approach

The SS-HORSE-NCSM method is generalized to the case of democratic decay into an odd number of fragments. This method is applied to the search for resonances in three-neutron system (trineutron) using ab initio No-Core Shell Model calculations with realistic nucleon-nucleon potentials. The $3/2^-$ and $1/2^-$ strongly overlapping resonances are predicted when softened $NN$ interactions are used and are preferred over the case where bare $NN$ interactions of the chiral effective field theory are used with no resonance obtained.

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arXiv2022

SS-HORSE Extension of the No-Core Shell Model: Application to Resonances in $^7{\mathrm He}$

Theoretical ab initio studies of resonances in the unbound ${\rm^{7}He}$ nucleus are presented. We perform no-core shell model calculations with $NN$ interactions Daejeon16 and JISP16 and utilize the SS-HORSE method to calculate the $S$ matrix for two-body channels $n{-}{\rm^{6}He}$ and $n{-}{\rm^{6}He^{*}}$ with ${\rm^{6}He}$ respectively in the ground and excited $2^{+}$ states as well as for the four-body democratic decay channel ${{\rm^{4}He}+n+n+n}$. The resonant energies and widths areobtained by numerical location of the $S$-matrix poles. We describe all experimentally known ${\rm^{7}He}$ resonances and suggest an interpretation of an observed wide resonance of unknown spin-parity.

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arXiv2022

Resonances in Exotic $^7$He Nucleus within the No-Core Shell Model

We present results of calculations of $n{-}{^6\rm He}$ elastic scattering phase shifts and resonances in ${^7\rm He}$. The calculations utilize the SS-HORSE method combined with ab initio no-core shell model calculations of the ${^7\rm He}$ and ${^6\rm He}$ nuclei with Daejeon16 and the JISP16 $NN$ interactions.

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arXiv2022

Effective operators for valence space calculations from the {\itshape ab initio} No-Core Shell Mode

In recent years, remarkable progress has been achieved in developing novel non-perturbative techniques for constructing valence space shell model Hamiltonians from realistic internucleon interactions. One of these methods is based on the Okubo--Lee--Suzuki (OLS) unitary transformation applied to no-core shell model (NCSM) solutions. In the present work, we implement the corresponding approach to solve for valence space effective electromagnetic operators. To this end, we use the NCSM results for $A=16-18$, obtained at $N_{\rm max}=4$, to derive a charge-dependent version of the effective interaction for the $sd$ shell, which allows us to exactly reproduce selected NCSM spectra of $^{18}$O, $^{18}$F and $^{18}$Ne within the two valence nucleon space. We then deduce effective single-particle matrix elements of electric quadrupole ($E2$) and magnetic dipole ($M1$) operators by matching them to the electromagnetic transitions and moments for $^{17}$O and $^{17}$F from the NCSM at $N_{\rm max}=4$. Thus, effective $E2$ and $M1$ operators are obtained as sets of single-particle matrix elements for the valence space ($sd$ shell) which allow us to reproduce the NCSM results for $A=17$ exactly. Systematic comparison of a large set of $sd$ shell results on quadrupole and magnetic dipole moments and transitions for $A=18$ using effective $E2$ and $M1$ operators that we derive from the full NCSM calculations demonstrates a remarkable agreement.

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arXiv2020

Improved description of light nuclei through chiral effective field theory at leading order

We propose an arrangement of the most commonly invoked version of the two-nucleon chiral potential such that the low-lying amplitude zero of the 1S0 partial wave is captured at leading order of the effective expansion. Adopting other partial waves from the LENPIC interaction, we show how this modification yields an improved description of ground-state energies and point-proton radii of three test nuclei.

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arXiv2019

Daejeon16 NN Interaction

We have developed a realistic nucleon-nucleon (NN) interaction, dubbed Daejeon16. We start from a SRG (similarity renormalization group) evolved chiral N3LO interaction. We then apply PETs (phase-equivalent transformations) to the SRG-evolved interaction. It turned out that the obtained in such a way Daejeon16 NN interaction provides a good description of various observables in light nuclei without NNN forces. In this contribution, we present our new results for some selected nuclei using the ab initio no-core shell model (NCSM) with the Daejeon16 interaction. One of the interesting results is that the ab initio NCSM with Daejeon16 clearly demonstrates the phenomenon of parity inversion in Be-11, i.e., the ground state in Be-11 has the spin-parity (1/2, +) in experiments contrary to the expectation from the conventional shell model

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arXiv2018

Nucleon-$α$ Scattering and Resonances in $^5$He and $^5$Li with JISP16 and Daejeon16 $NN$ interactions

The SS-HORSE approach to analysis of resonant states is generalized to the case of charged particle scattering utilizing analytical properties of partial scattering amplitudes and applied to the study of resonant states in the $^{5}$Li nucleus and non-resonant $s$-wave proton-$α$ scattering within the no-core shell model using the JISP16 and Daejeon16 $NN$ interactions. We present also the results of calculations of neutron-$α$ scattering and resonances in the $^{5}$He nucleus with Daejeon16 and compare with results published previously using JISP16.

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arXiv2018

Nucleon-deuteron scattering with the JISP16 potential

The nucleon-nucleon J-matrix Inverse Scattering Potential JISP16 is applied to elastic nucleon-deuteron (Nd) scattering and the deuteron breakup process at the lab. nucleon energies up to 135 MeV. The formalism of the Faddeev equations is used to obtain 3N scattering states. We compare predictions based on the JISP16 force with data and with results based on various NN interactions: the CD Bonn, the AV18, the chiral force with the semi-local regularization at the 5th order of the chiral expansion and with low-momentum interactions obtained from the CD Bonn force as well as with the predictions from the combination of the AV18 NN interaction and the Urbana IX 3N force. JISP16 provides a satisfactory description of some observables at low energies but strong deviations from data as well as from standard and chiral potential predictions with increasing energy. However, there are also polarization observables at low energies for which the JISP16 predictions differ from those based on the other forces by a factor of two. The reason for such a behavior can be traced back to the P-wave components of the JISP16 force. At higher energies the deviations can be enhanced by an interference with higher partial waves and by the properties of the JISP16 deuteron wave function. In addition, we compare the energy and angular dependence of predictions based on the JISP16 force with the results of the low-momentum forces obtained with different values of the momentum cutoff parameter. We found that such low-momentum forces can be employed to interpret the Nd elastic scattering data only below some specific energy which depends on the cutoff parameter. Since JISP16 is defined in a finite oscillator basis, it has properties similar to low momentum interactions and its application to the description of Nd scattering data is limited to a low momentum transfer region.

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arXiv2016

Shell Model States in the Continuum

We suggest a method for calculating scattering phase shifts and energies and widths of resonances which utilizes only eigenenergies obtained in variational calculations with oscillator basis and their dependence on oscillator basis spacing $\hbarΩ$. We make use of simple expressions for the $S$-matrix at eigenstates of a finite (truncated) Hamiltonian matrix in the oscillator basis obtained in the HORSE ($J$-matrix) formalism of quantum scattering theory. The validity of the suggested approach is verified in calculations with model Woods--Saxon potentials and applied to calculations of $nα$ resonances and non-resonant scattering using the no-core shell model.

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arXiv2016

N3LO NN interaction adjusted to light nuclei in ab exitu approach

We use phase-equivalent transformations to adjust off-shell properties of similarity renormalization group evolved chiral effective field theory NN interaction (Idaho N3LO) to fit selected binding energies and spectra of light nuclei in an ab exitu approach. We then test the transformed interaction on a set of additional observables in light nuclei to verify that it provides reasonable descriptions of these observables with an apparent reduced need for three- and many-nucleon interactions.

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arXiv2016

Prediction for a four-neutron resonance

We utilize various {\em ab initio} approaches to search for a low-lying resonance in the four-neutron ($4n$) system using the JISP16 realistic $NN$ interaction. Our most accurate prediction is obtained using a $J$-matrix extension of the No-Core Shell Model and suggests a $4n$ resonant state at an energy near $E_r = 0.8$ MeV with a width of approximately $Γ= 1.4$ MeV.

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