arXiv subjects

R. A. Sen'kov

Publications and source records attributed to R. A. Sen'kov.

At least 19 recordsLinked to original sources

arXiv2015

Shell-model calculation of neutrinoless double-$β$ decay of $^{76}$Ge

In this article we present a more detailed version of our recent Rapid Communication [Phys. Rev. C 90, 051301(R) (2014)] where we calculate the nuclear matrix elements for neutrinoless double-$β$ decay of $^{76}$Ge. For the calculations we use a novel method that has perfect convergence properties and allows one to obtain the nonclosure nuclear matrix elements for $^{76}$Ge with a 1% accuracy. We present a new way of calculation of the optimal closure energy, using this energy with the closure approximation provides the most accurate closure nuclear matrix elements. In addition, we present a new analysis of the heavy-neutrino-exchange nuclear matrix elements, and we compare occupation probabilities and Gamow-Teller strength with experimental data.

nucl-th↗

arXiv2014

Accurate shell-model nuclear matrix elements for neutrinoless double-beta decay

We investigate a novel method of accurate calculation of the neutrinoless double-$β$ decay shell-model nuclear matrix elements for the experimentally relevant case of $^{76}$Ge. We demonstrate that with the new method the nuclear matrix elements have perfect convergence properties and, using only the first 100 intermediate states of each spin, the matrix elements can be calculated with better than 1% accuracy. Based on the analysis of neutrinoless double-beta decays of $^{48}$Ca, $^{82}$Se, and $^{76}$Ge isotopes, we propose a new method to estimate the optimal values of the average closure energies at which the closure approximation gives the most accurate nuclear matrix elements. We also analyze the nuclear matrix elements for the heavy-neutrino-exchange mechanism, and we show that our method can be used to quench contributions from different intermediate spin states.

nucl-th↗

arXiv2014

Nuclear Structure Aspects of Neutrinoless Double Beta Decay

We decompose the neutrinoless double-beta decay matrix elements into sums of products over the intermediate nucleus with two less nucleons. We find that the sum is dominated by the J^pi=0^+ ground state of this intermediate nucleus for both the light and heavy neutrino decay processes. This provides a new theoretical tool for comparing and improving nuclear structure models. It also provides the connection to two-nucleon transfer experiments.

nucl-th↗

arXiv2014

Neutrinoless double-$β$ decay of ${}^{82}$Se in the shell model: beyond closure approximation

We recently proposed a new method to calculate the standard nuclear matrix elements for neutrinoless double-$β$ decay ($0νββ$) of ${}^{48}$Ca going beyond the closure approximation. Here we extend this analysis to the important case of ${}^{82}$Se, which was chosen as the base isotope for the upcoming SuperNEMO experiment. We demonstrate that using a mixed method that considers information from closure and non-closure approaches, one can get excellent convergence properties for the nuclear matrix elements, which allows one to avoid unmanageable computational costs. We show that in contrast with the closure approximation the mixed approach has a very weak dependence on the average closure energy. The matrix elements for the heavy neutrino-exchange mechanism that could contribute to the $0νββ$ decay of ${}^{82}$Se are also presented.

nucl-th↗

arXiv2013

Neutrinoless double beta decay of 48Ca in the shell model: Closure versus nonclosure approximation

Neutrinoless double-beta decay is a unique process that could reveal physics beyond the Standard Model. Essential ingredients in the analysis of neutrinoless double-beta rates are the associated nuclear matrix elements. Most of the approaches used to calculate these matrix elements rely on the closure approximation. Here we analyze the light neutrino-exchange matrix elements of 48Ca neutrinoless double-beta decay and test the closure approximation in a shell-model approach. We calculate the neutrinoless double-beta nuclear matrix elements for 48Ca using both the closure approximation and a nonclosure approach, and we estimate the uncertainties associated with the closure approximation. We demonstrate that the nonclosure approach has excellent convergence properties which allow us to avoid unmanageable computational cost. Combining the nonclosure and closure approaches we propose a new method of calculation for neutrinoless double-beta decay rates which can be applied to the neutrinoless double-beta decay rates of heavy nuclei, such as 76Ge or 82Se.

nucl-th↗

arXiv2013

Moments Method for Shell-Model Level Density

The modern form of the Moments Method applied to the calculation of the nuclear shell-model level density is explained and examples of the method at work are given. The calculated level density practically exactly coincides with the result of full diagonalization when the latter is feasible. The method provides the pure level density for given spin and parity with spurious center-of-mass excitations subtracted. The presence and interplay of all correlations leads to the results different from those obtained by the mean-field combinatorics.

nucl-th↗

arXiv2011

High-Performance Algorithm for Calculating Non-Spurious Spin- and Parity-Dependent Nuclear Level Densities

A new high-performance algorithm for calculating the spin- and parity-dependent shell model nuclear level densities using methods of statistical spectroscopy in the proton-neutron formalism was recently proposed. When used in valence spaces that cover more than one major harmonic oscillator shell, this algorithm mixes the genuine intrinsic states with spurious center-of-mass excitations. In this paper we present an advanced algorithm, based on the recently proposed statistical moments method, that eliminates the spurious states. Results for unnatural parity states of several sd-shell nuclei are presented and compared with those of exact shell model calculations and experimental data.

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arXiv2010

Unified BCS-like model of pairing and alpha-correlations

We construct a BCS-like model that combines nucleonic pairing correlations and possible quartic correlations of alpha-type in a single variational wave function and derive corresponding gap equations. In the approximation of large logarithms typical for the BCS approach, we show that the system reveals two possible types of a condensate which cannot coexist. If the alpha-type condensate prevails at N=Z, the growth of the neutron excess will naturally lead to the first order phase transition to the nucleon condensates.

nucl-th↗

arXiv2010

A High-Performance Algorithm to Calculate Spin- and Parity-Dependent Nuclear Level Densities

A new algorithm for calculating the spin- and parity-dependent shell model nuclear level densities using the moments method in the proton-neutron formalism is presented. A new, parallelized code based on this algorithm was developed and tested using up to 4000 cores for a set of nuclei from the sd-, pf-, and pf + g9/2 - model spaces. By comparing the nuclear level densities at low excitation energy for a given nucleus calculated in two model spaces, such as pf and pf + g9/2, one could estimate the ground state energy in the larger model space, which is not accessible to direct shell model calculations due to the unmanageable dimension. Examples for the ground state energies of for 64Ge and 68Se in the pf + g9/2 model space are presented.

nucl-th↗

arXiv2010

Transport Through Nanostructures with Asymmetric Coupling to the Leads

Using an approach to open quantum systems based on the effective non-Hermitian Hamiltonian, we fully describe transport properties for a paradigmatic model of a coherent quantum transmitter: a finite sequence of square potential barriers. We consider the general case of asymmetric external barriers and variable coupling strength to the environment. We demonstrate that transport properties are very sensitive to the degree of opening of the system and determine the parameters for maximum transmission at any given degree of asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian, we show a double transition to a super-radiant regime where the transport properties and the structure of resonances undergo a strong change. We extend our analysis to the presence of disorder and to higher dimensions.

cond-mat.mes-hall↗

arXiv2008

Many-Body Approximations in the sd-Shell Sandbox

A new theoretical approach is presented that combines the Hartree-Fock variational scheme with the exact solution of the pairing problem in the finite orbital space. Using this formulation in the sd-space as an example, we show that the exact pairing significantly improves the results for the ground state energy

nucl-th↗

arXiv2007

Schiff Theorem Revisited

We carefully rederive the Schiff theorem and prove that the usual expression of the Schiff moment operator is correct and should be applied for calculations of atomic electric dipole moments. The recently discussed corrections to the definition of the Schiff moment are absent.

nucl-th↗

arXiv2006

Black ring with two angular momenta

General regular black ring solution with two angular momenta is presented, found by the inverse scattering problem method. The mass, angular momenta and the event horizon volume are given explicitly as functions of the metric parameters.

hep-th↗

arXiv2006

Nuclear Schiff moment in nuclei with soft octupole and quadrupole vibrations

Nuclear forces violating parity and time reversal invariance (${\cal P},{\cal T}$-odd) produce ${\cal P},{\cal T}$-odd nuclear moments, for example, the nuclear Schiff moment. In turn, this moment can induce the electric dipole moment in the atom. The nuclear Schiff moment is predicted to be enhanced in nuclei with static quadrupole and octupole deformation. The analogous suggestion of the enhanced contribution to the Schiff moment from the soft collective quadrupole and octupole vibrations in spherical nuclei is tested in this article in the framework of the quasiparticle random phase approximation with separable quadrupole and octupole forces applied to the odd $^{217-221}$Ra and $^{217-221}$Rn isotopes. We confirm the existence of the enhancement effect due to the soft modes. However, in the standard approximation the enhancement is strongly reduced by a small weight of the corresponding "particle + phonon" component in a complicated wave function of a soft nucleus. The perspectives of a better description of the structure of heavy soft nuclei are discussed.

nucl-th↗

arXiv2005

On shielding of nuclear electric dipole moments in atoms

We demonstrate explicitly that some recent calculations of atomic electric dipole moments (EDM) are incomplete. A contribution overlooked therein is pointed out. When included, it cancels exactly the result of those calculations, and thus restores the standard conclusions for nuclear EDM in atoms.

hep-ph↗

arXiv2004

Effects of core polarization on the nuclear Schiff moment

Schiff moments were calculated for a set of nuclei with full account of core polarization effects. A finite range P and T violating weak nucleon-nucleon interaction has been used in the calculations. While in the absence of core polarization the Schiff moment depends on one combination of the weak interaction constants, in the presence of core polarization the Schiff moment depends on all three constants separately. The dominant contribution comes from isovector, $ΔT=1$, part of the weak interaction. The effects of core polarization were found to have in general a large effect on the Schiff moments.

nucl-th↗

arXiv2003

Schiff moment of the Mercury nucleus and the proton dipole moment

We calculated the contribution of internal nucleon electric dipole moments to the Schiff moment of $^{199}$Hg. The contribution of the proton electric dipole moment was obtained via core polarization effects that were treated in the framework of random phase approximation with effective residual forces. We derived a new upper bound $|d_p|< 5.4\times 10^{-24} e\cdot$cm of the proton electric dipole moment.

nucl-th↗

arXiv2003

P- and T-violating Schiff moment of the Mercury nucleus

The Schiff moment of the $^{199}$Hg nucleus was calculated using finite range P- and T-violating weak nucleon-nucleon interaction. Effects of the core polarization were considered in the framework of RPA with effective residual forces.

nucl-th↗