arXiv subjects

S. A. Rakityansky

Publications and source records attributed to S. A. Rakityansky.

At least 19 recordsLinked to original sources

arXiv2025

$R$-matrix type parametrization of the Jost function for extracting the resonance parameters from scattering data

A new method is proposed for fitting non-relativistic binary-scattering data and for extracting the parameters of possible quantum resonances in the compound system that is formed during the collision. The method combines the well-known $R$-matrix approach with the analysis based on the semi-analytic representation of the Jost functions. It is shown that such a combination has the advantages of both these approaches, namely, the number of the fitting parameters remains relatively small (as for the $R$-matrix approach) and the proper analytic structure of the $S$-matrix is preserved (as for the Jost function method). It is also shown that the new formalism, although closely related to the $R$-matrix method, has the benefit of no dependence on an arbitrary channel radius. The efficiency and accuracy of the proposed method are tested using a model single-channel potential. Artificial ``experimental'' data generated with this potential are fitted, and its known resonances are successfully recovered as zeros of the Jost function on the appropriate sheet of the Riemann surface of the energy.

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arXiv2024

Wave function of $^9$Be in the three-body (alpha-alpha-n)-model

A simple analytic expression of the three-body wave function describing the system $(ααn)$ in the ground state $\frac{3}{2}^-$ of ${}^9\mathrm{Be}$ is obtained. In doing this, it is assumed that the $α$ particles interact with each other via the $S$-wave Ali-Bodmer potential including the Coulomb term, and the neutron-$α$ forces act only in the $P$-wave state. This wave function is constructed by trial and error method via solving in this way a kind of inverse problem when the two-body $αα$ potential is recovered from a postulated three-body wave function. As a result, the wave function is an exact solution of the corresponding three-body Schrödinger equation for experimentally known binding energy and for the $αα$ potential whose difference from the Ali-Bodmer one is minimized by varying the adjustable parameters which the postulated wave function depends on.

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arXiv2019

Resonance states 0+ of the Boron isotope B8 from the Jost-matrix analysis of experimental data

The available R-matrix parametrization of experimental data on the excitation functions for the elastic and inelastic p-Be7 scattering at the collision energies up to 3.4 MeV is used to generate the corresponding partial-wave cross sections in the states with J^pi=0+. Thus obtained data are considered as experimental partial cross sections and are fitted using the semi-analytic two-channel Jost matrix with proper analytic structure and some adjustable parameters. Then the spectral points are sought as zeros of the Jost matrix determinant (which correspond to the S-matrix poles) at complex energies. The correct analytic structure makes it possible to calculate the fitted Jost matrix on any sheet of the Riemann surface whose topology involves not only the square-root but also the logarithmic branching caused by the Coulomb interaction. In this way, two overlapping 0+ resonances at the excitation energies ~1.79 MeV and ~1.96 MeV have been found.

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arXiv2018

Jost-matrix analysis of the resonance 5He*(3/2+) near the dt-threshold

Experimental data on the n-alpha and dt collisions in the quantum state J^pi=3/2+ near the dt-threshold are fitted using the semi-analytic multi-channel Jost matrix with proper analytic structure and some adjustable parameters. Then the spectral points are sought as zeros of the Jost matrix determinant (which correspond to the S-matrix poles) at complex energies. The correct analytic structure makes it possible to calculate the fitted Jost matrix on any sheet of the Riemann surface whose topology involves not only the square-root but also the logarithmic branching caused by the Coulomb interaction. Within a distance of 100,keV above the dt-threshold, three 3/2+ resonances are found on the non-physical sheet of the Riemann surface. Several S-matrix (shadow) poles on the other sheets of this surface are located as well.

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arXiv2016

Nuclear fusion induced by X-rays in a crystal

The nuclei that constitute a crystalline lattice, oscillate relative to each other with a very low energy that is not sufficient to penetrate through the Coulomb barriers separating them. An additional energy, which is needed to tunnel through the barrier and fuse, can be supplied by external electromagnetic waves (X-rays or the synchrotron radiation). Exposing to the X-rays the solid compound LiD (lithium-deuteride) for the duration of 111 hours, we have detected 88 events of the nuclear fusion d+Li6 ---> Be8*. Our theoretical estimate agrees with what we observed. One of possible applications of the phenomenon we found, could be the measurements of the rates of various nuclear reactions (not necessarily fusion) at extremely low energies inaccessible in accelerator experiments.

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arXiv2016

Extracting the resonance parameters from experimental data on scattering of charged particles

A new parametrization of the multi-channel S-matrix is used to fit scattering data and then to locate the resonances as its poles. The S-matrix is written in terms of the corresponding "in" and "out" Jost matrices which are expanded in the Taylor series of the collision energy E around an appropriately chosen energy E0. In order to do this, the Jost matrices are written in a semi-analytic form where all the factors (involving the channel momenta and Sommerfeld parameters) responsible for their "bad behaviour" (i.e. responsible for the multi-valuedness of the Jost matrices and for branching of the Riemann surface of the energy) are given explicitly. The remaining unknown factors in the Jost matrices are analytic and single-valued functions of the variable E and are defined on a simple energy plane. The expansion is done for these analytic functions and the expansion coefficients are used as the fitting parameters. The method is tested on a two-channel model, using a set of artificially generated data points with typical error bars and a typical random noise in the positions of the points.

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arXiv2012

A method for extracting the resonance parameters from experimental cross section

The matrix elements of the multi-channel Jost matrices are written in such a way that their dependencies on all possible odd powers of channel momenta are factorized explicitly. As a result the branching of the Riemann energy surface at all the channel thresholds is represented in them via exact analytic expressions. The remaining single-valued functions of the energy are expanded in the Taylor series near an arbitrary point on the real axis. Using the thus obtained Jost matrices, the $S$-matrix is constructed and then the scattering cross section is calculated, which therefore depends on the Taylor expansion coefficients. These coefficients are considered as the adjustable parameters that are optimized to fit a given set of experimental data. After finding the coefficients, the resonances are located as zeros of the Jost matrix determinant at complex energies. Within this approach the $S$-matrix has proper analytic structure. This enables us not only to locate multi-channel resonances but also to reproduce their partial widths as well as the scattering cross section in the channels for which the data are not available.

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arXiv2011

Analytic structure and power-series expansion of the Jost function for the two-dimensional problem

For a two-dimensional quantum mechanical problem, we obtain a generalized power-series expansion of the S-matrix that can be done near an arbitrary point on the Riemann surface of the energy, similarly to the standard effective range expansion. In order to do this, we consider the Jost-function and analytically factorize its momentum dependence that causes the Jost function to be a multi-valued function. The remaining single-valued function of the energy is then expanded in the power-series near an arbitrary point in the complex energy plane. A systematic and accurate procedure has been developed for calculating the expansion coefficients. This makes it possible to obtain a semi-analytic expression for the Jost-function (and therefore for the S-matrix) near an arbitrary point on the Riemann surface and use it, for example, to locate the spectral points (bound and resonant states) as the S-matrix poles. The method is applied to a model simlar to those used in the theory of quantum dots.

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arXiv2011

Analyzing the contribution of individual resonance poles of the S-matrix to the two-channel scattering

A two-channel problem is considered within a method based on first order differential equations that are equivalent to the corresponding Schrödinger equation but are more convenient for dealing with resonant phenomena. Using these equations, it is possible to directly calculate the Jost matrix for practically any complex value of the energy. The spectral points (bound and resonant states) can therefore be located in a rigorous way, namely, as zeros of the Jost matrix determinant. When calculating the Jost matrix, the differential equations are solved and thus, at the same time, the wave function is obtained with the correct asymptotic behavior that is embedded in the solution analytically. The method offers very accurate way of calculating not only total widths of resonances but their partial widths as well. For each pole of the S-matrix, its residue can be calculated rather accurately, which makes it possible to obtain the Mittag-Leffler type expansion of the S-matrix as a sum of the singular terms (representing the resonances) and the background term (contour integral). As an example, the two-channel model by Noro and Taylor is considered. It is demonstrated how the contributions of individual resonance poles to the scattering cross section can be analyzed using the Mittag-Leffler expansion and the Argand plot technique. This example shows that even poles situated far away from the physical real axis may give significant contributions to the cross section.

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arXiv2011

Pade approximation of the S-matrix as a way of locating quantum resonances and bound states

It is shown that the spectral points (bound states and resonances) generated by a central potential of a single-channel problem, can be found using rational parametrization of the S-matrix. To achieve this, one only needs values of the S-matrix along the real positive energy axis. No calculations of the S-matrix at complex energies or a complex rotation are necessary. The proposed method is therefore universal in that it is applicable to any potential (local, non-local, discontinuous, etc.) provided that there is a way of obtaining the S-matrix (or scattering phase-shifts) at real collision energies. Besides this, combined with any method that extracts the phase-shifts from the scattering data, the proposed rational parametrization technique would be able to do the spectral analysis using the experimental data.

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arXiv2011

Multi-channel analog of the effective-range expansion

Similarly to the standard effective range expansion that is done near the threshold energy, we obtain a generalized power-series expansion of the multi-channel Jost-matrix that can be done near an arbitrary point on the Riemann surface of the energy within the domain of its analyticity. In order to do this, we analytically factorize its momentum dependencies at all the branching points on the Riemann surface. The remaining single-valued matrix functions of the energy are then expanded in the power-series near an arbitrary point in the domain of the complex energy plane where it is analytic. A systematic and accurate procedure has been developed for calculating the expansion coefficients. This means that near an arbitrary point in the domain of physically interesting complex energies it is possible to obtain a semi-analytic expression for the Jost-matrix (and therefore for the S-matrix) and use it, for example, to locate the spectral points (bound and resonant states) as the S-matrix poles.

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arXiv2007

Three-body resonances Lambda-n-n and Lambda-Lambda-n

Possible bound and resonant states of the hypernuclear systems $Λnn$ and $ΛΛn$ are sought as zeros of the corresponding three-body Jost functions calculated within the framework of the hyperspherical approach with local two-body S-wave potentials describing the $nn$, $Λn$, and $ΛΛ$ interactions. Very wide near-threshold resonances are found for both three-body systems. The positions of these resonances turned out to be sensitive to the choice of the $Λn$-potential. Bound $Λnn$ and $ΛΛn$ states only appear if the two-body potentials are multiplied by a factor of $\sim 1.5$.

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arXiv2002

Near-Threshold Photoproduction of eta-mesons on Three-Nucleon Nuclei

A microscopic few-body description of near-threshold coherent photoproduction of the eta meson on tritium and 3He targets is given. The photoproduction cross-section is calculated using the Finite Rank Approximation (FRA) of the nuclear Hamiltonian. The results indicate a strong final state interaction of the eta meson with the residual nucleus. Sensitivity of the results to the choice of the eta N T-matrix is investigated.

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arXiv2001

Coherent Photoproduction of eta-mesons on Three-Nucleon Systems

A microscopic few-body description of near-threshold coherent photoproduction of the eta-meson on tritium and He3 targets is given. The photoproduction cross-section is calculated using the Finite Rank Approximation (FRA) of the nuclear Hamiltonian. The results indicate a strong final state interaction of the eta-meson with the residual nucleus. Sensitivity of the results to the choice of the eta-N T-matrix is investigated. The importance of obeying the two-body unitarity condition in the eta-N system is demonstrated.

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arXiv2001

Microscopic description of $η$-photoproduction on light nuclei

A microscopic four-body description of near-threshold coherent photoproduction of the $η$ meson on the (3N)-nuclei is given. The photoproduction cross-section is calculated using the Finite Rank Approximation (FRA) of the nuclear Hamiltonian. The results indicate that the final state interaction of the $η$ meson with the residual nucleus plays an important role in the photoproduction process. Sensitivity of the results to the choice of the $ηN$ T-matrix is investigated. The importance of obeying the condition of $ηN$ unitarity is demonstrated.

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arXiv2001

Low energy scattering and photoproduction of $η$-mesons on deuterons

Photo-production of $η$-mesons and their collisions with light nuclei are studied within the Alt-Grassberger-Sandhas (AGS) formalism for different parameters of $ηN$ interaction. A three-body resonance or a quasi-bound state is found close to the $ηd$ threshold. Expected manifestation of this resonant behavior of $ηd$ elastic scattering in various processes involving $ηd$-system in their final states was found in calculations of the $γd \to ηd$ reaction in the framework of modified AGS equations. These calculations revealed peaks in the energy dependence of the total cross-section almost at the same energies as in the elastic $ηd$ scattering. Meanwhile, such peaks did not appear in the impulse approximation (i.e. when $ηd$ resonance was not taken into account). This sheds some light on the nature of the enhancement of $η$ photo-production on deuteron observed recently in the low energy experiments.

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arXiv2001

Photoproduction of $η$-mesons off light nuclei

Photoproduction of $η$-mesons off deuteron is studied within the Alt-Grassberger-Sandhas formalism for different parameters of $ηN$ interaction. The calculations revealed peaks in the energy dependence of the total cross-section.

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arXiv1999

Faddeev-type calculation of eta-d threshold scattering

The scattering length for the eta-meson collision with deuteron is calculated on the basis of rigorous few-body equations (AGS) for various eta-N input. The results obtained strongly support the existence of a resonance or quasi-bound state close to the eta-d threshold.

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