arXiv subjects

Winfried Leidemann

Publications and source records attributed to Winfried Leidemann.

At least 19 recordsLinked to original sources

arXiv2025

The ${}^9$Be photodisintegration cross section within Cluster Effective Field Theory

A low-energy calculation of ${}^9$Be photodisintegration cross section is presented within an $\alpha\alpha n$ cluster approach. The $\alpha n$ and $\alpha\alpha$ contact interactions are derived from cluster effective field theory. The two-body potentials defined in momentum space are regularized by a Gaussian cutoff. The associated low-energy constants are found by comparing the calculated low-energy T-matrix with its effective range expansion. A three-body state-dependent potential is also introduced in the model. First, the ${}^9$Be three-body binding energy is studied within the non-symmetrized hyperspherical harmonics method. Then, the low-energy cross section is calculated via the Lorentz integral transform method, focussing on the dominant electric dipole transitions. A twofold evaluation of the nuclear current matrix element is presented, employing both the electric dipole transition operator (Siegert theorem) and the one-body convection current operator. This approach is adopted to allow for a discussion of the effect of the many-body currents.

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arXiv2019

On calculating response functions via their Lorentz integral transforms

The accuracy of reconstruction of a response function from its Lorentz integral transform is studied in an exactly solvable model. An inversion procedure is elaborated in detail and features of the procedure are studied. Unlike results in the literature pertaining to the same model, the response function is reconstructed from its Lorentz integral transform with rather high accuracy.

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arXiv2018

Relativistic effects in ab-initio electron-nucleus scattering

The electromagnetic responses obtained from Green's function Monte Carlo (GFMC) calculations are based on realistic treatments of nuclear interactions and currents. The main limitations of this method comes from its nonrelativistic nature and its computational cost, the latter hampering the direct evaluation of the inclusive cross sections as measured by experiments. We extend the applicability of GFMC in the quasielastic region to intermediate momentum transfers by performing the calculations in a reference frame that minimizes nucleon momenta. Additional relativistic effects in the kinematics are accounted for employing the two-fragment model. In addition, we developed a novel algorithm, based on the concept of first-kind scaling, to compute the inclusive electromagnetic cross section of $^4$He through an accurate and reliable interpolation of the response functions. A very good agreement is obtained between theoretical and experimental cross sections for a variety of kinematical setups. This offers a promising prospect for the data analysis of neutrino-oscillation experiments that requires an accurate description of nuclear dynamics in which relativistic effects are fully accounted for.

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arXiv2017

Ab initio calculations for non-strange and strange few-baryon systems

Concerning the non-strange particle systems the low-energy excitation spectra of the three- and four-body helium isotopes are studied. Objects of the study are the astrophysical $S$-factor $S_{12}$ of the radiative proton deuteron capture $d(p,γ)^3$He and the width of the $^4$He isoscalar monopole resonance.Both observables are calculated using the Lorentz integral transform (LIT) method. The LIT equations are solved via expansions of the LIT states on a specifically modified hyperspherical harmonics (HH) basis. It is illustrated that at low energies such a modification allows to work with much higher LIT resolutions than with an unmodified HH basis. It is discussed that this opens up the possibility to determine astrophysical $S$-factors as well as the width of low-lying resonances with the LIT method. In the sector of strange baryon systems binding energies of the hypernucleus $^3_Λ$H are calculated using a nonsymmetrized HH basis. The results are compared with those calculated by various other groups with different methods. For all the considered non-strange and strange baryon systems it is shown that high-precision results are obtained.

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arXiv2016

Determination of S-factors with the LIT method

The precise determination of astrophysical S-factors is essential for a detailed understanding of the nucleosynthesis in its various facets. It is discussed how the Lorentz integral transform (LIT) method can be applied for such a determination. The astrophysical S-factor for the proton-deuteron radiative capture is considered as test case. The importance of a specific many-body basis used for the LIT equation solution is pointed out. The excellent results of the test are discussed.

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arXiv2016

Calculation of the astrophysical S-factor S_12 with the Lorentz integral transform

The LIT approach is tested for the calculation of astrophysical S-factors. As an example the S-factor of the reaction 2H(p,gamma)3He is considered. It is discussed that a sufficiently high density of LIT states at low energies is necessary for a precise determination of S-factors. In particular it is shown that the hyperspherical basis is not very well suited for such a calculation and that a different basis system is much more advantageous. A comparison of LIT results with calculations, where continuum wave functions are explicitly used, shows that the LIT approach leads to reliable results. It is also shown how an error estimate of the LIT inversion can be obtained.

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arXiv2015

Energy Resolution with the Lorentz integral transform

A brief outline of the Lorentz Integral Transform (LIT) method is given. The method is well established and allows to treat reactions into the many-body continuum with bound-state like techniques. The energy resolution that can be achieved is studied by means of a simple two-body reaction. From the discussion it will become clear that the LIT method is an approach with a controlled resolution and that there is no principle problem to even resolve narrow resonances in the many-body continuum. As an example the isoscalar monopole resonance of 4He is considered. The importance of the choice of a proper basis for the expansion of the LIT states is pointed out. Employing such a basis a width of 180(70) keV is found for the 4He isoscalar monopole resonance when using a simple central nucleon-nucleon potential model.

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arXiv2014

Is the first excited state of the $α$-particle a breathing mode?

The isoscalar monopole excitation of 4He is studied within a few-body ab initio approach. We consider the transition density to the low-lying and narrow 0+ resonance, as well as various sum rules and the strength energy distribution itself at different momentum transfers q. Realistic nuclear forces of chiral and phenomenological nature are employed. Various indications for a collective breathing mode are found: i) the specific shape of the transition density, ii) the high degree of exhaustion of the non-energy-weighted sum rule at low q and iii) the complete dominance of the resonance peak in the excitation spectrum. For the incompressibility K of the alpha-particle values between 20 and 30 MeV are found.

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arXiv2013

Theoretical study of the ${}^{4}\rm{He}(γ,p)^3\rm{H}$ and ${}^{4}\rm{He}(γ,n)^3\rm{He}$ reactions

{\it Ab initio} calculation of the total cross section for the reactions $^{4}\rm{He}(γ,p)^3\rm{H}$ and $^{4}\rm{He}(γ,n)^3\rm{He}$ is presented, using state-of-the-art nuclear forces. The Lorentz integral transform (LIT) method is applied, which allows exact treatment of the final state interaction (FSI). The dynamic equations are solved using the effective interaction hyperspherical harmonics (EIHH) method. In this calculation of the cross sections the three-nucleon force is fully taken into account, except in the source term of the LIT equation for the FSI transition matrix element.

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arXiv2012

Nonsymmetrized hyperspherical harmonics with realistic NN potentials

The Schroedinger equation is solved for an A-nucleon system using an expansion of the wave function in nonsymmetrized hyperspherical harmonics. Our approach is both an extension and a modification of the formalism developed by Gattobigio et al.. The extension consists in the inclusion of spin and isospin degrees of freedom such that a calculation with more realistic NN potential models becomes possible, whereas the modification allows a much simpler determination of the fermionic ground state. The approach is applied to four- and six-body nuclei (4He, 6Li) with various NN potential models. It is shown that the results for ground-state energy and radius agree well with those from the literature.

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arXiv2012

Modern Ab Initio Approaches and Applications in Few-Nucleon Physics with A \ge 4

We present an overview of the evolution of ab initio methods for few-nucleon systems with A \ge 4, tracing the progress made that today allows precision calculations for these systems. First a succinct description of the diverse approaches is given. In order to identify analogies and differences the methods are grouped according to different formulations of the quantum mechanical many-body problem. Various significant applications from the past and present are described. We discuss the results with emphasis on the developments following the original implementations of the approaches. In particular we highlight benchmark results which represent important milestones towards setting an ever growing standard for theoretical calculations. This is relevant for meaningful comparisons with experimental data. Such comparisons may reveal whether a specific force model is appropriate for the description of nuclear dynamics.

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arXiv2011

Transverse electron scattering response function of 3He in the quasi-elastic peak region and beyond with Delta isobar degrees of freedom

The 3He transverse electron scattering response function R_T(q,omega) is calculated in the quasi-elastic peak region and beyond for momentum transfers q = 500, 600 and 700 MeV/c. Distinct from our previous work for these kinematics where we included meson exchange currents and relativistic corrections we now additionally include Delta isobar currents (Delta-IC). The Delta-IC contribution increases the quasi-elastic peak height by about 5% and leads to an excellent agreement with experimental data in the whole peak region. In addition it is shown that effects due to the three-nucleon force largely cancel those due to the Delta-IC in the peak region. Finally, we have found that Delta-IC are important for three-body break-up reactions in the so-called dip region. This could explain why in a previous study of such a reaction, where Delta degrees of freedom were not included, no agreement between experimental and theoretical results could be obtained.

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arXiv2011

Photon Scattering with the Lorentz Integral Transform Method

The application of the Lorentz integral transform (LIT) method to photon scattering off nuclei is presented in general. As an example, elastic photon scattering off the deuteron in the unretarded dipole approximation is considered using the LIT method. The inversion of the integral transform is discussed in detail paying particular attention to the high-energy contributions in the resonance term. The obtained E1-polarizabilities are compared to results from the literature. The corresponding theoretical cross section is confronted with experimental results confirming, as already known from previous studies, that the E1-contribution is the most important one at lower energies.

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arXiv2010

Frame dependence of 3He transverse (e,e') response functions at intermediate momentum transfers

The transverse electron scattering response function of 3He was recently studied by us in the quasi-elastic peak region for momentum transfers q between 500 and 700 MeV/c. Those results, obtained using the Active Nucleon Breit frame (ANB), are here supplemented by calculations in the laboratory, Breit and ANB frames using the two-fragment model discussed in our earlier work on the frame dependence of the the longitudinal response function R_L(q,omega). We find relatively frame independent results and good agreement with experiment especially for the lower momentum transfers. This agreement occurs when we neglect an omega-dependent piece of the one-body current relativistic correction. An inclusion of this term leads however to a rather pronounced frame dependence at q=700 MeV/c. A discussion of this term is given here. This report also includes a correction to our previous ANB results for R_T(q,omega).

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arXiv2010

Transverse electron scattering response function of $^3$He with $Δ$-isobar degrees of freedom

A calculation of the $^3$He transverse $(e,e')$ inclusive response function, $R_T$, which includes $Δ$ degrees of freedom is performed using the Lorentz integral transform method. The resulting coupled equations are treated in impulse approximation, where the $NNN$ and $NNΔ$ channels are solved separately. As NN and NNN potentials we use the Argonne V18 and UrbanaIX models respectively. Electromagnetic currents include the $Δ$-isobar currents, one-body N-currents with relativistic corrections and two-body currents consistent with the Argonne V18 potential. $R_T$ is calculated for the breakup threshold region at momentum transfers near 900 MeV/c. Our results are similar to those of Deltuva {\it et al.} in that large $Δ$-isobar current contributions are found. However we find that these are largely canceled by the relativistic contribution from the one-body N-currents. Finally a comparison is made between theoretical results and experimental data.

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arXiv2009

Improved transverse (e,e') response function of 3He at intermediate momentum transfers

The transverse electron scattering response function of 3He is studied in the quasi-elastic peak region for momentum transfers between 500 and 700 MeV/c. A conventional description of the process leads to results at a substantial variation with experiment. To improve the results, the present calculation is done in a reference frame (the ANB or Active Nucleon Breit frame) which diminishes the influence of relativistic effects on nuclear states. The laboratory frame response function is then obtained via a kinematics transformation. In addition, a one-body nuclear current operator is employed that includes all leading order relativistic corrections. Multipoles of this operator are listed. It is shown that the use of the ANB frame leads to a sizable shift of the quasi-elastic peak to lower energy and, contrary to the relativistic current, also to an increase of the peak height. The additionally considered meson exchange current contribution is quite small in the peak region. In comparison with experiment one finds an excellent agreement of the peak positions. The peak height agrees well with experiment for the lowest considered momentum transfer (500 MeV/c), but tends to be too high for higher momentum transfer (10% at 700 MeV/c).

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arXiv2009

Search for three-nucleon force effects on the longitudinal response function of 4He

A detailed study of the 4He longitudinal response function R_L(ω,q) is performed at different kinematics, with particular emphasis on the role of three-nucleon forces. The effects shown are the results of an ab initio calculation where the full four-body continuum dynamics is considered via the Lorentz integral transform method. The contributions of the various multipoles to the longitudinal response function are analyzed and integral properties of the response are discussed in addition. The Argonne V18 nucleon-nucleon interaction and two different three-nucleon force models (Urbana IX, Tucson-Melbourne') are used. At lower momentum transfer (q<= 200$ MeV/c) three-nucleon forces play an important role. One even finds a dependence of R_L on the three-nucleon force model itself with differences up to 10%. Thus a Rosenbluth separation of the inclusive electron scattering cross section of 4He at low momentum transfer would be of high value in view of a discrimination between different three-nucleon force models.

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arXiv2009

Threshold 3He and 3H Transverse Electron Scattering Response Functions

The threshold transverse response functions R_T(q,omega) for 3He and 3H are calculated using the AV18 nucleon-nucleon potential, the UrbanaIX three-body force, and the Coulomb potential. Final states are completely taken into account via the Lorentz integral transform technique. Consistent two-body pi- and rho-meson exchange currents as deduced using the Arenhövel-Schwamb technique are included. The convergence of the method is shown and a comparison of the corresponding MEC contribution is made to that of a consistent MEC for the meson theoretical r-space BonnA potential. The response R_T is calculated in the threshold region at q=174, 324, and 487 MeV/c and compared with available data. The strong MEC contributions in the threshold region are nicely confirmed by the data at q=324 and 487 MeV/c although some differences between theoretical and experimental results remain. A comparison is also made with other calculations, where the same theoretical input is used. The agreement is generally rather good, but leaves also some space for further improvement.

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