arXiv subjects

S. Bayegan

Publications and source records attributed to S. Bayegan.

At least 19 recordsLinked to original sources

arXiv2021

Halo EFT calculation of charge form factor for two-neutron ${}^{6}\textrm{He}$ halo nucleus: two-body resonant P-wave interaction

We take a new look at $^{6}\textrm{He}$ halo nucleus and set up a halo effective field theory at low energies to calculate the charge form factor of ${}^{6}\textrm{He}$ system with resonant P-wave interaction. P-wave Lagrangian has been introduced and the charge form factor of ${}^{6}\textrm{He}$ halo nucleus has been obtained at Leading-Order. In this study, the mean-square charge radius of ${}^{6}\textrm{He}$ nucleus relative to ${}^{4}\textrm{He}$ core and the root-mean-square (r.m.s) charge radius of ${}^{6}\textrm{He}$ nucleus have been estimated as $\langle r_{E}^{2}\rangle=1.408\; \textrm{fm}^{2}$ and $\langle r_{E}^{2}\rangle_{{}^{6}He}^{\frac{1}{2}}=2.058 \;\textrm{fm}$, respectively. We have compared our results with the other available theoretical and experimental data.

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arXiv2020

A Novel Regularization Scheme for Nucleon-Nucleon Lattice Simulations with Effective Field Theory

We propose a new regularization scheme to study the bound state of two-nucleon systems in Lattice Effective Field Theory. Inspired by continuum EFT calculation, we study an exponential regulator acting on the leading-order (LO) and next-to-leading order (NLO) interactions, consisting of local contact terms. By fitting the low-energy coefficients (LECs) to deuteron binding energy and the asymptotic normalization coefficient (ANC) on a lattice simulation, we extract the effective range expansion (ERE) parameters in the $^3S_1$ channel to order $p^2$. We explore the impact of different powers of the regulator on the extracted ERE parameters for the lattice spacing $a=1.97$ fm. Moreover, we investigate how the implementation of the regularization scheme improves the predicted ERE parameters on the lattice spacing in the range of $1.4 \le a \le 2.6$ fm. Our numerical analysis indicates that for lattice spacing greater than $2$ fm, the predicted observables are very close to the experimental data.

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arXiv2017

Comment on "Six-body bound system calculations in the case of effective $α-$core structure" [Eur. Phys. J. Plus (2016) 131: 240]

The authors argue that the calculated $^6$He binding energies by the solution of the coupled Yakubovsky integral equation in a partial wave decomposition reported by E. Ahmadi Pouya and A. A. Rajabi [Eur. Phys. J. Plus (2016) 131: 240] are incorrect. The formalism of the paper has serious mistakes and the numerical results are not reproducible and cannot be validated.

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arXiv2017

Comment on "Three-dimensional study of the six-body bound-state for the case of effective three-body configuration model" [Int. J. Mod. Phys. E 25, 9 (2016) 1650072]

The authors argue that the calculated $^6$He binding energies by the solution of the coupled Faddeev-Yakubovsky integral equation in a Three-dimensional scheme reported by E. Ahmadi Pouya and A. A. Rajabi [Int. J. Mod. Phys. E 25, 9 (2016) 1650072] are incorrect. The formalism of the paper has serious mistakes and the numerical results are quite misleading because such a calculation even with small grids for Jacobi momenta and the angle variables leads to a huge memory of 37.8 PB (petabyte) which cannot even be achieved on present supercomputers.

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arXiv2014

New insight into $nd\rightarrow$ $^3Hγ$ process at thermal energy with pionless effective field theory

We take a new look at the neutron radiative capture by a deuteron at thermal energy with the pionless effective field theory (EFT($π\!\!\!/$)) approach. We present in detail the calculation of $nd\rightarrow$ $^3Hγ$ amplitudes for incoming doublet and quartet channels leading to the formation of a triton fully in the projection method based on the cluster-configuration space approach. In the present work, we consider all possible one-body and two-body photon interaction diagrams. In fact, additional diagrams that make significant changes in the results of the calculation of the total cross section in the $nd\rightarrow$ $^3Hγ$ process are included in this study. The properly normalized triton wave function is calculated and taken into consideration. We compare the cross section of the dominant magnetic M1-transition of $nd\rightarrow$ $^3Hγ$ up to next-to-next-to-leading order $\textrm{N}^2\textrm{LO}$ with the results of the previous model-dependent theoretical calculations and experimental data. The more acceptable results for cross section $σ^{(2)}_{tot}=0.297\;(\textrm{LO})+0.124\;(\textrm{NLO})+0.048\;(\textrm{N}^2\textrm{LO})=[0.469\pm0.033]\:\textrm{mb}$ show order by order convergence and cutoff independence. No three-body currents are needed to renormalize observables up to $\textrm{N}^2\textrm{LO}$ in this process.

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arXiv2014

Photon asymmetries in $nd\rightarrow$ $^3Hγ$ using EFT($π/!/!/!/$) approach

The parity-violating Lagrangian of the weak nucleon-nucleon ($NN$) interaction in the pionless effective field theory (EFT($//!/!/!/pi$)) approach contains five independent unknown low-energy coupling constants (LECs). The photon asymmetry with respect to neutron polarization in $np\rightarrow$ $dγ$ $A^{np}_γ$, the circular polarization of outgoing photon in $np\rightarrow$ $dγ$ $P^{np}_γ$, the neutron spin rotation in hydrogen $\frac{1}ρ\frac{dϕ^{np}}{dl}$, the neutron spin rotation in deuterium $\frac{1}ρ\frac{dϕ^{nd}}{dl}$ and the circular polarization of $γ$-emission in $nd\rightarrow$ $^3Hγ$ $P^{nd}_γ$ are the parity-violating observables which have been recently calculated in terms of parity-violating LECs in the EFT($//!/!/!/pi$) framework. We obtain the LECs by matching the parity-violating observables to the Desplanques, Donoghue, and Holstein (DDH) best value estimates. Then, we evaluate photon asymmetry with respect to the neutron polarization $a^{nd}_γ$ and the photon asymmetry in relation to deuteron polarization $A^{nd}_γ$ in $nd\rightarrow$ $^3Hγ$ process. We finally compare our EFT($//!/!/!/pi$) photon asymmetries results with the experimental values and the previous calculations based on the DDH model.

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arXiv2013

Parity-violating photon circular polarization in $nd\rightarrow$ $^3Hγ$ with effective field theory at thermal energy

We study the parity-violating effects in $nd\rightarrow$ $^3Hγ$ process using pionless effective field theory (EFT($//!/!/!/pi$)) at thermal energy. For the weak $NN$ interaction the parity-violating Lagrangian contains five independent low-energy coupling constants (LECs). One can fix the coupling constants by comparison of the calculated observables with a sufficient number of experimental data. So, we need five observables to obtain LECs. One of the five parity-violating observables is the photon circular polarization ($P_γ$) in $nd\rightarrow$ $^3Hγ$ process. We calculate $P_γ$ at the leading order in terms of LECs. We compare our results with the previous $P_γ$ calculation based on the model of Desplanques, Donoghue, and Holstein.

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arXiv2012

Triton photodisintegration in three-dimensional approach

Two- and three- particles photodisintegration of the triton is investigated in a three-dimensional (3D) Faddeev approach. For this purpose the Jacobi momentum vectors for three particles system and spin-isospin quantum numbers of the individual nucleons are considered. Based on this picture the three-nucleon Faddeev integral equations with the two-nucleon interaction are formulated without employing the partial wave decomposition. The single nucleon current as well as $π-$ and $ρ-$ like exchange currents are used in an appropriate form to be employed in 3D approach. The exchange currents are derived from AV18 NN force. The two-body t-matrix, Deuteron and Triton wave functions are calculated in the 3D approach by using AV18 potential. Benchmarks are presented to compare the total cross section for the two- and three- particles photodisintegration in the range of $E_γ<30 MeV$. The 3D Faddeev approach shows promising results.

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arXiv2011

A New Expression for 3N Bound State Faddeev Equation in a 3D Approach

A spin-isospin dependent three-dimensional approach has been applied for formulation of the three-nucleon bound state and a new expression for Faddeev equation based on three-nucleon free basis state has been obtained. Then the three-nucleon wave function has been obtained as a function of five independent variables.

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arXiv2011

Solutions of the bound state Faddeev-Yakubovsky equations in three dimensions by using NN and 3N potential models

A recently developed three-dimensional approach (without partial-wave decomposition) is considered to investigate solutions of Faddeev-Yakubovsky integral equations in momentum space for three- and four-body bound states, with the inclusion of three-body forces. In the calculations of the binding energies, spin-dependent nucleon-nucleon (NN) potential models (named, S$_{3}$, MT-I/III, YS-type and P$_{5.5}$GL) are considered along with the scalar two-meson exchange three-body potential. Good agreement of the presently reported results with the ones obtained by other techniques are obtained, demonstrating the advantage of an approach in which the formalism is much more simplified and easy to manage for direct computation.

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arXiv2010

A Spin-Isospin Dependent 3N Scattering Formalism in a 3D Faddeev Scheme

We have introduced a spin-isospin dependent three-dimensional approach for formulation of the three-nucleon scattering. Faddeev equation is expressed in terms of vector Jacobi momenta and spin-isospin quantum numbers of each nucleon. Our formalism is based on connecting the transition amplitude $T$ to momentum-helicity representations of the two-body $t$-matrix and the deuteron wave function. Finally the expressions for nucleon-deuteron elastic scattering and full breakup process amplitudes are presented.

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arXiv2010

Triton Photodisintegration with Effective Field Theory

Effective field theory (EFT) has been recently used for the calculation of neutron-deuteron radiative capture at very low energies.We present here the use of EFT to calculate the two-body photodisintegration of the triton, considering the three-body force. The calculated cross section shows sharp rising from threshold to maximum about 0.88 mb at 13 MeV and decreasing slightly to about 0.81 mb at 19 MeV, in agreement with the experimental data. Our results are in good agreement with the experimental data and the other calculations using modern realistic two- and three-nucleon forces, like AV18/UrbanaIX potential.

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arXiv2010

3D calculation of Tucson-Melbourne 3NF effect in triton binding energy

As an application of the new realistic three-dimensional (3D) formalism reported recently for three-nucleon (3N) bound states, an attempt is made to study the effect of three-nucleon forces (3NFs) in triton binding energy in a non partial wave (PW) approach. The spin-isospin dependent 3N Faddeev integral equations with the inclusion of 3NFs, which are formulated as function of vector Jacobi momenta, specifically the magnitudes of the momenta and the angle between them, are solved with Bonn-B and Tucson-Melbourne NN and 3N forces in operator forms which can be incorporated in our 3D formalism. The comparison with numerical results in both, novel 3D and standard PW schemes, shows that non PW calculations avoid the very involved angular momentum algebra occurring for the permutations and transformations and it is more efficient and less cumbersome for considering the 3NF.

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arXiv2010

Photodisintegration of $^3H$ in a three dimensional Faddeev approach

An interaction of a photon with $^3H$ is invstigated based on a three dimensional Faddeev approach. In this approach the three-nucleon Faddeev equations with two-nucleon interactions are formulated with consideration of the magnitude of the vector Jacobi momenta and the angle between them with the inclusion of the spin-isospin quantum numbers, without employing a partial wave decomposition. In this formulation the two body t-matrices and triton wave function are calculated in the three dimensional approach using AV18 potential. In the first step we use the standard single nucleon current in this article.

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arXiv2009

Three-dimensional approach for construction of low-momentum effective interaction from realistic potentials

The low-momentum effective interaction $V_{low k}$ has been formulated in the three-dimensional momentum-helicity representation as a function of the magnitude of momentum vectors and the angle between them. As an application, AV18 potential has been used in the model space of Lee-Suzuki method and it has been shown that the low-momentum effective interaction, $V_{low k}$ reproduces the same two-body observables obtained by the bare potential $V_{NN}$.

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arXiv2009

The three dimensional calculations of NN bound and scattering states with chiral potential up to N3LO

The recently developed chiral nucleon-nucleon ($NN$) potential by Epelbaum \emph{et al.} has been employed to study the two-nucleon bound and scattering states. Chiral $NN$ potential up to next-to-next-to-next-to leading order (N$^3$LO) is used to calculated the np differential cross section and deuteron binding energy in a realistic three dimensional approach. The obtained results based on this helicity representation are compared to the standard partial wave (PW) results. This comparison shows that the 3D approach provides the same accuracy in the description of $NN$ observables and the results are in close agreement with available experimental data.

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arXiv2008

A Realistic Formalism for 4N Bound State in a Three-Dimensional Yakubovsky Scheme

A spin-isospin dependent Three-Dimensional formalism based on the momentum vectors for the four-nucleon bound state is presented. The four-nucleon Yakubovsky equations with two- and three-nucleon interactions are formulated as a function of the vector Jacobi momenta. Our formalism, according to the number of spin-isospin states that one takes into account, leads to only a strictly finite number of the coupled three-dimensional integral equations to be solved. The evaluation of the transition and permutation operators as well as the coordinate transformations due to considering the continuous angle variables instead of the discrete angular momentum quantum numbers are less complicated in comparison with partial wave representation. With respect to partial wave the present formalism with the smaller number of equations leads to higher dimensionality of the integral equations. We have concluded that Three-Dimensional formalism is less cumbersome for considering the three-nucleon forces.

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arXiv2008

Towards a three dimensional solution for 3N bound states with 3NFs

After a brief discussion about the necessity of using the 3D approach, we present the non PW formalism for 3N bound state with the inclusion of 3N force (3NF). As an example the evaluation of 3NF matrix elements, which appear in the obtained coupled three dimensional integral equations, for $2π$-exchange Tucson-Melbourne 3NF show how would be this formalism efficient and less cumbersome in comparison with the PW formalism.

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