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N. Barnea

Publications and source records attributed to N. Barnea.

At least 37 records · Page 2Linked to original sources
arXiv2018

Zemach moments and radii of 2,3H and 3,4He

We present benchmark calculations of Zemach moments and radii of 2,3H and 3,4He using various few-body methods. Zemach moments are required to interpret muonic atom data measured by the CREMA collaboration at the Paul Scherrer Institute. Conversely, radii extracted from spectroscopic measurements can be compared with ab initio computations, posing stringent constraints on the nuclear model. For a given few-body method, different numerical procedures can be applied to compute these quantities. A detailed analysis of the numerical uncertainties entering the total theoretical error is presented. Uncertainties from the few-body method and the calculational procedure are found to be smaller than the dependencies on the dynamical modeling and the single nucleon inputs, which are found to be <= 2%. When relativistic corrections and two-body currents are accounted for, the calculated moments and radii are in very good agreement with the available experimental data.

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arXiv2018

The nuclear contacts and short range correlations in nuclei

Atomic nuclei are complex strongly interacting systems and their exact theoretical description is a long-standing challenge. An approximate description of nuclei can be achieved by separating its short and long range structure. This separation of scales stands at the heart of the nuclear shell model and effective field theories that describe the long-range structure of the nucleus using a mean- field approximation. We present here an effective description of the complementary short-range structure using contact terms and stylized two-body asymptotic wave functions. The possibility to extract the nuclear contacts from experimental data is presented. Regions in the two-body momentum distribution dominated by high-momentum, close-proximity, nucleon pairs are identified and compared to experimental data. The amount of short-range correlated (SRC) nucleon pairs is determined and compared to measurements. Non-combinatorial isospin symmetry for SRC pairs is identified. The obtained one-body momentum distributions indicate dominance of SRC pairs above the nuclear Fermi-momentum.

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arXiv2017

Eta-mesic nuclei

In this contribution we report on theoretical studies of $η$ nuclear quasi-bound states in few- and many-body systems performed recently by the Jerusalem-Prague Collaboration [1-5]. Underlying energy-dependent $ηN$ interactions are derived from coupled-channel models that incorporate the $N^*(1535)$ resonance. The role of self-consistent treatment of the strong energy dependence of subthreshold $ηN$ amplitudes is discussed. Quite large downward energy shift together with rapid decrease of the $ηN$ amplitudes below threshold result in relatively small binding energies and widths of the calculated $η$ nuclear bound states. We argue that the subthreshold behavior of $ηN$ scattering amplitudes is crucial to conclude whether $η$ nuclear states exist, in which nuclei the $η$ meson could be bound and if the corresponding widths are small enough to allow detection of these $η$ nuclear states in experiment.

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arXiv2017

Onset of $η$-nuclear binding in a pionless EFT approach

$ηNNN$ and $ηNNNN$ bound states are explored in stochastic variational method (SVM) calculations within a pionless effective field theory (EFT) approach at leading order. The theoretical input consists of regulated $NN$ and $NNN$ contact terms, and a regulated energy dependent $ηN$ contact term derived from coupled-channel models of the $N^{\ast}(1535)$ nucleon resonance plus a regulated $ηNN$ contact term. A self consistency procedure is applied to deal with the energy dependence of the $ηN$ subthreshold input, resulting in a weak dependence of the calculated $η$-nuclear binding energies on the EFT regulator. It is found, in terms of the $ηN$ scattering length $a_{ηN}$, that the onset of binding $η\,^3$He requires a minimal value of Re$\,a_{ηN}$ close to 1 fm, yielding then a few MeV $η$ binding in $η\,^4$He. The onset of binding $η\,^4$He requires a lower value of Re$\,a_{ηN}$, but exceeding 0.7 fm.

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arXiv2017

Onset of $η$-meson binding in the He isotopes

The onset of binding $η$(548) mesons in nuclei is studied in the He isotopes by doing precise $ηNNN$ and $ηNNNN$ few-body stochastic variational method calculations for two semi-realistic $NN$ potentials and two energy dependent $ηN$ potentials derived from coupled-channel models of the $N^{\ast}(1535)$ nucleon resonance. The energy dependence of the $ηN$ subthreshold input is treated self consistently. It is found that a minimal value of the real part of the $ηN$ scattering length $a_{ηN}$ close to 1 fm is required to bind $η$ mesons in $^3$He, yielding then a few MeV $η$ binding in $^4$He. The onset of $η$-meson binding in $^4$He requires that Re$\,a_{ηN}$ exceeds 0.7 fm approximately. These results compare well with results of recent $ηNNN$ and $ηNNNN$ pionless effective field theory calculations. Related optical-model calculations are also discussed.

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arXiv2017

Benchmark Results for Few-Body Hypernuclei

The Non-Symmetrized Hyperspherical Harmonics method (NSHH) is introduced in the hypernuclear sector and benchmarked with three different ab-initio methods, namely the Auxiliary Field Diffusion Monte Carlo method, the Faddeev-Yakubovsky approach and the Gaussian Expansion Method. Binding energies and hyperon separation energies of three- to five-body hypernuclei are calculated by employing the two-body $Λ$N component of the phenomenological Bodmer-Usmani potential, and a hyperon-nucleon interaction simulating the scattering phase shifts given by NSC97f. The range of applicability of the NSHH method is briefly discussed.

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arXiv2016

Electric dipole polarizability from first principles calculations

The electric dipole polarizability quantifies the low-energy behaviour of the dipole strength and is related to critical observables such as the radii of the proton and neutron distributions. Its computation is challenging because most of the dipole strength lies in the scattering continuum. In this paper we combine integral transforms with the coupled-cluster method and compute the dipole polarizability using bound-state techniques. Employing different interactions from chiral effective field theory, we confirm the strong correlation between the dipole polarizability and the charge radius, and study its dependence on three-nucleon forces. We find good agreement with data for the 4He, 40Ca, and 16O nuclei, and predict the dipole polarizability for the rare nucleus 22O.

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arXiv2015

Nuclear structure corrections to the Lamb shift in $μ^3$He$^+$ and $μ^3$H

Measuring the 2S-2P Lamb shift in a hydrogen-like muonic atom allows one to extract its nuclear charge radius with a high precision that is limited by the uncertainty in the nuclear structure corrections. The charge radius of the proton thus extracted was found to be 7-sigma away from the CODATA value, in what has become the yet unsolved "proton radius puzzle". Further experiments currently aim at the isotopes of hydrogen and helium: the precise extraction of their radii may provide a hint at the solution of the puzzle. We present the first ab initio calculation of nuclear structure corrections, including the nuclear polarization correction, to the 2S-2P transition in $μ^3$He$^+$ and $μ^3$H, and assess solid theoretical error bars. Our predictions reduce the uncertainty in the nuclear structure corrections to the level of a few percents and will be instrumental to the on-going $μ^3$He$^+$ experiment. We also support the mirror $μ\,^3$H system as a candidate for further probing of the nucleon polarizabilities and shedding more light on the puzzle.

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arXiv2015

Charge, neutron, and weak size of the atomic nucleus

What is the size of the atomic nucleus? This deceivably simple question is difficult to answer. While the electric charge distributions in atomic nuclei were measured accurately already half a century ago, our knowledge of the distribution of neutrons is still deficient. In addition to constraining the size of atomic nuclei, the neutron distribution also impacts the number of nuclei that can exist and the size of neutron stars. We present an ab initio calculation of the neutron distribution of the neutron-rich nucleus $^{48}$Ca. We show that the neutron skin (difference between radii of neutron and proton distributions) is significantly smaller than previously thought. We also make predictions for the electric dipole polarizability and the weak form factor; both quantities are currently targeted by precision measurements. Based on ab initio results for $^{48}$Ca, we provide a constraint on the size of a neutron star.

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arXiv2015

Few-body calculations of $η$-nuclear quasibound states

We report on precise hyperspherical-basis calculations of $ηNN$ and $ηNNN$ quasibound states, using energy dependent $ηN$ interaction potentials derived from coupled-channel models of the $S_{11}$ $N^{\ast}(1535)$ nucleon resonance. The $ηN$ attraction generated in these models is too weak to generate a two-body bound state. No $ηNN$ bound-state solution was found in our calculations in models where Re $a_{ηN}\lesssim 1$ fm, with $a_{ηN}$ the $ηN$ scattering length, covering thereby the majority of $N^{\ast}(1535)$ resonance models. A near-threshold $ηNNN$ bound-state solution, with $η$ separation energy of less than 1 MeV and width of about 15 MeV, was obtained in the 2005 Green-Wycech model where Re $a_{ηN}\approx 1$ fm. The role of handling self consistently the subthreshold $ηN$ interaction is carefully studied.

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arXiv2015

Giant and pigmy dipole resonances in 4He, 16,22O, and 40Ca from chiral nucleon-nucleon interactions

We combine the coupled-cluster method and the Lorentz integral transform for the computation of inelastic reactions into the continuum. We show that the bound-state-like equation characterizing the Lorentz integral transform method can be reformulated based on extensions of the coupled-cluster equation-of-motion method, and we discuss strategies for viable numerical solutions. Starting from a chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order, we compute the giant dipole resonances of 4He, 16,22O and 40Ca, truncating the coupled-cluster equation-of-motion method at the two-particle-two-hole excitation level. Within this scheme, we find a low-lying E1 strength in the neutron-rich 22O nucleus, which compares fairly well with data from [Leistenschneider et al. Phys. Rev. Lett. 86, 5442 (2001)]. We also compute the electric dipole polariziability in 40Ca. Deficiencies of the employed Hamiltonian lead to overbinding, too small charge radii and a too small electric dipole polarizability in 40Ca.

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arXiv2013

Effective Field Theory for Lattice Nuclei

We show how nuclear effective field theory (EFT) and ab initio nuclear-structure methods can turn input from lattice quantum chromodynamics (LQCD) into predictions for the properties of nuclei. We argue that pionless EFT is the appropriate theory to describe the light nuclei obtained in recent LQCD simulations carried out at pion masses much heavier than the physical pion mass. We solve the EFT using the effective-interaction hyperspherical harmonics and auxiliary-field diffusion Monte Carlo methods. Fitting the three leading-order EFT parameters to the deuteron, dineutron and triton LQCD energies at $m_π\approx 800$ MeV, we reproduce the corresponding alpha-particle binding and predict the binding energies of mass-5 and 6 ground states.

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arXiv2013

The isoscalar monopole resonance of the alpha particle: a prism to nuclear Hamiltonians

We present an ab-initio study of the isoscalar monopole excitations of 4He using different realistic nuclear interactions, including modern effective field theory potentials. In particular we concentrate on the transition form factor $F_{\cal M}$ to the narrow $0^+$ resonance close to threshold. F_M exhibits a strong potential model dependence, and can serve as a kind of prism to distinguish among different nuclear force models. Comparing to the measurements obtained from inelastic electron scattering off 4He, one finds that the state-of-the-art theoretical transition form factors are at variance with experimental data, especially in the case of effective field theory potentials. We discuss some possible reasons for such discrepancy, which still remains a puzzle.

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arXiv2012

Matter and charge radius of 6He in the hyperspherical-harmonics approach

We present ab-initio calculations of the binding energy and radii of the two-neutron halo nucleus 6He using two-body low-momentum interactions based on chiral effective field theory potentials. Calculations are performed via a hyperspherical harmonics expansion where the convergence is sped up introducing an effective interaction for non-local potentials. The latter is essential to reach a satisfactory convergence of the extended matter radius and of the point-proton radius. The dependence of the results on the resolution scale is studied. A correlation is found between the radii and the two-neutron separation energy. The importance of three-nucleon forces is pointed out comparing our results and previous calculations to experiment.

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arXiv2012

Realistic calculations of Kbar-N-N, Kbar-N-N-N, and Kbar-Kbar-N-N quasibound states

Binding energies and widths of three-body KbarNN, and of four-body KbarNNN and KbarKbarNN nuclear quasibound states are calculated in the hyperspherical basis, using realistic NN potentials and subthreshold energy dependent chiral KbarN interactions. Results of previous K^-pp calculations are reproduced and an upper bound is placed on the binding energy of a K^-d quasibound state. A self consistent handling of energy dependence is found to restrain binding, keeping the calculated four-body ground-state binding energies to relatively low values of about 30 MeV. The lightest strangeness -2 particle-stable Kbar nuclear cluster is most probably KbarKbarNN. The calculated Kbar N -> pi Y conversion widths range from approximately 30 MeV for the KbarNNN ground state to approximately 80 MeV for the KbarKbarNN ground state.

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arXiv2010

The effective interaction hyperspherical harmonics method for non-local potentials

A different formulation of the effective interaction hyperspherical harmonics (EIHH) method, suitable for non-local potentials, is presented. The EIHH method for local interactions is first shortly reviewed to point out the problems of an extension to non-local potentials. A viable solution is proposed and, as an application, results on the ground-state properties of 4- and 6-nucleon systems are presented. One finds a substantial acceleration in the convergence rate of the hyperspherical harmonics series. Perspectives for an application to scattering cross sections, via the Lorentz transform method are discussed.

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arXiv2010

New integrals in few-body problems

This work is concerned with multi-dimensional integrals, which are making their appearance in few-body atomic and nuclear physics. It is shown that the relevant two- and three-dimensional integrals can be reduced to one-dimensional form. This implies that the internal one- and two-dimensional integrals can be evaluated in explicit analytic form in term of the familiar generalized hypergeometric functions. Some of the integrals are presented here for the first time.

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