arXiv subjects

N. Barnea

Publications and source records attributed to N. Barnea.

At least 19 recordsLinked to original sources

arXiv2024

A=3 (e,e') $x_B \geq 1$ cross-section ratios and the isospin structure of short-range correlations

We study the relation between measured high-$x_B$, high-$Q^2$, Helium-3 to Tritium, $(e,e')$ inclusive-scattering cross-section ratios and the relative abundance of high-momentum neutron-proton ($np$) and proton-proton ($pp$) short-range correlated (SRC) nucleon pairs in three-body ($A=3$) nuclei. Analysis of this data using a simple pair-counting cross-section model suggested a much smaller $np/pp$ ratio than previously measured in heavier nuclei, questioning our understanding of $A=3$ nuclei and, by extension, all other nuclei. Here we examine this finding using spectral-function-based cross-section calculations, with both an \textit{ab initio} $A=3$ spectral function and effective Generalized Contact Formalism (GCF) spectral functions using different nucleon-nucleon interaction models. The \textit{ab initio} calculation agrees with the data, showing good understanding of the structure of $A=3$ nuclei. An 8\% uncertainty on the simple pair-counting model, as implied by the difference between it and the \textit{ab initio} calculation, gives a factor of 5 uncertainty in the extracted $np/pp$ ratio. Thus we see no evidence for the claimed ``unexpected structure in the high-momentum wavefunction for hydrogen-3 and helium-3''.

nucl-th↗

arXiv2023

In-medium $Λ$ isospin impurity from charge symmetry breaking in the ${_Λ^4}{\rm H}-{_Λ^4}{\rm He}$ mirror hypernuclei

The $Λ$ separation energies in the mirror hypernuclei ${_Λ^4}{\rm H}-{_Λ^4}{\rm He}$ exhibit large charge symmetry breaking (CSB). Analyzing this CSB within pionless effective field theory while using partially conserved baryon-baryon SU(3) flavor symmetry, we deduce a $Λ-Σ^0$ induced in-medium admixture amplitude ${\cal A}_{I=1}\approx 1.5\%$ in the dominantly isospin $I=0$ $Λ$ hyperon. Our results confirm the free-space value ${\cal A}^{(0)}_{I=1}$ inferred directly within the SU(3) baryon octet by Dalitz and von-Hippel in 1964 and reaffirmed in a recent QCD+QED lattice calculation. Furthermore, exploring the consequences of SU(3) flavor symmetry on the $Λ$-nucleon interaction, we find that CSB is expected to impact the $S=1$ and $S=0$ spin channels in opposite directions, with the latter dominating by an order of magnitude. These observations explain a recent deduction of $Λ$-nucleon CSB strengths.

nucl-th↗

arXiv2023

The Present and Future of QCD

This White Paper presents the community inputs and scientific conclusions from the Hot and Cold QCD Town Meeting that took place September 23-25, 2022 at MIT, as part of the Nuclear Science Advisory Committee (NSAC) 2023 Long Range Planning process. A total of 424 physicists registered for the meeting. The meeting highlighted progress in Quantum Chromodynamics (QCD) nuclear physics since the 2015 LRP (LRP15) and identified key questions and plausible paths to obtaining answers to those questions, defining priorities for our research over the coming decade. In defining the priority of outstanding physics opportunities for the future, both prospects for the short (~ 5 years) and longer term (5-10 years and beyond) are identified together with the facilities, personnel and other resources needed to maximize the discovery potential and maintain United States leadership in QCD physics worldwide. This White Paper is organized as follows: In the Executive Summary, we detail the Recommendations and Initiatives that were presented and discussed at the Town Meeting, and their supporting rationales. Section 2 highlights major progress and accomplishments of the past seven years. It is followed, in Section 3, by an overview of the physics opportunities for the immediate future, and in relation with the next QCD frontier: the EIC. Section 4 provides an overview of the physics motivations and goals associated with the EIC. Section 5 is devoted to the workforce development and support of diversity, equity and inclusion. This is followed by a dedicated section on computing in Section 6. Section 7 describes the national need for nuclear data science and the relevance to QCD research.

hep-ph↗

arXiv2022

Consequences of increased hypertriton binding for $s$-shell $Λ$-hypernuclear systems

Consequences of increasing the binding energy of the hypertriton ground state ${_Λ^3}{\rm H}(J^P={\frac{1}{2}}^+)$ from the emulsion value $B^{\rm EMUL}_Λ({_Λ^3}{\rm H}_{\rm g.s.})$=0.13$\pm$0.05 MeV to the STAR value $B^{\rm STAR}_Λ(^{3}_Λ{\rm H}) = (0.41\pm 0.12 \pm 0.11)$ MeV are studied for $s$-shell hypernuclei within a pionless EFT approach at leading order, constrained by the binding energies of the $0^+$ and $1^+$ ${_Λ^4} {\rm H}$ states. The stochastic variational method is used in bound-state calculations, whereas the inverse analytic continuation in the coupling constant method is used to locate $S$-matrix poles of continuum states. It is found that the $Λnn({\frac{1}{2}}^+)$ resonance becomes broader and less likely to be observed experimentally, whereas the ${_Λ^3}{\rm H}({\frac{3}{2}}^+)$ spin-flip virtual state moves closer to the $Λd$ threshold to become a shallow bound state for specific $ΛN$ interaction strengths. The effect of such a near-threshold ${_Λ^3}{\rm H}({\frac{3}{2}}^+)$ state on femtoscopic studies of $Λ$-deuteron correlations, and its lifetime if bound, are discussed. Increasing $B_Λ({_Λ^3} {\rm H}_{\rm g.s.})$ moderately, up to $\sim$0.5 MeV, hardly affects calculated values of $B_Λ({_Λ^5}{\rm He})$.

nucl-th↗

arXiv2021

Measurement of the α-particle monopole transition form factor challenges theory: a low-energy puzzle for nuclear forces?

We perform a systematic study of the $α$-particle excitation from its ground state $0_1^+$ to the $0_2^+$ resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad $Q^2$-range (from $0.5$ to $5.0$ fm$^{-2}$). The precision of the new data dramatically superseeds that of older sets of data, each covering only a portion of the $Q^2$-range. The new data allow the determination of two coefficients in a low-momentum expansion leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations we observe that modern nuclear forces, including those derived within chiral effective field theory which are well tested on a variety of observables, fail to reproduce the excitation of the $α$-particle.

nucl-ex↗

arXiv2021

Removing center of mass effects in response function and sum rule calculations based on the harmonic oscillator basis

Response functions are at the heart of any comparison of theory with experiment in studies of the nuclear dynamics with electroweak probes. Calculations performed in the laboratory frame often suffer from center of mass contaminations that need to be removed. By confining the system in a harmonic oscillator, we derive a set of analytical formulas to subtract the center of mass effects from calculations of response functions and associated sum rules. After a general analytical derivation, we first deal specifically with the longitudinal response function appearing in electron scattering and provide expressions for the center of mass correcting functions. Next, we present a proof of principle study for the case of the electric dipole sum rules in a two-body problem with a numerical implementation of our formalism. These steps pave the way to applying the proposed method to heavier nuclei in the future.

nucl-th↗

arXiv2021

Extracing the number of short-range corerlated nucleon pairs from inclusive electron scattering data

The extraction of the relative abundances of short-range correlated (SRC) nucleon pairs from inclusive electron scattering is studied using the generalized contact formalism (GCF) with several nuclear interaction models. GCF calculations can reproduce the observed scaling of the cross-section ratios for nuclei relative to deuterium at high-$x_B$ and large-$Q^2$, $a_2=(σ_A/A)/(σ_d/2)$. In the non-relativistic instant-form formulation, the calculation is very sensitive to the model parameters and only reproduces the data using parameters that are inconsistent with ab-initio many-body calculations. Using a light-cone GCF formulation significantly decreases this sensitivity and improves the agreement with ab-initio calculations. The ratio of similar mass isotopes, such as $^{40}$Ca and $^{48}$Ca, should be sensitive to the nuclear asymmetry dependence of SRCs, but is found to also be sensitive to low-energy nuclear structure. Thus the empirical association of SRC pair abundances with the measured $a_2$ values is only accurate to about $20\%$. Improving this will require cross-section calculations that reproduce the data while properly accounting for both nuclear structure and relativistic effects.

nucl-th↗

arXiv2021

Nature of the $Λnn$ $(J^π=1/2^+, I=1)$ and ${\rm ^3_ΛH^*} (J^π=3/2^+, I=0)$ states

The nature of the $Λnn$ and ${\rm ^3_ΛH^*} (J^π=3/2^+,~I=0)$ states is investigated within a pionless effective field theory at leading order, constrained by the low energy $ΛN$ scattering data and hypernuclear 3- and 4-body data. Bound state solutions are obtained using the stochastic variational method, the continuum region is studied by employing two independent methods - the inverse analytic continuation in the coupling constant method and the complex scaling method. Our calculations yield both the $Λnn$ and ${\rm ^3_ΛH^*}$ states unbound. We conclude that the excited state ${\rm ^3_ΛH^*}$ is a virtual state and the $Λnn$ pole located close to the three-body threshold in a complex energy plane could convert to a true resonance with Re$(E)>0$ for some considered $ΛN$ interactions. Finally, the stability of resonance solutions is discussed and limits of the accuracy of performed calculations are assessed.

nucl-th↗

arXiv2021

Scale and Scheme Independence and Position-Momentum Equivalence of Nuclear Short-Range Correlations

Ab-initio Quantum Monte Carlo (QMC) calculations of nuclei from deuterium to 40Ca, obtained using four different phenomenological and local chiral nuclear potentials, are analyzed using the Generalized Contact Formalism (GCF). We extract spin- and isospin-dependent "nuclear contact terms" for each interaction in both coordinate and momentum space. The extracted contact terms, that count the number of short-range correlated (SRC) pairs with different quantum numbers, are dependent on the nuclear interaction model used in the QMC calculation. However, the ratios of contact terms for a nucleus A to deuterium (for spin-1 pn pairs) or to 4He (for all NN pairs) are independent of the nuclear interaction model and are the same for both short-distance and high-momentum pairs. This implies that the relative abundance of short-range pairs in the nucleus is a long-range (mean-field) quantity that is insensitive to the short-distance nature of the nuclear force. Measurements of exclusive (e,e'NN) pair breakup processes are instead more sensitive to short-range dynamics

nucl-th↗

arXiv2020

On the width of the $K^-$D atom ground state

Experiments at DA$Φ$NE-Frascati and at J-PARC are scheduled to produce $K^-$D atoms and observe their X-ray cascade down to the 1$S$ ground state (g.s.), thereby measuring its strong-interaction width and shift away from a purely Coulomb state. A width $Γ_{1S}\lesssim 1$ keV will ensure good resolution of the X-ray transitions feeding the 1$S$ g.s. Here we study the expected $K^-$D 1$S$ g.s. width from the perspective of global fits to level shifts and widths in heavier kaonic atoms across the periodic table, using $K^-$ nuclear optical potentials constructed from $\bar K N$ chiral interaction models. Special attention is paid to the subthreshold energy at which the $\bar K N$ subsystem interacts in the $K^-$D atomic g.s. Within this approach we predict strong-interaction upward level shift of close to 700 eV and width of about 1.2 to 1.3 keV for the $K^-$D atom 1$S$ g.s., in fair agreement with genuinely three-body $K^-$D atom calculations. Comparison is made with $π^-$D atom phenomenology.

nucl-th↗

arXiv2020

Probing the core of the strong nuclear interaction

The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of Quantum Chromodynamics (QCD). However, as these equations cannot be solved directly, physicists resort to describing nuclear interactions using effective models that are well constrained at typical inter-nucleon distances in nuclei but not at shorter distances. This limits our ability to describe high-density nuclear matter such as in the cores of neutron stars. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations thereby accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta above 400 MeV/c. As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor-force to a predominantly spin-independent scalar-force. These results demonstrate the power of using such measurements to study the nuclear interaction at short-distances and also support the use of point-like nucleons with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of atomic nuclei.

nucl-ex↗

arXiv2020

$Λ^*$ matter and its stability

We performed calculations of nuclear systems composed solely of $Λ^*$ hyperons, aiming at exploring the possibility of existence of absolutely stable $Λ^*$ matter. We considered $Λ^*$ interaction strengths compatible with the $Λ^*Λ^*$ binding energy $B_{Λ^*Λ^*}$ given by the $\bar{K}N$ interaction model by Yamazaki and Akaishi [1]. We found that the binding energy per $Λ^*$ saturates at values well below 100 MeV for mass number $A\geq120$. The $Λ^*$ matter is thus highly unstable against strong interaction decay.

nucl-th↗

arXiv2020

Generalized Contact Formalism Analysis of the $^4$He$(e,e'pN)$ Reaction

Measurements of short-range correlations in exclusive $^4$He$(e,e'pN)$ reactions are analyzed using the Generalized Contact Formalism (GCF). We consider both instant-form and light-cone formulations with both the AV18 and local N2LO(1.0) nucleon-nucleon ($NN$) potentials. We find that kinematic distributions, such as the reconstructed pair opening angle, recoil neutron momentum distribution, and pair center of mass motion, as well as the measured missing energy, missing mass distributions, are all well reproduced by GCF calculations. The missing momentum dependence of the measured $^4$He$(e,e'pN)$ / $^4$He$(e,e'p)$ cross-section ratios, sensitive to nature of the $NN$ interaction at short-distacnes, are also well reproduced by GCF calculations using either interaction and formulation. This gives credence to the GCF scale-separated factorized description of the short-distance many-body nuclear wave-function.

nucl-th↗

arXiv2020

The Continuum Spectrum of Hypernuclear Trios

The spectrum of hypernuclear trios composed of a $Λ$ baryon and two nucleons is the subject of an ongoing experimental campaign, aiming to study the interaction of the $Λ$ particle with a neutron, and the 3-body $Λ$-nucleon-nucleon force. In this manuscript we utilize baryonic effective field theory at leading order, constrained to reproduce the available low energy light hypernuclear data, to study the continuum spectrum of such hypernuclear trios. Using the complex scaling method and the inverse analytic continuation in the coupling constant method we find the existence of a virtual state in the $Λn p$ $J^π=\frac{3}{2}^{+}$ channel, leading to cross-section enhancement near threshold. For the $Λn n$ $J^π=\frac{1}{2}^{+}$ channel we predict a resonance state. Depending, however, on the value of the $ΛN$ scattering length, the resonance pole moves from the physical to the unphysical complex energy sheet within the experimental bounds.

nucl-th↗

arXiv2019

The onset of $ΛΛ$ hypernuclear binding

Binding energies of light, $A\leq 6$, $ΛΛ$ hypernuclei are calculated using the stochastic variational method in a pionless effective field theory (EFT) approach at leading order with the purpose of assessing critically the onset of binding in the strangeness S=-2 hadronic sector. The EFT input in this sector consists of (i) a $ΛΛ$ contact term constrained by the $ΛΛ$ scattering length $a_{ΛΛ}$, using a range of values compatible with $ΛΛ$ correlations observed in relativistic heavy ion collisions, and (ii) a $ΛΛN$ contact term constrained by the only available $A\leq 6$ $ΛΛ$ hypernucler binding energy datum of $^{6}_{ΛΛ}$He. The recently debated neutral three-body and four-body systems $^{3}_{ΛΛ}$n and $^{4}_{ΛΛ}$n are found unbound by a wide margin. A relatively large value of $|a_{ΛΛ}| \gtrsim 1.5$ fm is needed to bind $^{4}_{ΛΛ}$H, thereby questioning its particle stability. In contrast, the particle stability of the $A=5$ $ΛΛ$ hypernuclear isodoublet $^{5}_{ΛΛ} $H--$^{5}_{ΛΛ}$He is robust, with $Λ$ separation energy of order 1 MeV.

nucl-th↗

arXiv2019

Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei

We measured the triple coincidence A(e,e'np) and A(e,e'pp) reactions on carbon, aluminum, iron, and lead targets at Q2 > 1.5 (GeV/c)2, xB > 1.1 and missing momentum > 400 MeV/c. This was the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for Single-Charge Exchange effects decreased the SRC pairs ratio to ~ 3%, which is lower than previous results. Comparisons to theoretical Generalized Contact Formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale- and scheme-dependence in cross section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.

nucl-ex↗

arXiv2019

On the Stability of $Λ(1405)$ Matter

A hypothesis of absolutely stable strange hadronic matter composed of $Λ(1405)$ baryons, here denoted $Λ^*$, is tested within many-body calculations performed using the Relativistic Mean-Field approach. In our calculations, we employed the $Λ^*Λ^*$ interaction compatible with the $Λ^*Λ^*$ binding energy $B_{Λ^*Λ^*}=40$~MeV given by the phenomenological energy-independent $\bar{K}N$ interaction model by Yamazaki and Akaishi (YA). We found that the binding energy per $Λ^*$, as well as the central density in $Λ^*$ many-body systems saturates for mass number $A\geq120$, leaving $Λ^*$ aggregates highly unstable against strong interaction decay. Moreover, we confronted the YA interaction model with kaonic atom data and found that it fails to reproduce the $K^-$ single-nucleon absorption fractions at rest from bubble chamber experiments.

nucl-th↗

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.

nucl-th↗