arXiv subjects

S. Pastore

Publications and source records attributed to S. Pastore.

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

Local chiral interactions and magnetic structure of few-nucleon systems

The magnetic form factors of $^2$H, $^3$H, and $^3$He, deuteron photodisintegration cross sections at low energies, and deuteron threshold electrodisintegration cross sections at backward angles in a wide range of momentum transfers, are calculated with the chiral two-nucleon (and three-nucleon) interactions including $Δ$ intermediate states that have recently been constructed in configuration space. The $A\,$=$\,$3 wave functions are obtained from hyperspherical-harmonics solutions of the Schrödinger equation. The electromagnetic current includes one- and two-body terms, the latter induced by one- and two-pion exchange (OPE and TPE, respectively) mechanisms and contact interactions. The contributions associated with $Δ$ intermediate states are only retained at the OPE level, and are neglected in TPE loop (tree-level) corrections to two-body (three-body) current operators. Expressions for these currents are derived and regularized in configuration space for consistency with the interactions. The low-energy constants that enter the contact few-nucleon systems. The predicted form factors and deuteron electrodisintegration cross section are in excellent agreement with experiment for momentum transfers up to 2--3 fm$^{-1}$. However, the experimental values for the deuteron photodisintegration cross section are consistently underestimated by theory, unless use is made of the Siegert form of the electric dipole transition operator. A complete analysis of the results is provided, including the clarification of the origin of the aforementioned discrepancy.

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arXiv2018

Local chiral interactions, the tritium Gamow-Teller matrix element, and the three-nucleon contact term

The Gamow-Teller (GT) matrix element contributing to tritium $β$ decay is calculated with trinucleon wave functions obtained from hyperspherical-harmonics solutions of the Schrödinger equation with the chiral two- and three-nucleon interactions including $Δ$ intermediate states that have recently been constructed in configuration space. Predictions up to N3LO in the chiral expansion of the axial current (with $Δ$'s) overestimate the empirical value by 1--4 \%. By exploiting the relation between the low-energy constant (LEC) in the contact three-nucleon interaction and two-body axial current, we provide new determinations of the LECs $c_D$ and $c_E$ that characterize this interaction by fitting the trinucleon binding energy and tritium GT matrix element. Some of the implications that the resulting models of three-nucleon interactions have on the spectra of light nuclei and the equation of state of neutron matter are briefly discussed. We also provide a partial analysis, which ignores $Δ$'s, of the contributions due to loop corrections in the axial current at N4LO. Finally, explicit expressions for the axial current up to N4LO have been derived in configuration space, which other researchers in the field may find useful.

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arXiv2018

A new leading contribution to neutrinoless double-beta decay

Within the framework of chiral effective field theory we discuss the leading contributions to the neutrinoless double-beta decay transition operator induced by light Majorana neutrinos. Based on renormalization arguments in both dimensional regularization with minimal subtraction and a coordinate-space cutoff scheme, we show the need to introduce a leading-order short-range operator, missing in all current calculations. We discuss strategies to determine the finite part of the short-range coupling by matching to lattice QCD or by relating it via chiral symmetry to isospin-breaking observables in the two-nucleon sector. Finally, we speculate on the impact of this new contribution on nuclear matrix elements of relevance to experiment.

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arXiv2017

Neutrinoless double beta decay matrix elements in light nuclei

We present the first ab initio calculations of neutrinoless double beta decay matrix elements in $A=6$-$12$ nuclei using Variational Monte Carlo wave functions obtained from the Argonne $v_{18}$ two-nucleon potential and Illinois-7 three-nucleon interaction. We study both light Majorana neutrino exchange and potentials arising from a large class of multi-TeV mechanisms of lepton number violation. Our results provide benchmarks to be used in testing many-body methods that can be extended to the heavy nuclei of experimental interest. In light nuclei we have also studied the impact of two-body short range correlations and the use of different forms for the transition operators, such as those corresponding to different orders in chiral effective theory.

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arXiv2017

Quantum Monte Carlo calculations of weak transitions in $A\,$=$\,$6--10 nuclei

Ab initio calculations of the Gamow-Teller (GT) matrix elements in the $β$ decays of $^6$He and $^{10}$C and electron captures in $^7$Be are carried out using both variational and Green's function Monte Carlo wave functions obtained from the Argonne $v_{18}$ two-nucleon and Illinois-7 three-nucleon interactions, and axial many-body currents derived from either meson-exchange phenomenology or chiral effective field theory. The agreement with experimental data is excellent for the electron captures in $^7$Be, while theory overestimates the $^6$He and $^{10}$C data by $\sim 2\%$ and $\sim 10\%$, respectively. We show that for these systems correlations in the nuclear wave functions are crucial to explain the data, while many-body currents increase by $\sim 2$--$3\%$ the one-body GT contributions. These findings suggest that the longstanding $g_A$-problem, i.e., the systematic overprediction ($\sim 20 \%$ in $A\le 18$ nuclei) of GT matrix elements in shell-model calculations, may be resolved, at least partially, by correlation effects.

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arXiv2016

Nuclear Axial Currents in Chiral Effective Field Theory

Two-nucleon axial charge and current operators are derived in chiral effective field theory up to one loop. The derivation is based on time-ordered perturbation theory, and accounts for cancellations between the contributions of irreducible diagrams and the contributions due to non-static corrections from energy denominators of reducible diagrams. Ultraviolet divergencies associated with the loop corrections are isolated in dimensional regularization. The resulting axial current is finite and conserved in the chiral limit, while the axial charge requires renormalization. A complete set of contact terms for the axial charge up to the relevant order in the power counting is constructed.

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arXiv2014

Quantum Monte Carlo calculations of electromagnetic transitions in 8Be with meson-exchange currents derived from chiral effective field theory

We report quantum Monte Carlo calculations of electromagnetic transitions in 8Be. The realistic Argonne v18 two-nucleon and Illinois-7 three-nucleon potentials are used to generate the ground state and nine excited states, with energies that are in excellent agreement with experiment. A dozen M1 and eight E2 transition matrix elements between these states are then evaluated. The E2 matrix elements are computed only in impulse approximation, with those transitions from broad resonant states requiring special treatment. The M1 matrix elements include two-body meson-exchange currents derived from chiral effective field theory, which typically contribute 20--30% of the total expectation value. Many of the transitions are between isospin-mixed states; the calculations are performed for isospin-pure states and then combined with empirical mixing coefficients to compare to experiment. Alternate mixings are also explored. In general, we find that transitions between states that have the same dominant spatial symmetry are in reasonable agreement with experiment, but those transitions between different spatial symmetries are often underpredicted.

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arXiv2014

An update of muon capture on hydrogen

The successful precision measurement of the rate of muon capture on a proton by the MuCap Collaboration allows for a stringent test of the current theoretical understanding of this process. Chiral perturbation theory, which is a low-energy effective field theory that preserves the symmetries and the pattern of symmetry breaking in the underlying theory of QCD, offers a systematic framework for describing $μp$ capture and provides a basic test of QCD at the hadronic level. We describe how this effective theory with no free parameters reproduces the measured capture rate. A recent study has addressed new sources of uncertainties that were not considered in the previous works, and we review to what extent these uncertainties are now under control. Finally, the rationale for studying muon capture on the deuteron and some recent theoretical developments regarding this process are discussed.

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arXiv2013

The muon capture rate on hydrogen and the values of $g_A$ and $g_{πNN}$

Motivated by the recent developments in the determination of the experimental values of the nucleon axial-vector coupling constant $g_A$ and the pion-nucleon coupling constant $g_{πNN}$, we carry out a heavy-baryon chiral perturbation calculation of the hyperfine-singlet $μp$ capture rate $Γ_0$ to next-to-next-to-leading order (N$^2$LO), with the use of the latest values of $g_A$ and $g_{πNN}$. The calculated N$^2$LO value is $Γ_0^{\rm theor}(μ^- p \to ν_μn) =718 \pm 7 \,s^{-1}$, where the estimated N$^3$LO contribution dominates the error. This value is in excellent agreement with the experimental value reported by the MuCap Collaboration.

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arXiv2013

Charge-symmetry breaking forces and isospin mixing in 8Be

We report Green's function Monte Carlo calculations of isospin-mixing (IM) matrix elements for the 2+, 1+, and 3+ T=0,1 pairs of states at 16--19 MeV excitation in 8Be. The realistic Argonne v18 (AV18) two-nucleon and Illinois-7 three-nucleon potentials are used to generate the nuclear wave functions. Contributions from the full electromagnetic interaction and strong class III charge-symmetry-breaking (CSB) components of the AV18 potential are evaluated. We also examine two theoretically more complete CSB potentials based on rho-omega mixing, tuned to give the same neutron-neutron scattering length as AV18. The contribution of these different CSB potentials to the 3H-3He, 7Li-7Be, and 8Li-8B isovector energy differences is evaluated and reasonable agreement with experiment is obtained. Finally, for the 8Be IM calculation we add the small class IV CSB terms coming from one-photon, one-pion, and one-rho exchange, as well as rho-omega mixing. The expectation values of the three CSB models vary by up to 20% in the isovector energy differences, but only by 10% or less in the IM matrix element. The total matrix element gives 85--90% of the experimental IM value of -145 keV for the 2+ doublet, with about two thirds coming from the Coulomb interaction. We also report the IM matrix element to the first 2+ state at 3 MeV excitation, which is the final state for various tests of the Standard Model for beta-decay.

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arXiv2013

Electromagnetic transition from the 4$^+$ to 2$^+$ resonance in $^8$Be measured via the radiative capture in $^4$He+$^4$He

An earlier measurement on the 4$^+$ to 2$^+$ radiative transition in $^8$Be provided the first electromagnetic signature of its dumbbell-like shape. However, the large uncertainty in the measured cross section does not allow a stringent test of nuclear structure models. The present paper reports a more elaborate and precise measurement for this transition, via the radiative capture in the $^4$He+$^4$He reaction, improving the accuracy by about a factor of three. The {\it ab initio} calculations of the radiative transition strength with improved three-nucleon forces are also presented. The experimental results are compared with the predictions of the alpha cluster model and {\it ab initio} calculations.

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arXiv2013

Quantum Monte Carlo calculations of electromagnetic moments and transitions in A <= 9 nuclei with meson-exchange currents derived from chiral effective field theory

Quantum Monte Carlo calculations of electromagnetic moments and transitions are reported for A <= 9 nuclei. The realistic Argonne v18 two-nucleon and Illinois-7 three-nucleon potentials are used to generate the nuclear wave functions. Contributions of two-body meson-exchange current (MEC) operators are included for magnetic moments and M1 transitions. The MEC operators have been derived in both a standard nuclear physics approach and a chiral effective field theory formulation with pions and nucleons including up to one-loop corrections. The two-body MEC contributions provide significant corrections and lead to very good agreement with experiment. Their effect is particularly pronounced in the A=9, T=3/2 systems, in which they provide up to 20% (40 %) of the total predicted value for the 9Li (9C) magnetic moment.

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arXiv2013

GFMC calculations of electromagnetic moments and M1 transitions in $A\leq 9$ nuclei

We present recent Green's function Monte Carlo calculations of magnetic moments and M1 transitions in $A \leq 9$ nuclei, which include corrections arising from two-body meson-exchange electromagnetic currents. Two-body effects provide significant corrections to the calculated observables, bringing them in excellent agreement with the experimental data. In particular, we find that two-body corrections are especially large in the A=9, $T=3/2$ systems, in which they account for up to $\sim 20%$ ($\sim 40 %$) of the total predicted value for the $^9$Li ($^9$C) magnetic moment.

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arXiv2012

Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory

The objectives of the present work are twofold. The first is to address and resolve some of the differences present in independent, chiral-effective-field-theory (\chiEFT) derivations up to one loop, recently appeared in the literature, of the nuclear charge and current operators. The second objective is to provide a complete set of \chiEFT predictions for the structure functions and tensor polarization of the deuteron, for the charge and magnetic form factors of 3He and 3H, and for the charge and magnetic radii of these few-nucleon systems. The calculations use wave functions derived from high-order chiral two- and three-nucleon potentials and Monte Carlo methods to evaluate the relevant matrix elements. Predictions based on conventional potentials in combination with \chiEFT charge and current operators are also presented. There is excellent agreement between theory and experiment for all these observables for momentum transfers up to q< 2.0-2.5 (1/fm); for a subset of them, this agreement extends to momentum transfers as high as q~5-6 (1/fm). A complete analysis of the results is provided.

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arXiv2012

Quantum Monte Carlo calculations of magnetic moments and M1 transitions in $A \le 9$ nuclei

We present Quantum Monte Carlo calculations of magnetic moments and M1 transitions in $A\le 9$ nuclei which take into account contributions of two-body electromagnetic currents. The Hamiltonian utilized to generate the nuclear wave functions includes the realistic Argonne-{\it v}$_{18}$ two-nucleon and the Illinois-7 three-nucleon interactions. The nuclear two-body electromagnetic currents are derived from a pionful chiral effective field theory including up to one-loop corrections. These currents involve unknown Low Energy Constants which have been fixed so as to reproduce a number of experimental data for the two- and three-nucleon systems, such as $np$ phase shifts and deuteron, triton, and $^3$He magnetic moments. This preliminary study shows that two-body contributions provide significant corrections which are crucial to bring the theory in agreement with the experimental data in both magnetic moments and M1 transitions.

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arXiv2011

The two-nucleon electromagnetic charge operator in chiral effective field theory ($χ$EFT) up to one loop

The electromagnetic charge operator in a two-nucleon system is derived in chiral effective field theory ($χ$EFT) up to order $e\, Q$ (or N4LO), where $Q$ denotes the low-momentum scale and $e$ is the electric charge. The specific form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for the non-static terms in the corresponding potentials. We show that different prescriptions lead to unitarily equivalent potentials and accompanying charge operators. Thus, provided a consistent set is adopted, predictions for physical observables will remain unaffected by the non-uniqueness associated with these off-the-energy-shell effects.

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arXiv2010

Thermal neutron captures on $d$ and $^3$He

We report on a study of the $nd$ and $n\,^3$He radiative captures at thermal neutron energies, using wave functions obtained from either chiral or conventional two- and three-nucleon realistic potentials with the hyperspherical harmonics method, and electromagnetic currents derived in chiral effective field theory up to one loop. The predicted $nd$ and $n\,^3$He cross sections are in good agreement with data, but exhibit a significant dependence on the input Hamiltonian. A comparison is also made between these and new results for the $nd$ and $n\,^3$He cross sections obtained in the conventional framework for both potentials and currents.

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