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Effects of. Delta. -isobar degrees of freedom on the reactions sup 3 He(n,. gamma. ) sup 4 He and sup 3 He(p,e sup +. nu. sub e ) sup 4 He at low-energy

Description: The cross sections of the radiative {sup 3}He(n,{gamma}){sup 4}He and weak {sup 3}He(p,e{sup +}{nu}{sub e}){sup 4}He capture reactions at thermal neutron and keV proton energies have been calculated with the Variational Monte Carlo method. The ground state and low-energy continuum wave functions have been determined variationally from a realistic Hamiltonian, and include both nucleon and {Delta}-isobar degrees of freedom. The electroweak transition operator contains one- and two-body components in the N + {Delta} Hilbert space.
Date: January 1, 1991
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Effects of {Delta}-isobar degrees of freedom on the reactions {sup 3}He(n,{gamma}){sup 4}He and {sup 3}He(p,e{sup +}{nu}{sub e}){sup 4}He at low-energy

Description: The cross sections of the radiative {sup 3}He(n,{gamma}){sup 4}He and weak {sup 3}He(p,e{sup +}{nu}{sub e}){sup 4}He capture reactions at thermal neutron and keV proton energies have been calculated with the Variational Monte Carlo method. The ground state and low-energy continuum wave functions have been determined variationally from a realistic Hamiltonian, and include both nucleon and {Delta}-isobar degrees of freedom. The electroweak transition operator contains one- and two-body components in the N + {Delta} Hilbert space.
Date: December 31, 1991
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

A fresh look at (e,e{prime}) scattering from light nuclei: A GFMC approach

Description: Green`s function Monte Carlo methods are used to calculate, in light nuclei, the Euclidean proton response, defined as the Laplace transform of the longitudinal response function measured in inclusive electron scattering. The calculations are based on a realistic Hamiltonian containing two- and three-nucleon interactions. Final-state-interaction effects are exactly included in this approach. The calculated Euclidean proton response functions for the {sup 4}He nucleus are found to be in excellent agreement with those obtained from an analysis of the Bates and Saclay longitudinal data.
Date: August 1, 1992
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

The hep Astrophysical Factor

Description: The S-factor for the {sup 3}He(p,e+ve){sup 4}He reaction has been recently calculated using realistic calculations and currents. The present talk summarizes the main results of that calculation.
Date: April 1, 2001
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Electro-weak reactions for astrophysics

Description: The status of ''ab initio'' microscopic calculations of the {sup 2}H(p,{gamma}){sup 3}He and {sup 3}He(p,e{sup +}{nu}{sub e}){sup 4}He reactions is reviewed. The methods used to generate accurate nuclear ground- and scattering-state wave functions, and to construct realistic electro-weak transition operators are described. The uncertainties in the theoretical predictions, particularly those relevant to the p-{sup 3}He weak capture, are discussed. For the dp radiative capture, the theoretical results are compared with the TUNL data in the energy range 0--100 keV.
Date: June 1, 2000
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Relativistic nuclear Hamiltonian and currents to (v/c){sub 2}.

Description: Relativistic Hamiltonians are defines as the sum of relativistic one-body kinetic energies and many-body interactions and their boost corrections. The calculation of the latter from commutation relations of the Poincare group is reviewed. It is shown that the most important terms can be understood from classical relativistic mechanics. The constraints of relativistic covariance on the charge and current densities are examined. Nuclear charge and current operators that satisfy them up to order (1/m){sup 2} are derived.
Date: August 1, 1995
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Electromagnetic Structure of Few-Nucleon Systems: a Critical Review

Description: Our current understanding of the structure of nuclei with up to A=8, including energy spectra, electromagnetic form factors, and capture reactions, is critically reviewed within the context of a realistic approach to nuclear dynamics based on two- and three-nucleon interactions and associated electromagnetic currents.
Date: October 1, 2000
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Interactions, Currents and the Structure of Few-Nucleon Systems

Description: The current understanding of the structure of nuclei with up to A=8, including energy spectra, electromagnetic form factors, and weak transitions, is reviewed within the context of a realistic approach to nuclear dynamics based on two- and three-nucleon interactions and associated electro-weak currents. Low-energy radiative and weak capture reactions of astrophysical relevance involving these light systems are also discussed.
Date: October 1, 2000
Creator: Schiavilla, R.
Partner: UNT Libraries Government Documents Department

The Charge Form Factors of the Three- and Four-Body Nuclei

Description: The charge form factors of 3H, 3He, and 4He are calculated using the Monte Carlo method and variational ground-state wave functions obtained for the Argonne two-nucleon and Urbana-VII three-nucleon interactions. The model for the charge density operator contains the two-body exchange contributions of longest range. With some spread due to the uncertainty in the electromagnetic form factors of the nucleon the calculated charge form factors are in good agreement with the empirical values over the whole experimentally covered range of momentum transfer.
Date: January 1, 1990
Creator: Schiavilla, R.; Pandharipande, V.R. & Riska, D.O.
Partner: UNT Libraries Government Documents Department

Theoretical study of the radiative capture reactions {sup 2}H(n,{gamma}){sup 3}H and {sup 2}H(p,{gamma}){sup 3}He at low energies

Description: Correlated Hyperspherical Harmonics wave functions with {Delta}-isobar admixtures obtained from realistic interactions are used to study the thermal neutron radiative capture on deuterium, and the {sup 2}H({rvec p},{gamma}){sup 3}He and p({rvec d},{gamma}){sup 3}He reactions in the center of mass energy range 0-100 keV. The nuclear electromagnetic current includes one and two-body components. Results for the {sup 2}H({rvec d},{gamma}){sup 3}H cross section and photon polarization parameter, as well as for the energy dependence of the astrophysical factor and angular distributions of the differential cross section, vector and tensor analyzing powers, and photon linear polarization coefficient of the {sup 2}H({rvec p},{gamma}){sup 3}He and p({rvec d},{gamma}){sup 3}He reactions are reported. Large effects due to two-body currents, in particular the long-range ones associated with the tensor component of the nucleon-nucleon interaction, are observed in the photon polarization parameter and vector analyzing power. Good, quantitative agreement between theory and experiment is found for all observables, with the exception of the vector analyzing power for which the calculated values underestimate the data by about 30%.
Date: February 1, 1996
Creator: Viviani, M.; Schiavilla, R. & Kievsky, A.
Partner: UNT Libraries Government Documents Department

Electromagnetic structure of the trinucleons

Description: The electromagnetic form factors of the trinucleons {sup 3}H and {sup 3}He are calculated with wave functions obtained with the Argonne AV18 two-nucleon and Urbana IX three-nucleon interactions. Full account is taken of the two-body currents required by current conservation with the AV18 interaction as well as those associated with N-Delta transition currents and the currents of Delta resonance components in the wave functions. Explicit three-nucleon current operators associated with the two-pion exchange three-nucleon interaction arising from irreducible S-wave pion-nucleon scattering is constructed and shown to have very little effect on the calculated magnetic form factors. The calculated magnetic form factor of {sup 3}H, and charge form factors of both {sup 3}H and {sup 3}He are in satisfactory agreement with the experimental data. However, the position of the zero in the magnetic form factor of {sup 3}He is slightly underpredicted.
Date: May 1, 1998
Creator: Marcucci, L.E.; Riska, D.O. & Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Parity-Violating Electron Deuteron Scattering

Description: The longitudinal asymmetry due to Z{sup 0} exchange is calculated in quasi-elastic electron-deuteron scattering at momentum transfers {vert_bar}Q{sup 2}{vert_bar} {approx_equal} 0.1 GeV{sup 2} relevant for the SAMPLE experiment. The deuteron and pn scattering-state wave functions are obtained from solutions of a Schroedinger equation with the Argonne v{sub 18} potential. Electromagnetic and weak neutral one- and two-nucleon currents are included in the calculation. The two-nucleon currents of pion range are shown to be identical to those derived in Chiral Perturbation Theory. The results indicate that two-body contributions to the asymmetry are small ({approx_equal} 0.2%) around the quasi-elastic peak, but become relatively more significant ({approx_equal} 3%) in the high-energy wing of the quasi-elastic peak.
Date: September 1, 2000
Creator: Diaconescu, L.; Schiavilla, R. & Kolck, M. van
Partner: UNT Libraries Government Documents Department

Neutron Charge Form Factor at Large Q{sup 2 }

Description: The neutron charge form factor G{sub En}(q) is determined from an analysis of the deuteron quadrupole form factor F{sub C2}(q) data. Recent calculations, based on a variety of different model interactions and currents, indicate that the contributions associated with the uncertain two-body operators of shorter range are relatively small for F{sub C2}(q), even at large momentum transfer q. Hence, G{sub En}(q) can be extracted from F{sub C2}(q) at large q{sup 2} without undue systematic uncertainties from theory.
Date: September 1, 2001
Creator: Schiavilla, R. & Sick, I.
Partner: UNT Libraries Government Documents Department

Parity-Violating Interaction Effects I: The Longitudinal Asymmetry in pp Elastic Scattering

Description: The proton-proton parity-violating longitudinal asymmetry is calculated in the lab-energy range 0--350 MeV, using a number of different, latest-generation strong-interaction potentials--Argonne {nu}{sub 18}, Bonn-2000, and Nijmegen-I--in combination with a weak-interaction potential consisting of rho- and omega-meson exchanges--the model known as DDH. The complete scattering problem in the presence of parity-conserving, including Coulomb, and parity-violating potentials is solved in both configuration- and momentum-space. The predicted parity-violating asymmetries are found to be only weakly dependent upon the input strong-interaction potential adopted in the calculation. Values for the rho- and omega-meson weak coupling constants h{sub {rho}}{sup pp} and h{sub {omega}}{sup pp} are determined by reproducing the measured asymmetries at 13.6 MeV, 45 MeV, and 221 MeV.
Date: September 1, 2001
Creator: Carlson, J.; Schiavilla, R.; Brown, V. & Gibson, B.
Partner: UNT Libraries Government Documents Department

Weak Transitions in A=6 and 7 Nuclei

Description: The {sup 6}He beta decay and {sup 7}Be electron capture processes are studied using variational Monte Carlo wave functions, derived from a realistic Hamiltonian consisting of the Argonne upsilon{sub 18} two-nucleon and Urbana-IX three-nucleon interactions. The model for the nuclear weak axial current includes one- and two-body operators with the strength of the leading two-body term - associated with Delta-isobar excitation of the nucleon -- adjusted to reproduce the Gamow-Teller matrix element in tritium Beta-decay. The measured half-life of {sup 6}He is under-predicted by theory by {approx_equal}8%, while that of {sup 7}Be for decay into the ground and first excited states of {sup 7}Li is over-predicted by {approx_equal} 9%. However, the experimentally known branching ratio for these latter processes is in good agreement with the calculated value. Two-body axial current contributions lead to a {approx_equal}1.7% (4.4%) increase in the value of the Gamow-Teller matrix element of {sup 6}He ({sup 7}Be), obtained with one-body currents only, and slightly worsen (appreciably improve) the agreement between the calculated and measured half-life. Corrections due to retardation effects associated with the finite lepton momentum transfers involved in the decays, as well as contributions of suppressed transitions induced by the weak vector charge and axial current operators, have also been calculated and found to be negligible. The approximate character of the variational wave functions employed here is presumable at the origin of the present unsatisfactory situation between theory and experiment.
Date: January 1, 2001
Creator: Schiavilla, R. & Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Summary of Few Nucleon Working Group

Description: Systems of A {ge} 2 nucleons are arguably the most interesting area of application of hadronic effective field theories (EFTs), because of a great confluence of factors absent in A {le} 1 systems: the theoretical challenge of combining a momentum expansion and a resummation to produce bound states, and the abundance of experimental data. A {ge} 2 systems are also the newest playground for hadronic EFTs, even though Weinberg's seminal papers 1 date from almost ten years ago. The working group on few-body systems at Chiral Dynamics 2000 attests that this field is now viewed as part of mainstream Chiral Perturbation Theory (ChPT). Few-nucleon systems provide, more generally, a unique testing ground for the simple, traditional picture of the nucleus as a system of interacting nucleons. The nucleon-nucleon (NN) interaction, as revealed by pp and np scattering experiments and the deuteron's properties, has a very rich structure. In light nuclear systems, with only a few degrees of freedom, it is possible to obtain accurate solutions for a wide variety of nuclear properties directly from realistic models of the NN interaction. Within this deceptively simple picture, we can test our understanding of nuclear structure and dynamics over a wide range of energy, from the few keV of astrophysical relevance to the MeV regime of nuclear spectra to the tens to hundreds of MeV measured in nuclear response experiments. Through the advances in computational techniques and facilities, the last few years have witnessed dramatic progress in the theory of light nuclei, as well as a variety of intriguing new experimental results. Important advances have occurred in studies of the spectra and structure of light nuclei, hadronic scattering, the response of light nuclei to external probes, and electroweak reactions involving few-nucleon systems at very low energy. By the Chiral Dynamics Workshop in Mainz ...
Date: May 1, 2001
Creator: Schiavilla, R.; Kolck, V. & Weller, H.
Partner: UNT Libraries Government Documents Department

A six-body calculation of the alpha-deuteron radiative capture cross section

Description: The authors have computed the cross section for the process d + alpha -> {sup 6}Li + gamma at the low energies relevant for primordial nucleosynthesis and comparison with laboratory data. The final state is a six-body wave function generated by the variational Monte Carlo method from the Argonne v-18 and Urbana IX potentials, including improved treatment of large-particle-separation behavior. The initial state is built up from the alpha-particle and deuteron ground-state solutions for these potentials, with phenomenological descriptions of scattering and cluster distortions. The dominant E2 cross section is in reasonable agreement with the laboratory data. Including center-of-energy and other small corrections, the authors obtain an E1 contribution which is larger than the measured contribution at 2 MeV by a factor of three. They calculate explicitly the impulse-approximation M1 contribution, which is expected to be very small, and obtain a result consistent with zero. They find little reason to suspect that the cross section is large enough to produce significant {sup 6}Li in the big bang.
Date: September 1, 2000
Creator: Nollett, K. M.; Wiringa, R. B. & Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Longitudinal and Transverse Quasi-Elastic Response Functions of Light Nuclei

Description: The {sup 3}He and {sup 4}He longitudinal and transverse response functions are determined from an analysis of the world data on quasi-elastic inclusive electron scattering. The corresponding Euclidean response functions are derived and compared to those calculated with Green's function Monte Carlo methods, using realistic interactions and currents. Large contributions associated with two-body currents are found, particularly in the {sup 4}He transverse response, in agreement with data. The contributions of two-body charge and current operators in the {sup 3}He, {sup 4}He, and {sup 6}Li response functions are also studied via sum-rule techniques. A semi-quantitative explanation for the observed systematics in the excess of transverse quasi-elastic strength, as function of mass number and momentum transfer, is provided. Finally, a number of model studies with simplified interactions, currents, and wave functions is carried out to elucidate the role played, in the full calculation, by tensor interactions and correlations.
Date: June 1, 2001
Creator: Carlson, J.; Jourdan, J.; Schiavilla, R. & Sick, I.
Partner: UNT Libraries Government Documents Department

Structure and dynamics of few-nucleon systems

Description: Few-nucleon physics is a field rich with high-quality experimental data and possibilities for accurate calculations of strongly-correlated quantum systems. In this article the authors discuss the traditional model of the nucleus as a system of interacting nucleons and outline many recent experimental results and theoretical developments in the field of few-nucleon physics. They present discussions of nuclear structure and spectra, clustering and correlations, elastic and inelastic electromagnetic form factors, low energy electroweak reactions, and nuclear scattering and response in the quasi-elastic regime. Through a review of the rich experimental data and a variety of theoretical developments, a coherent description of the nuclear strong and electroweak interaction properties emerges. In this article, they attempt to provide some insight into the practice and possibilities of in few-nucleon physics today.
Date: May 1, 1997
Creator: Schiavilla, R. & Carlson, J.
Partner: UNT Libraries Government Documents Department

An accurate nucleon-nucleon potential with charge-independence breaking

Description: The authors present a new high-quality nucleon-nucleon potential with explicit charge dependence and charge asymmetry, which they designate Argonne {upsilon}{sub 18}. The model has a charge-independent part with fourteen operator components that is an updated version of the Argonne {upsilon}{sub 14} potential. Three additional charge-dependent and one charge-asymmetric operators are added, along with a complete electromagnetic interaction. The potential has been fit directly to the Nijmegen pp and np scattering data base, low-energy nn scattering parameters, and deuteron binding energy. With 40 adjustable parameters it gives a {chi}{sup 2} per datum of 1.09 for 4,301 pp and np data in the range 0--350 MeV.
Date: August 1, 1994
Creator: Wiringa, R. B.; Stoks, V. G. J. & Schiavilla, R.
Partner: UNT Libraries Government Documents Department

Monte Carlo approaches to the few-nucleon continuum

Description: Variational and Green's Function Monte Carlo methods are reviewed as applied to the study of the few-nucleon continuum at low- and intermediate- energies. Results recently obtained for the radiative and weak capture reactions n + {sup 3}He {yields} {sup 4}He + {gamma} and p + {sup 3}He {yields} {sup 4}He + e{sup +} + {nu}{sub e}, the {sup 5}He P-wave resonances, and the inclusive and exclusive electron scattering reactions on {sup 3}H and the helium isotopes are summarized.
Date: October 1, 1994
Creator: Schiavilla, R.; Carlson, J. & Wiringa, R.
Partner: UNT Libraries Government Documents Department

Many-body currents and the strange-quark content of {sup 4}He

Description: Meson-exchange current (MEC) contributions to the parity-violating (PV) asymmetry for elastic scattering of polarized electrons from {sup 4}He are calculated over a range of momentum transfer using Monte Carlo methods and a variational {sup 4}He ground state wavefunction. The results indicate that MEC's generate a negligible contribution to the asymmetry at low-{vert_bar}{anti q}{vert_bar}, where a determination of the nucleon's mean square strangeness radius could be carried out at CEBAF. At larger values of momentum transfer--beyond the first diffraction minimum--two-body corrections from the {rho}-{pi} strangeness charge operator enter the asymmetry at a potentially observable level, even in the limit of vanishing strange-quark matrix elements of the nucleon. For purposes of constraining the nucleon's strangeness electric form factor, theoretical uncertainties associated with these MEC contributions do not appear to impose serious limitations.
Date: April 1, 1994
Creator: Musolf, M.J.; Schiavilla, R. & Donnelly, T.W.
Partner: UNT Libraries Government Documents Department

Elastic e-d Scattering Data and the Deuteron Wave Function

Description: What range of momentum components in the deuteron wave function are available ed elastic scattering data sensitive to? This question is addressed within the context of a model calculation of the deuteron form factors, based on realistic interactions and currents. It is shown that the data on the A(q), B(q), and T{sub 20}(q) observables at q {le} 6 fm{sup -1} essentially probe momentum components up to {approx} 4m{sub {pi}}.
Date: June 1, 2002
Creator: Schiavilla, R. & Pandharipande, V.R.
Partner: UNT Libraries Government Documents Department