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Relativistic effects in the binding energies of few-body nuclei

Description: We have examined the consequences of Poincare invariance for the binding-energy calculations of few-body nuclei. Nonrelativistic Hamiltonians have been considered in an attempt to fit simultaneously the binding energies of 2-, 3-, 4-body nuclei and that of nuclear matter. Even with reasonable three-body forces it appears to be difficult to simultaneously fit the binding energies of 3- and 4-body nuclei. The size of the discrepancy is of the order of possible relativistic effects. After elimination of the center-of-mass motion the two-body Schroedinger equation can always be interpreted as a relativistic equation. Given a two-body mass operator (i.e. a two-body Hamiltonian for zero total momentum) it is possible to construct a consistent relativistic multi-body dynamics with a nonrelativistic limit. The relativistic effects can be calculated in first-order perturbation theory using an expansion in inverse powers of the nucleon mass.
Date: January 1, 1983
Creator: Coester, F. & Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Quantum Monte Carlo calculations for light nuclei.

Description: Quantum Monte Carlo calculations of ground and low-lying excited states for nuclei with A {le} 8 are made using a realistic Hamiltonian that fits NN scattering data. Results for more than 40 different (J{pi}, T) states, plus isobaric analogs, are obtained and the known excitation spectra are reproduced reasonably well. Various density and momentum distributions and electromagnetic form factors and moments have also been computed. These are the first microscopic calculations that directly produce nuclear shell structure from realistic NN interactions.
Date: October 23, 1998
Creator: Wiringa, R. B.
Partner: UNT Libraries Government Documents Department

Microscopic calculations of nuclear structure and nuclear correlations

Description: A major goal in nuclear physics is to understand how nuclear structure comes about from the underlying interactions between nucleons. This requires modelling nuclei as collections of strongly interacting particles. Using realistic nucleon-nucleon potentials, supplemented with consistent three-nucleon potentials and two-body electroweak current operators, variational Monte Carlo methods are used to calculate nuclear ground-state properties, such as the binding energy, electromagnetic form factors, and momentum distributions. Other properties such as excited states and low-energy reactions are also calculable with these methods.
Date: January 1, 1992
Creator: Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Monte Carlo calculations of few-body and light nuclei

Description: A major goal in nuclear physics is to understand how nuclear structure comes about from the underlying interactions between nucleons. This requires modelling nuclei as collections of strongly interacting particles. Using realistic nucleon-nucleon potentials, supplemented with consistent three-nucleon potentials and two-body electroweak current operators, variational Monte Carlo methods are used to calculate nuclear ground-state properties, such as the binding energy, electromagnetic form factors, and momentum distributions. Other properties such as excited states and low-energy reactions are also calculable with these methods.
Date: January 1, 1992
Creator: Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Monte Carlo calculations of few-body and light nuclei

Description: A major goal in nuclear physics is to understand how nuclear structure comes about from the underlying interactions between nucleons. This requires modelling nuclei as collections of strongly interacting particles. Using realistic nucleon-nucleon potentials, supplemented with consistent three-nucleon potentials and two-body electroweak current operators, variational Monte Carlo methods are used to calculate nuclear ground-state properties, such as the binding energy, electromagnetic form factors, and momentum distributions. Other properties such as excited states and low-energy reactions are also calculable with these methods.
Date: February 1, 1992
Creator: Wiringa, R. B.
Partner: UNT Libraries Government Documents Department

Nucleon-nucleon interactions

Description: Nucleon-nucleon interactions are at the heart of nuclear physics, bridging the gap between QCD and the effective interactions appropriate for the shell model. We discuss the current status of {ital NN} data sets, partial-wave analyses, and some of the issues that go into the construction of potential models. Our remarks are illustrated by reference to the Argonne {ital v}{sub 18} potential, one of a number of new potentials that fit elastic nucleon-nucleon data up to 350 MeV with a {Chi}{sup 2} per datum near 1. We also discuss the related issues of three-nucleon potentials, two-nucleon charge and current operators, and relativistic effects. We give some examples of calculations that can be made using these realistic descriptions of {ital NN} interactions. We conclude with some remarks on how our empirical knowledge of {ital NN} interactions may help constrain models at the quark level, and hence models of nucleon structure.
Date: December 31, 1996
Creator: Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Quantum Monte Carlo calculations for light nuclei

Description: Quantum Monte Carlo calculations of ground and low-lying excited states for nuclei with A {le} 8 have been made using a realistic Hamiltonian that fits NN scattering data. Results for more than two dozen different (J{sup {pi}}, T) p-shell states, not counting isobaric analogs, have been obtained. The known excitation spectra of all the nuclei are reproduced reasonably well. Density and momentum distributions and various electromagnetic moments and form factors have also been computed. These are the first microscopic calculations that directly produce nuclear shell structure from realistic NN interactions.
Date: October 1, 1997
Creator: Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Quantum Monte Carlo Calculations for Light Nuclei

Description: Quantum Monte Carlo calculations of ground and low-lying excited states for nuclei with A {le} 8 are made using a realistic Hamiltonian that fits NN scattering data. Results for more than 30 different (j{sup {prime}}, T) states, plus isobaric analogs, are obtained and the known excitation spectra are reproduced reasonably well. Various density and momentum distributions and electromagnetic form factors and moments have also been computed. These are the first microscopic calculations that directly produce nuclear shell structure from realistic NN interactions.
Date: August 1, 1998
Creator: Wiringa, R. 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

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

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

Charge-independence-breaking in the triton

Description: We find the effect of the observed charge-independence-breaking in {sup 1}S{sub 0} nucleon-nucleon scattering on the binding energy of the triton to be of order 80 keV. When corrections for this effect are made in an 18-channel momentum-space Faddeev calculation, we find the Paris and Argonne v{sub 14} potentials triton binding energies that differ by only 20 keV.
Date: January 1, 1990
Creator: Gloeckle, W.; Lee, T.S.H. & Wiringa, R.B.
Partner: UNT Libraries Government Documents Department

Femtometer toroidal structures in nuclei

Description: The two-nucleon density distributions in states with isospin T=0, spin S=1 and projection M{sub S}=0 and {+-}1 are studied in {sup 2}H, {sup 3,4}He, {sup 6,7}Li and {sup 16}O. The equidensity surfaces for M{sub S}=0 distributions are found to be toroidal in shape, while those of M{sub S}={+-}1 have dumbbell shapes at large density. The dumbbell shapes are generated by rotating tori. The toroidal shapes indicate that the tensor correlations have near maximal strength at r < 2 fm in all these nuclei. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron, the quasi-deuteron model, and the dp, dd and {alpha}d L=2 (D-wave) components in {sup 3}He, {sup 4}He and {sup 6}Li. The toroidal distribution has a maximum-density diameter of {approximately}1 fm and a half-maximum density thickness of {approximately}0.9 fm. Many realistic models of nuclear forces predict these values, which are supported by the observed electromagnetic form factors of the deuteron, and also predicted by classical Skyrme effective Lagrangians, related to QCD in the limit of infinite colors. Due to the rather small size of this structure, it could have a revealing relation to certain aspects of QCD.
Date: March 1, 1996
Creator: Forest, J.; Pandharipande, V.; Pieper, S.; Wiringa, R.B.; Shiavilla, R. & Arriaga, A.
Partner: UNT Libraries Government Documents Department

Variational theory of nuclear and neutron matter

Description: In these lectures we will discuss attempts to solve the A = 3 to {infinity} nuclear many-body problems with the variational method. We choose the form of a variational wave function {Chi}{sub v}(1, 2{hor ellipsis}A) to describe the ground state. The {Chi}{sub v} and the ground-state energy E{sub v} are obtained by minimizing E{sub v} = {l angle}{Chi}{sub v}{vert bar}H{vert bar}{Chi}{sub v}{r angle}/{l angle}{Chi}{sub v}{vert bar}{Chi}{sub v}{r angle} with respect to variations in {Chi}{sub v}. If the form of the variational wave function is chosen properly we can expect {Chi}{sub v} {approx} {Chi}{sub 0} and E{sub v} {approx} E{sub 0} where {Chi}{sub 0} and E{sub 0} are the exact ground-state wave function and energy. In general E{sub v} {ge} E{sub 0} in variational calculations. 63 refs., 11 figs.
Date: June 1, 1989
Creator: Pandharipande, V.R.; Wiringa, R.B. (Illinois Univ., Urbana, IL (USA). Dept. of Physics & Argonne National Lab., IL (USA))
Partner: UNT Libraries Government Documents Department