A. Arriaga

Femtometer toroidal structures in nuclei

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.

Evaluation of L1 and L2 minimum norm performances on EEG localizations.

OBJECTIVE In this work we study the performance of minimum norm methods to estimate the localization of brain electrical activity. These methods are based on the simplest forms of L(1) and L(2) norm estimates and are applied to simulated EEG data. The influence of several factors like the number of electrodes, grid density, head model, the number and depth of the sources and noise levels was taken into account. The main objective of the study is to give information about the dependence, on these factors, of the localization sources, to allow for proper interpretation of the data obtained in real EEG records. METHODS For the tests we used simulated dipoles and compared the localizations predicted by the L(1) and L(2) norms with the location of these point-like sources. We varied each parameter separately and evaluated the results. RESULTS From this work we conclude that, the grid should be constructed with approximately 650 points, so that the information about the orientation of the sources is preserved, especially for L(2) norm estimates; in favorable noise conditions, both L(1) and L(2) norm approaches are able to distinguish between more than one point-like sources. CONCLUSIONS The critical dependence of the results on the noise level and source depth indicates that regularized and weighted solutions should be used. Finally, all these results are valid both for spherical and for realistic head models.

Three-body correlations in few-body nuclei

A detailed comparison of Faddeev and variational wave functions for {sup 3}H, calculated with realistic nuclear forces, has been made to study the form of three-body correlations in few-body nuclei. Three new three-body correlations for use in variational wave functions have been identified, which substantially reduce the difference with the Faddeev wave function. The difference between the variational upper bound and the Faddeev binding energy is reduced by half, to typically {lt}2%. These three-body correlations also produce a significant lowering of the variational binding energy for {sup 4}He and larger nuclei.

Quantum Monte Carlo Studies of Relativistic Effects in Light Nuclei

Relativistic Hamiltonians are defined as the sum of relativistic one-body kinetic energy, two- and three-body potentials and their boost corrections. In this work the authors use the variational Monte Carlo method to study two kinds of relativistic effects in the binding energy of {sup 3}H and {sup 4}He. The first is due to the nonlocalities in the relativistic kinetic energy and relativistic one-pion exchange potential (OPEP), and the second is from boost interaction. The OPEP contribution is reduced by about 15% by the relativistic nonlocality, which may also have significant effects on pion exchange currents. However, almost all of this reduction is canceled by changes in the kinetic energy and other interaction terms, and the total effect of the nonlocalities on the binding energy is very small. The boost interactions, on the other hand, give repulsive contributions of 0.4 (1.9) MeV in {sup 3}H ({sup 4}He) and account for 37% of the phenomenological part of the three-nucleon interaction needed in the nonrelativistic Hamiltonians.

Relativistic calculation of deuteron threshold electrodisintegration at backward angles

The threshold electrodisintegration of the deuteron at backward angles is studied in instant form Hamiltonian dynamics, including a relativistic one-pion-exchange potential (OPEP) with off-shell terms as predicted by pseudovector coupling of pions to nucleons. The bound and scattering states are obtained in the center-of-mass frame, and then boosted from it to the Breit frame, where the evaluation of the relevant matrix elements of the electromagnetic current operator is carried out. The latter includes, in addition to one-body, also two-body terms due to pion exchange, as obtained, consistently with the OPEP, in pseudovector pion-nucleon coupling theory. In order to estimate the magnitude of the relativistic effects we perform, for comparison, the calculation with a nonrelativistic phase-equivalent Hamiltonian and consistent one-body and two-body pion-exchange currents. Our results for the electrodisintegration cross section show that, in the calculations using one-body currents, relativistic corrections become significant (i.e., larger than 10%) only at high momentum transfer

Relativistic effects and quasipotential equations

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Relativistic NN scattering equations without partial-wave decomposition

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Variational Monte Carlo calculations of the 2H(d,γ)4He reaction at low energies

{Q}^{2}\ensuremath{\simeq}40

4He D-State Effects in the 2H(d/γ) 4He Reaction at Low Energies

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Relativistic N N scattering equations without partial wave decomposition

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