1,497 research outputs found
The nuclear pseudospin symmetry along an isotopic chain
We investigate the isospin dependence of pseudospin symmetry in the chain of
tin isotopes (from Sn until Sn). Using a Woods-Saxon
parametrization of the nuclear potential for these isotopes we study in detail
the effect of the vector-isovector and Coulomb potentials in the energy
splittings of neutron and proton pseudospin partners in the isotopic chain. We
conclude that the realization of nuclear pseudospin symmetry does not change
considerably with the mass number, and is always favored for neutrons. We also
find that the potential accounts for essentially all the pseudospin
isospin asymmetry observed and that the Coulomb potential plays a negligible
role in this asymmetry. This can be explained by the dynamical nature of
pseudospin symmetry in nuclei, namely the dependence of the pseudospin
splittings on the shape of the nuclear mean-field potential.Comment: 4 pages, 4 figures, to be published in Brazilian Journal of Physic
Tensor coupling and pseudospin symmetry in nuclei
In this work we study the contribution of the isoscalar tensor coupling to
the realization of pseudospin symmetry in nuclei. Using realistic values for
the tensor coupling strength, we show that this coupling reduces noticeably the
pseudospin splittings, especially for single-particle levels near the Fermi
surface. By using an energy decomposition of the pseudospin energy splittings,
we show that the changes in these splittings come by mainly through the changes
induced in the lower radial wave function for the low-lying pseudospin
partners, and by changes in the expectation value of the pseudospin-orbit
coupling term for surface partners. This allows us to confirm the conclusion
already reached in previous studies, namely that the pseudospin symmetry in
nuclei is of a dynamical nature.Comment: 11 pages, 5 figures, uses REVTeX macro
Dynamical nature of the nuclear pseudospin and its isospin asymmetry
Pseudospin symmetry in nuclei is investigated by solving the Dirac equation
with Woods-Saxon scalar and vector radial potentials. We relate the pseudospin
interaction with a pseudospin-orbit term in a Schroedinger-like equation for
the lower component of the Dirac spinor. We show that this term gives a large
contribution to the energy splittings of pseudospin partners, so that the near
pseudospin degeneracy arises from a significant cancellation among the
different terms in that equation. This is a manifestation of the dynamical
character of this symmetry in the nucleus. We analyze the isospin dependence of
the pseudospin symmetry in a nuclear isotope chain by including a
vector-isovector potential V_rho and a Coulomb potential and conclude that
V_rho gives the main contribution to the observed pseudospin isospin asymmetry.Comment: 4 pages, 2 figures, uses World Scientific style file. Contribution
presented at the VIII International Workshop on Hadron Physics, Bento
Goncalves, RS, Brazil, April 14-19, 2002. To be published by World Scientific
in the proceedings of the "International Workshop on Hadron Physics 2002
Spin and pseudospin symmetries in the antinucleon spectrum of nuclei
Spin and pseudospin symmetries in the spectra of nucleons and antinucleons
are studied in a relativistic mean-field theory with scalar and vector
Woods-Saxon potentials, in which the strength of the latter is allowed to
change. We observe that, for nucleons and antinucleons, the spin symmetry is of
perturbative nature and it is almost an exact symmetry in the physical region
for antinucleons. The opposite situation is found in the pseudospin symmetry
case, which is better realized for nucleons than for antinucleons, but is of
dynamical nature and cannot be viewed in a perturbative way both for nucleons
and antinucleons. This is shown by computing the spin-orbit and
pseudospin-orbit couplings for selected spin and pseudospin partners in both
spectra.Comment: 8 figures, uses revtex 4.1 macro
White box radial basis function classifiers with component selection for clinical prediction models
Objective: To propose a new flexible and sparse classifier that results in interpretable decision support systems. Methods: Support vector machines (SVMs) for classification are very powerful methods to obtain classifiers for complex problems. Although the performance of these methods is consistently high and non-linearities and interactions between variables can be handled efficiently when using non-linear kernels such as the radial basis function (RBF) kernel, their use in domains where interpretability is an issue is hampered by their lack of transparency. Many feature selection algorithms have been developed to allow for some interpretation but the impact of the different input variables on the prediction still remains unclear. Alternative models using additive kernels are restricted to main effects, reducing their usefulness in many applications. This paper proposes a new approach to expand the RBF kernel into interpretable and visualizable components, including main and two-way interaction effects. In order to obtain a sparse model representation, an iterative l-regularized parametric model using the interpretable components as inputs is proposed. Results: Results on toy problems illustrate the ability of the method to select the correct contributions and an improved performance over standard RBF classifiers in the presence of irrelevant input variables. For a 10-dimensional x-or problem, an SVM using the standard RBF kernel obtains an area under the receiver operating characteristic curve (AUC) of 0.947, whereas the proposed method achieves an AUC of 0.997. The latter additionally identifies the relevant components. In a second 10-dimensional artificial problem, the underlying class probability follows a logistic regression model. An SVM with the RBF kernel results in an AUC of 0.975, as apposed to 0.994 for the presented method. The proposed method is applied to two benchmark datasets: the Pima Indian diabetes and the Wisconsin Breast Cancer dataset. The AUC is in both cases comparable to those of the standard method (0.826 versus 0.826 and 0.990 versus 0.996) and those reported in the literature. The selected components are consistent with different approaches reported in other work. However, this method is able to visualize the effect of each of the components, allowing for interpretation of the learned logic by experts in the application domain. Conclusions: This work proposes a new method to obtain flexible and sparse risk prediction models. The proposed method performs as well as a support vector machine using the standard RBF kernel, but has the additional advantage that the resulting model can be interpreted by experts in the application domain. © 2013 Elsevier B.V
Flexibility of expressive timing in repeated musical performances
Performances by soloists in the Western classical tradition are normally highly prepared, yet must sound fresh and spontaneous. How do musicians manage this? We tested the hypothesis that they achieve the necessary spontaneity by varying the musical gestures that express their interpretation of a piece. We examined the tempo arches produced by final slowing at the ends of phrases in performances of J. S. Bach’s No. 6 (Prelude) for solo cello (12 performances) and the Italian Concerto (Presto) for solo piano (eight performances). The performances were given by two experienced concert soloists during a short time period (3½ months for the Prelude, 2 weeks for the Presto) after completing their preparations for public performance. We measured the tempo of each bar or half-bar, and the stability of tempo across performances (difference of the tempo of each bar/half bar from each of the other performances). There were phrase arches for both tempo and stability with slower, less stable tempi at beginnings and ends of phrases and faster, more stable tempi mid-phrase. The effects of practice were complex. Tempo decreased overall with practice, while stability increased in some bars and decreased in others. One effect of practice may be to imbue well-learned, automatic motor sequences with freshness and spontaneity through cognitive control at phrase boundaries where slower tempi and decreased stability provide opportunities for slower cognitive processes to modulate rapid automatic motor sequences
Ventilator-associated pneumonia prevention: one good turn does not always deserve another
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