arXiv subjects

Chang Xu

Publications and source records attributed to Chang Xu.

429 records · Page 24Linked to original sources

arXiv2013

A Survey on Multi-view Learning

In recent years, a great many methods of learning from multi-view data by considering the diversity of different views have been proposed. These views may be obtained from multiple sources or different feature subsets. In trying to organize and highlight similarities and differences between the variety of multi-view learning approaches, we review a number of representative multi-view learning algorithms in different areas and classify them into three groups: 1) co-training, 2) multiple kernel learning, and 3) subspace learning. Notably, co-training style algorithms train alternately to maximize the mutual agreement on two distinct views of the data; multiple kernel learning algorithms exploit kernels that naturally correspond to different views and combine kernels either linearly or non-linearly to improve learning performance; and subspace learning algorithms aim to obtain a latent subspace shared by multiple views by assuming that the input views are generated from this latent subspace. Though there is significant variance in the approaches to integrating multiple views to improve learning performance, they mainly exploit either the consensus principle or the complementary principle to ensure the success of multi-view learning. Since accessing multiple views is the fundament of multi-view learning, with the exception of study on learning a model from multiple views, it is also valuable to study how to construct multiple views and how to evaluate these views. Overall, by exploring the consistency and complementary properties of different views, multi-view learning is rendered more effective, more promising, and has better generalization ability than single-view learning.

cs.LG↗

arXiv2013

Extracting the nuclear symmetry potential and energy from neutron-nucleus scattering data

According to the Hugenholtz-Van Hove theorem, the symmetry energy and its density slope can be decomposed uniquely in terms of the single-nucleon potential in asymmetric nuclear matter which, at normal density, can be constrained by the nucleon optical model potential extracted from analyzing the nucleon-nucleus scattering data. To more accurately extract information on the symmetry energy and its density slope at normal density from neutron-nucleus scattering data, going beyond the well-known Lane potential, we include consistently the second order terms in isospin asymmetry in both the optical model potential and the symmetry energy decomposition. We find that the strength of the second-order symmetry potential $U_{sym,2}$ in asymmetric nuclear matter is significant compared to the first-order one U_{sym,1}$, especially at high nucleon momentum. While the $U_{sym,1}$ at normal density decreases with nucleon momentum, the $U_{sym,2}$ is found to have the opposite behavior. Moreover, we discuss effects of the $U_{sym,1}$ and $U_{sym,2}$ on determining the density dependence of the symmetry energy, and we find that the available neutron-nucleus scattering data favor a softer density dependence of the symmetry energy at normal density with a negligible contribution from the $U_{sym,2}$.

nucl-th↗

arXiv2013

Convex hulls of planar random walks with drift

Denote by $L_n$ the length of the perimeter of the convex hull of $n$ steps of a planar random walk whose increments have finite second moment and non-zero mean. Snyder and Steele showed that $n^{-1} L_n$ converges almost surely to a deterministic limit, and proved an upper bound on the variance $Var [ L_n] = O(n)$. We show that $n^{-1} Var [L_n]$ converges and give a simple expression for the limit, which is non-zero for walks outside a certain degenerate class. This answers a question of Snyder and Steele. Furthermore, we prove a central limit theorem for $L_n$ in the non-degenerate case.

math.PR↗

arXiv2012

Probing Nuclear Symmetry Energy and its Imprints on Properties of Nuclei, Nuclear Reactions, Neutron Stars and Gravitational Waves

Significant progress has been made in recent years in constraining nuclear symmetry energy at and below the saturation density of nuclear matter using data from both terrestrial nuclear experiments and astrophysical observations. However, many interesting questions remain to be studied especially at supra-saturation densities. In this lecture note, after a brief summary of the currently available constraints on nuclear symmetry energy near the saturation density we first discuss the relationship between the symmetry energy and the isopin and momentum dependence of the single-nucleon potential in isospin-asymmetric nuclear medium. We then discuss several open issues regarding effects of the tensor force induced neutron-proton short-range correlation (SRC) on nuclear symmetry energy. Finally, as an example of the impacts of nuclear symmetry energy on properties of neutron stars and gravitational waves, we illustrate effects of the high-density symmetry energy on the tidal polarizability of neutron stars in coalescing binaries.

nucl-th↗

arXiv2012

Delineating effects of tensor force on the density dependence of nuclear symmetry energy

In this talk, we report results of our recent studies to delineate effects of the tensor force on the density dependence of nuclear symmetry energy within phenomenological models. The tensor force active in the isosinglet neutron-proton interaction channel leads to appreciable depletion/population of nucleons below/above the Fermi surface in the single-nucleon momentum distribution in cold symmetric nuclear matter (SNM). We found that as a consequence of the high momentum tail in SNM the kinetic part of the symmetry energy $E^{kin}_{sym}(ρ)$ is significantly below the well-known Fermi gas model prediction of approximately $12.5 (ρ/ρ_0)^{2/3}$. With about 15% nucleons in the high momentum tail as indicated by the recent experiments at J-Lab by the CLAS Collaboration, the $E^{kin}_{sym}(ρ)$ is negligibly small. It even becomes negative when more nucleons are in the high momentum tail in SNM. These features have recently been confirmed by three independent studies based on the state-of-the-art microscopic nuclear many-body theories. In addition, we also estimate the second-order tensor force contribution to the potential part of the symmetry energy. Implications of these findings in extracting information about nuclear symmetry energy from nuclear reactions are discussed briefly.

nucl-th↗

arXiv2012

Single-nucleon potential decomposition of the nuclear symmetry energy

The nuclear symmetry energy $E_{sym}(ρ)$ and its density slope $L(ρ)$ can be decomposed analytically in terms of the single-nucleon potential in isospin asymmetric nuclear matter. Using three popular nuclear effective interaction models which have been extensively used in nuclear structure and reaction studies, namely, the isospin and momentum dependent MDI interaction model, the Skyrme Hartree-Fock approach and the Gogny Hartree-Fock approach, we analyze the contribution of different terms in the single-nucleon potential to the $E_{sym}(ρ)$ and $L(ρ)$. Our results show that the observed different density behaviors of $E_{sym}(ρ)$ for different interactions are essentially due to the variation of the symmetry potential $U_{sym,1}(ρ,k)$. Furthermore, we find that the contribution of the second-order symmetry potential $U_{sym,2}(ρ,k)$ to the $L(ρ)$ generally cannot be neglected. Moreover, our results demonstrate that the magnitude of the $U_{sym,2}(ρ,k)$ is generally comparable with that of $U_{sym,1}(ρ,k)$, indicating the second-order symmetry potential $U_{sym,2}(ρ,k)$ may have significant corrections to the widely used Lane approximation to the single-nucleon potential in extremely neutron(proton)-rich nuclear matter.

nucl-th↗

arXiv2011

Tensor force induced isospin-dependence of short-range nucleon-nucleon correlation and high-density behavior of nuclear symmetry energy

Quantitative information on the tensor force induced isospin-dependence of short-range nucleon-nucleon correlation (SRC) extracted from recent J-Lab experiments was used in constraining the high-momentum tails of single nucleon momentum distributions in both symmetric nuclear matter (SNM) and pure neutron matter (PNM). Its effects on the Equations of State of SNM and PNM as well as the nuclear symmetry energy are investigated. It is found that the tensor force induced isospin-dependence of SRC softens significantly the nuclear symmetry energy especially at supra-saturation densities.

nucl-th↗

arXiv2011

Imprints of Nuclear Symmetry Energy on Properties of Neutron Stars

Significant progress has been made in recent years in constraining the density dependence of nuclear symmetry energy using terrestrial nuclear laboratory data. Around and below the nuclear matter saturation density, the experimental constraints start to merge in a relatively narrow region. At supra-saturation densities, there are, however, still large uncertainties. After summarizing the latest experimental constraints on the density dependence of nuclear symmetry energy, we highlight a few recent studies examining imprints of nuclear symmetry energy on the binding energy, energy release during hadron-quark phase transitions as well as the $w$-mode frequency and damping time of gravitational wave emission of neutron stars.

nucl-th↗

arXiv2010

Nuclear symmetry energy and its density slope at normal density extracted from global nucleon optical potentials

Based on the Hugenholtz-Van Hove theorem, it is shown that both the symmetry energy E$_{sym}(ρ)$ and its density slope $L(ρ)$ at normal density $ρ_0$ are completely determined by the global nucleon optical potentials that can be extracted directly from nucleon-nucleus scatterings, (p,n) charge exchange reactions and single-particle energy levels of bound states. Adopting a value of $m^*/m=0.7$ for the nucleon effective k-mass in symmetric nuclear matter at $ρ_0$ and averaging all phenomenological isovector nucleon potentials constrained by world data available in the literature since 1969, the best estimates of $E_{sym}(ρ_0)=31.3$ MeV and $L(ρ_0)=52.7$ MeV are simultaneously obtained. Uncertainties involved in the estimates are discussed.

nucl-th↗

arXiv2010

Why is the nuclear symmetry energy so uncertain at supra-saturation densities?

Within the interacting Fermi gas model for isospin asymmetric nuclear matter, effects of the in-medium three-body interaction and the two-body short-range tensor force due to the $ρ$ meson exchange as well as the short-range nucleon correlation on the high-density behavior of the nuclear symmetry energy are demonstrated respectively in a transparent way. Possible physics origins of the extremely uncertain nuclear symmetry energy at supra-saturation densities are discussed.

nucl-th↗

arXiv2010

Analytical relations between nuclear symmetry energy and single-nucleon potentials in isospin asymmetric nuclear matter

Using the Hugenholtz-Van Hove theorem, we derive general expressions for the quadratic and quartic symmetry energies in terms of single-nucleon potentials in isospin asymmetric nuclear matter. These analytical relations are useful for gaining deeper insights into the microscopic origins of the uncertainties in our knowledge on nuclear symmetry energies especially at supra-saturation densities. As examples, the formalism is applied to two model single-nucleon potentials that are widely used in transport model simulations of heavy-ion reactions.

nucl-th↗

arXiv2010

An improved single particle potential for transport model simulations of nuclear reactions induced by rare isotope beams

Taking into account more accurately the isospin dependence of nucleon-nucleon interactions in the in-medium many-body force term of the Gogny effective interaction, new expressions for the single nucleon potential and the symmetry energy are derived. Effects of both the spin(isospin) and the density dependence of nuclear effective interactions on the symmetry potential and the symmetry energy are examined. It is shown that they both play a crucial role in determining the symmetry potential and the symmetry energy at supra-saturation densities. The improved single nucleon potential will be useful for simulating more accurately nuclear reactions induced by rare isotope beams within transport models.

nucl-th↗

arXiv2010

Tiling for Performance Tuning on Different Models of GPUs

The strategy of using CUDA-compatible GPUs as a parallel computation solution to improve the performance of programs has been more and more widely approved during the last two years since the CUDA platform was released. Its benefit extends from the graphic domain to many other computationally intensive domains. Tiling, as the most general and important technique, is widely used for optimization in CUDA programs. New models of GPUs with better compute capabilities have, however, been released, new versions of CUDA SDKs were also released. These updated compute capabilities must to be considered when optimizing using the tiling technique. In this paper, we implement image interpolation algorithms as a test case to discuss how different tiling strategies affect the program's performance. We especially focus on how the different models of GPUs affect the tiling's effectiveness by executing the same program on two different models of GPUs equipped testing platforms. The results demonstrate that an optimized tiling strategy on one GPU model is not always a good solution when execute on other GPU models, especially when some external conditions were changed.

cs.DC↗

arXiv2006

Branching ratios of $α$-decay to excited states of even-even nuclei

We make a systematic calculation on $α$-decay branching ratios to members of ground-state rotational band and to excited $0^{+}$ states of even-even nuclei by a simple barrier penetration approach. The branching ratios to the excited states of daughter nucleus are determined by the $α$-decay energy, the angular momentum of $α$-particle, and the excitation probability of the daughter nucleus. Our calculation covers isotopic chains from Hg to Fm in the mass regions 180$<$A$<$202 and A$\geq$224. The calculated branching ratios of $α$-transitions are consistent with the experimental data. Some useful predictions are made for future experiments.

nucl-th↗

arXiv2006

Half-lives of $α$-emitters approaching the N=Z line

The half-lives of newly observed $α$-emitters $^{105}$Te and $^{109}$Xe [Seweryniak \textit{et al.}, Phys. Rev. C \textbf{73}, 061301(R) (2006); Liddick \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 082501 (2006)] are investigated by the density-dependent cluster model. The half-lives of $α$-emitters close to the N=Z line are also studied in a unified framework where the influence of the nuclear deformation is properly taken into account. Good agreement between model and data is obtained and some possible $α$-emitters are suggested for future experiments.

nucl-th↗