616 research outputs found
Low-lying isovector monopole resonances
The mass difference between the even-even isobaric nuclei having the valence
nucleons on the same degenerate level is attributed to a Josephson-type
interaction between pairs of protons and pairs of neutrons. This interaction
can be understood as an isospin symmetry-breaking mean field for a
four-particle interaction separable in the two particles-two holes channel. The
strength of this mean field is estimated within an o(5) algebraic model, by
using the experimental value of the inertial parameter for the collective
isorotation induced by the breaking of the isospin symmetry. In superfluid
nuclei, the presumed interaction between the proton and neutron condensates
leads to coupled oscillations of the BCS gauge angles, which should appear in
the excitation spectrum as low-lying isovector monopole resonances.Comment: 16 pages/LaTex + 1 PostScript figure; related to cond-mat/9904242,
math-ph/000500
Potential Models and Lattice Gauge Current-Current Correlators
We compare current-current correlators in lattice gauge calculations with
correlators in different potential models, for a pseudoscalar charmonium in the
quark-gluon plasma. An important ingredient in the evaluation of the
current-current correlator in the potential model is the basic principle that
out of the set of continuum states, only resonance states and Gamow states with
lifetimes of sufficient magnitudes can propagate as composite objects and can
contribute to the current-current correlator. When the contributions from the
bound states and continuum states are properly treated, the potential model
current-current correlators obtained with the potential proposed in Ref. [11]
are consistent with the lattice gauge correlators. The proposed potential model
thus gains support to be a useful tool to complement lattice gauge calculations
for the study of states at high temperatures.Comment: 18 pages, 4 figures, to be published in Physcial Review
Spin-driven spatial symmetry breaking of spinor condensates in a double-well
The properties of an F=1 spinor Bose-Einstein condensate trapped in a
double-well potential are discussed using both a mean-field two-mode approach
and a simplified two-site Bose-Hubbard Hamiltonian. We focus in the region of
phase space in which spin effects lead to a symmetry breaking of the system,
favoring the spatial localization of the condensate in one well. To model this
transition we derive, using perturbation theory, an effective Hamiltonian that
describes N/2 spin singlets confined in a double-well potential.Comment: 12 pages, 5 figure
Partial Dynamical Symmetries in the g9/2 Shell-Progress and Puzzles
We present analytic proofs of the properties of four particle states in the
g9/2 shell which have seniority v=4 and angular momentum I=4 or 6.We show in
particular that the number of pairs with angular momentum I is equal to one for
these states
Configuration mixing calculation for complete low-lying spectra with the mean-field Hamiltonian
We propose a new theoretical approach to ground and low-energy excited states
of nuclei extending the nuclear mean-field theory. It consists of three steps:
stochastic preparation of many Slater determinants, the parity and angular
momentum projection, and diagonalization of the generalized eigenvalue
problems. The Slater determinants are constructed in the three-dimensional
Cartesian coordinate representation capable of describing arbitrary shape of
nuclei. We examine feasibility and usefulness of the method by applying the
method with the BKN interaction to light 4N-nuclei, 12C, 16O, and 20Ne. We
discuss difficulties of keeping linear independence for basis states projected
on good parity and angular momentum and present a possible prescription.Comment: 12 pages, revtex
Three-body correlations and finite-size effects in the Moore--Read states on a sphere
Two- and three-body correlations in partially filled degenerate fermion
shells are studied numerically for various interactions between the particles.
Three distinct correlation regimes are defined, depending on the short-range
behavior of the pair pseudopotential. For pseudopotentials similar to those of
electrons in the first excited Landau level, correlations at half-filling have
a simple three-body form consisting of the maximum avoidance of the triplet
state with the smallest relative angular momentum R_3=3. In analogy to the
superharmonic criterion for Laughlin two-body correlations, their occurrence is
related to the form of the three-body pseudopotential at short range. The
spectra of a model three-body repulsion are calculated, and the zero-energy
Moore--Read ground state, its +-e/4-charged quasiparticles, and the
magnetoroton and pair-breaking bands are all identified. The quasiparticles are
correctly described by a composite fermion model appropriate for Halperin's
p-type pairing with Laughlin correlations between the pairs. However, the
Moore--Read ground state, and specially its excitations, have small overlaps
with the corresponding Coulomb eigenstates when calculated on a sphere. The
reason lies in surface curvature which affects the form of pair pseudopotential
for which the "R_3>3" three-body correlations occur. In finite systems, such
pseudopotential must be slightly superharmonic at short range (different from
Coulomb pseudopotential). However, the connection with the three-body
pseudopotential is less size-dependent, suggesting that the Moore--Read state
and its excitations are a more accurate description for experimental nu=5/2
states than could be expected from previous calculations.Comment: 12 pages, 12 figures, submitted to PR
Number of states with fixed angular momentum for identical fermions and bosons
We present in this paper empirical formulas for the number of angular
momentum I states for three and four identical fermions or bosons. In the cases
with large I we prove that the number of states with the same and n
but different J is identical if for fermions
and for bosons, and that the number of states is also identical
for the same but different n and J if min(n, 2J+1 -
n) for fermions and for min(n, 2J) for bosons. Here , n is the particle number, and J refers to the angular momentum of
a single-particle orbit for fermions, or the spin L carried by bosons.Comment: 9 pages, no figure
Matching in Selective and Balanced Representation Space for Treatment Effects Estimation
The dramatically growing availability of observational data is being
witnessed in various domains of science and technology, which facilitates the
study of causal inference. However, estimating treatment effects from
observational data is faced with two major challenges, missing counterfactual
outcomes and treatment selection bias. Matching methods are among the most
widely used and fundamental approaches to estimating treatment effects, but
existing matching methods have poor performance when facing data with high
dimensional and complicated variables. We propose a feature selection
representation matching (FSRM) method based on deep representation learning and
matching, which maps the original covariate space into a selective, nonlinear,
and balanced representation space, and then conducts matching in the learned
representation space. FSRM adopts deep feature selection to minimize the
influence of irrelevant variables for estimating treatment effects and
incorporates a regularizer based on the Wasserstein distance to learn balanced
representations. We evaluate the performance of our FSRM method on three
datasets, and the results demonstrate superiority over the state-of-the-art
methods.Comment: Proceedings of the 29th ACM International Conference on Information
and Knowledge Management (CIKM '20
Shell-model description of monopole shift in neutron-rich Cu
Variations in the nuclear mean-field, in neutron-rich nuclei, are
investigated within the framework of the nuclear shell model. The change is
identified to originate mainly from the monopole part of the effective two-body
proton-neutron interaction. Applications for the low-lying states in odd- Cu
nuclei are presented. We compare the results using both schematic and realistic
forces. We also compare the monopole shifts with the results obtained from
large-scale shell-model calculations, using the same realistic interaction, in
order to study two-body correlations beyond the proton mean-field variations.Comment: Phys. Rev. C (in press
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