410 research outputs found
Specific heat of quasi-2D antiferromagnetic Heisenberg models with varying inter-planar couplings
We have used the stochastic series expansion (SSE) quantum Monte Carlo (QMC)
method to study the three-dimensional (3D) antiferromagnetic Heisenberg model
on cubic lattices with in-plane coupling J and varying inter-plane coupling
J_perp < J. The specific heat curves exhibit a 3D ordering peak as well as a
broad maximum arising from short-range 2D order. For J_perp << J, there is a
clear separation of the two peaks. In the simulations, the contributions to the
total specific heat from the ordering across and within the layers can be
separated, and this enables us to study in detail the 3D peak around T_c (which
otherwise typically is dominated by statistical noise). We find that the peak
height decreases with decreasing J_perp, becoming nearly linear below J_perp =
0.2J. The relevance of these results to the lack of observed specific heat
anomaly at the ordering transition of some quasi-2D antiferromagnets is
discussed.Comment: 7 pages, 8 figure
Computational Nuclear Physics and Post Hartree-Fock Methods
We present a computational approach to infinite nuclear matter employing
Hartree-Fock theory, many-body perturbation theory and coupled cluster theory.
These lectures are closely linked with those of chapters 9, 10 and 11 and serve
as input for the correlation functions employed in Monte Carlo calculations in
chapter 9, the in-medium similarity renormalization group theory of dense
fermionic systems of chapter 10 and the Green's function approach in chapter
11. We provide extensive code examples and benchmark calculations, allowing
thereby an eventual reader to start writing her/his own codes. We start with an
object-oriented serial code and end with discussions on strategies for porting
the code to present and planned high-performance computing facilities.Comment: 82 pages, to appear in Lecture Notes in Physics (Springer), "An
advanced course in computational nuclear physics: Bridging the scales from
quarks to neutron stars", M. Hjorth-Jensen, M. P. Lombardo, U. van Kolck,
Editor
A Naturally Narrow Positive Parity Theta^+
We present a consistent color-flavor-spin-orbital wave function for a
positive parity Theta^+ that naturally explains the observed narrowness of the
state. The wave function is totally symmetric in its flavor-spin part and
totally antisymmetric in its color-orbital part. If flavor-spin interactions
dominate, this wave function renders the positive parity Theta^+ lighter than
its negative parity counterpart. We consider decays of the Theta^+ and compute
the overlap of this state with the kinematically allowed final states. Our
results are numerically small. We note that dynamical correlations between
quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be
published in Phys. Rev. D, includes numerical estimates of decay width
Production of cascade hypernuclei via the (K-,K+) reaction within a quark-meson coupling model
We study the production of bound cascade hypernuclei via the (K-,K+) reaction
on 12C and 28Si targets within a covariant effective Lagrangian model,
employing the cascade bound state spinors derived from the latest quark-meson
coupling model as well as Dirac single particle wave functions. The K+-cascade
production vertex is described by excitation, propagation and decay of Lambda
and Sigma resonance states in the initial collision of a K- meson with a target
proton in the incident channel. The parameters of the resonance vertices are
fixed by describing the available data on total and differential cross sections
for the cascade production in elementary (K-,K+) reaction. We find that both
the elementary and hypernuclear production cross sections are dominated by the
contributions from the Lambda(1520) intermediate resonant state. The 0 degree
differential cross sections for the formation of simple s-state cascade
particle-hole states peak at a beam momentum around 1.0 GeV/c, with a value in
excess of 1 mub.Comment: 17 pages, 8 figures, version accepted for publication in Nucl. Phys.
Excited Baryons in Lattice QCD
We present first results for the masses of positive and negative parity
excited baryons calculated in lattice QCD using an O(a^2)-improved gluon action
and a fat-link irrelevant clover (FLIC) fermion action in which only the
irrelevant operators are constructed with APE-smeared links. The results are in
agreement with earlier calculations of N^* resonances using improved actions
and exhibit a clear mass splitting between the nucleon and its chiral partner.
An correlation matrix analysis reveals two low-lying J^P=(1/2)^- states with a
small mass splitting. The study of different Lambda interpolating fields
suggests a similar splitting between the lowest two Lambda1/2^- octet states.
However, the empirical mass suppression of the Lambda^*(1405) is not evident in
these quenched QCD simulations, suggesting a potentially important role for the
meson cloud of the Lambda^*(1405) and/or a need for more exotic interpolating
fields.Comment: Correlation matrix analysis performed. Increased to 400
configurations. 22 pages, 13 figures, 15 table
Role of the Delta (1232) in DIS on polarized He and extraction of the neutron spin structure function
We consider the effect of the transitions and in deep inelastic scattering on polarized He on the extraction
of the neutron spin structure function . Making the natural
assumption that these transitions are the dominant non-nucleonic contributions
to the renormalization of the axial vector coupling constant in the A=3 system,
we find that the effect of increases by % in the range , where our considerations are applicable
and most of the data for exist.Comment: 23 pages, 6 figures, revte
Z^* Resonances: Phenomenology and Models
We explore the phenomenology of, and models for, the Z^* resonances, the
lowest of which is now well established, and called the Theta. We provide an
overview of three models which have been proposed to explain its existence
and/or its small width, and point out other relevant predictions, and potential
problems, for each. The relation to what is known about KN scattering,
including possible resonance signals in other channels, is also discussed.Comment: 29 pages, uses RevTeX4; expanded version (published form
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Nucleon and hadron structure changes in the nuclear medium and impact on observables
We review the effect of hadron structure changes in a nuclear medium using
the quark-meson coupling (QMC) model, which is based on a mean field
description of non-overlapping nucleon (or baryon) bags bound by the
self-consistent exchange of scalar and vector mesons. This approach leads to
simple scaling relations for the changes of hadron masses in a nuclear medium.
It can also be extended to describe finite nuclei, as well as the properties of
hypernuclei and meson-nucleus deeply bound states. It is of great interest that
the model predicts a variation of the nucleon form factors in nuclear matter.
We also study the empirically observed, Bloom-Gilman (quark-hadron) duality.
Other applications of the model include subthreshold kaon production in heavy
ion collisions, D and D-bar meson production in antiproton-nucleus collisions,
and J/Psi suppression. In particular, the modification of the D and D-bar meson
properties in nuclear medium can lead to a large J/Psi absorption cross
section, which explains the observed J/Psi suppression in relativistic heavy
ion collisions.Comment: 143 pages, 77 figures, references added, a review article accepted in
Prog. Part. Nucl. Phy
Is Area-Wide Pest Management Useful? The Case of Citrus Greening.
Citrus greening currently poses a severe threat to citrus production worldwide. No treatment or management strategy is yet available to cure the disease. Scientists recommend controlling the vector of the disease, and area-wide pest management has been proposed as a superior alternative to individual pest management. We analyzed a unique dataset of farm-level citrus yields that allowed us to test this hypothesis. We found that yields of blocks located in an area with higher participation in coordinated sprays were 28%, 73% and 98% percent higher in 2012/13, 2013/14, and 2014/15, respectively, compared to the yields of blocks under the same management but located in an area with lower participation; providing evidence on the efficiency of a well-performing pest management area to deal with HLB. However, participation in CHMAs has not been commensurate with this evidence. We present survey data that provide insights about producers’ preferences and attitudes toward the area-wide pest management program. Despite the economic benefit we found area-wide pest management can provide, the strategic uncertainty involved in relying on neighbors seems to impose too high of a cost for most growers, who end up not coordinating sprays
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