943 research outputs found
Refining tree recruitment models
We used a micrometeorological dispersal model to simulate seed and seedling distributions derived from subcanopy balsam fir (Abies balsamea (L.) Mill.) source trees in a trembling aspen (Populus tremuloides Michx.) dominated forest. Our first objective was to determine the effect of substituting basal area for cone production as a proxy for seed output. The results showed that the r2 from the regression of predicted versus observed densities increased by ∼5% for seeds and ∼15% for seedling simulations. Our second objective was to determine the effects of changing the median horizontal wind speed. The median speed in this forest environment varies according to the proportion of leaves abscised. For values of the median expected wind speed between the extremes of leafless and full-canopy forests, the r2 of predicted versus observed varied between 0.35 and 0.49 for seeds and between 0.33 and 0.62 for seedling simulations. We demonstrated that the simple one-dimensional model can have added precision if the dispersal parameters are chosen so as to allow more fine-scale variation
The Destruction of Tori in Volume-Preserving Maps
Invariant tori are prominent features of symplectic and volume preserving
maps. From the point of view of chaotic transport the most relevant tori are
those that are barriers, and thus have codimension one. For an -dimensional
volume-preserving map, such tori are prevalent when the map is nearly
"integrable," in the sense of having one action and angle variables. As
the map is perturbed, numerical studies show that the originally connected
image of the frequency map acquires gaps due to resonances and domains of
nonconvergence due to chaos. We present examples of a three-dimensional,
generalized standard map for which there is a critical perturbation size,
, above which there are no tori. Numerical investigations to find
the "last invariant torus" reveal some similarities to the behavior found by
Greene near a critical invariant circle for area preserving maps: the crossing
time through the newly destroyed torus appears to have a power law singularity
at , and the local phase space near the critical torus contains
many high-order resonances.Comment: laTeX, 16 figure
Scalar hairy black holes and solitons in asymptotically flat spacetimes
A numerical analysis shows that a class of scalar-tensor theories of gravity
with a scalar field minimally and nonminimally coupled to the curvature allows
static and spherically symmetric black hole solutions with scalar-field hair in
asymptotically flat spacetimes. In the limit when the horizon radius of the
black hole tends to zero, regular scalar solitons are found. The asymptotically
flat solutions are obtained provided that the scalar potential of the
theory is not positive semidefinite and such that its local minimum is also a
zero of the potential, the scalar field settling asymptotically at that
minimum. The configurations for the minimal coupling case, although unstable
under spherically symmetric linear perturbations, are regular and thus can
serve as counterexamples to the no-scalar-hair conjecture. For the nonminimal
coupling case, the stability will be analyzed in a forthcoming paper.Comment: 7 pages, 10 postscript figures, file tex, new postscript figs. and
references added, stability analysis revisite
Nilpotent normal form for divergence-free vector fields and volume-preserving maps
We study the normal forms for incompressible flows and maps in the
neighborhood of an equilibrium or fixed point with a triple eigenvalue. We
prove that when a divergence free vector field in has nilpotent
linearization with maximal Jordan block then, to arbitrary degree, coordinates
can be chosen so that the nonlinear terms occur as a single function of two
variables in the third component. The analogue for volume-preserving
diffeomorphisms gives an optimal normal form in which the truncation of the
normal form at any degree gives an exactly volume-preserving map whose inverse
is also polynomial inverse with the same degree.Comment: laTeX, 20 pages, 1 figur
On Black Hole Scalar Hair in Asymptotically Anti de Sitter Spacetimes
The unexpected discovery of hairy black hole solutions in theories with
scalar fields simply by considering asymptotically Anti de-Sitter, rather than
asymptotically flat, boundary conditions is analyzed in a way that exhibits in
a clear manner the differences between the two situations.
It is shown that the trivial Schwarzschild Anti de Sitter becomes unstable in
some of these situations, and the possible relevance of this fact for the
ADS/CFT conjecture is pointed out.Comment: 12 pages. Published versio
Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions
Our previous point-contact Andreev reflection studies of the heavy-fermion
superconductor CeCoIn using Au tips have shown two clear features: reduced
Andreev signal and asymmetric background conductance [1]. To explore their
physical origins, we have extended our measurements to point-contact junctions
between single crystalline heavy-fermion metals and superconducting Nb tips.
Differential conductance spectra are taken on junctions with three
heavy-fermion metals, CeCoIn, CeRhIn, and YbAl, each with different
electron mass. In contrast with Au/CeCoIn junctions, Andreev signal is not
reduced and no dependence on effective mass is observed. A possible explanation
based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et
al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc.
Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press)
(cond-mat/0606535).Comment: 2 pages, 2 figures, submitted to the SCES conference, Houston, Texas,
USA, May 13-18, 200
The Fall of Stringy de Sitter
Kachru, Kallosh, Linde, & Trivedi recently constructed a four-dimensional de
Sitter compactification of IIB string theory, which they showed to be
metastable in agreement with general arguments about de Sitter spacetimes in
quantum gravity. In this paper, we describe how discrete flux choices lead to a
closely-spaced set of vacua and explore various decay channels. We find that in
many situations NS5-brane meditated decays which exchange NSNS 3-form flux for
D3-branes are comparatively very fast.Comment: 35 pp (11 pp appendices), 5 figures, v3. fixed minor typo
The Equivalence Principle and the Constants of Nature
We briefly review the various contexts within which one might address the
issue of ``why'' the dimensionless constants of Nature have the particular
values that they are observed to have. Both the general historical trend, in
physics, of replacing a-priori-given, absolute structures by dynamical
entities, and anthropic considerations, suggest that coupling ``constants''
have a dynamical nature. This hints at the existence of observable violations
of the Equivalence Principle at some level, and motivates the need for improved
tests of the Equivalence Principle.Comment: 12 pages; invited talk at the ISSI Workshop on the Nature of Gravity:
Confronting Theory and Experiment in Space, Bern, Switzerland, 6-10 October
2008; to appear in Space Science Review
Aspects of hairy black holes in spontaneously-broken Einstein-Yang-Mills systems: Stability analysis and Entropy considerations
We analyze (3+1)-dimensional black-hole space-times in spontaneously broken
Yang-Mills gauge theories that have been recently presented as candidates for
an evasion of the scalar-no-hair theorem. Although we show that in principle
the conditions for the no-hair theorem do not apply to this case, however we
prove that the `spirit' of the theorem is not violated, in the sense that there
exist instabilities, in both the sphaleron and gravitational sectors. The
instability analysis of the sphaleron sector, which was expected to be unstable
for topological reasons, is performed by means of a variational method. As
shown, there exist modes in this sector that are unstable against linear
perturbations. Instabilities exist also in the gravitational sector. A method
for counting the gravitational unstable modes, which utilizes a
catastrophe-theoretic approach is presented. The r\^ole of the catastrophe
functional is played by the mass functional of the black hole. The Higgs vacuum
expectation value (v.e.v.) is used as a control parameter, having a critical
value beyond which instabilities are turned on. The (stable) Schwarzschild
solution is then understood from this point of view. The catastrophe-theory
appproach facilitates enormously a universal stability study of non-Abelian
black holes, which goes beyond linearized perturbations. Some elementary
entropy considerations are also presented...Comment: Latex file, 50 pages, 2 figures (included as PS files at the end:
plot1.ps, plot2.ps
Nab: Measurement Principles, Apparatus and Uncertainties
The Nab collaboration will perform a precise measurement of 'a', the
electron-neutrino correlation parameter, and 'b', the Fierz interference term
in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS,
using a novel electric/magnetic field spectrometer and detector design. The
experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will
provide an independent measurement of lambda = G_A/G_V, the ratio of
axial-vector to vector coupling constants of the nucleon. Nab also plans to
perform the first ever measurement of 'b' in neutron decay, which will provide
an independent limit on the tensor weak coupling.Comment: 12 pages, 6 figures, 1 table, talk presented at the International
Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to
appear in Nucl. Instrum. Meth. in Physics Research
- …