1,329 research outputs found
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
Predicting the optimal amount of time to spend learning before designating protected habitat for threatened species
1. Deciding when to protect threatened species habitat when complete knowledge about the habitat extent is uncertain is a common problem in conservation. More accurate habitat mapping improves conservation outcomes once that habitat is protected. However, delaying protection to improve accuracy can lead to species decline or, at worst, local extinction when threats to that habitat continue unabated before protection is implemented. Hence, there is a trade-off between gaining knowledge and taking conservation action. 2. We quantified this trade-off and determined the optimal time to spend learning about a species' habitat before protecting that habitat. We used a range of hypothetical learning curves to model improvements in the accuracy of predicted habitat over time, and receiver operating characteristic (ROC) curves to model the corresponding increase in the proportion of habitat protected. We used rates of habitat loss to model the impact of delaying habitat protection and derived analytical solutions to the problem for different types of learning curves. 3. We illustrate our approach using two threatened species, the koala Phascolarctos cinereus in Australia and northern abalone Haliotis kamtschatkana in Canada. Our approach confirms that when impacts of threatening processes are incurred rapidly, the need for timely protection is high, and the optimal time to spend learning is short for all learning curves. When the rate of habitat loss is low, we benefit from better habitat identification, and the optimal time to protect is sensitive to assumptions about how we learn and the proportion of non-habitat we are willing to protect unnecessarily. 4. Navigating the trade-off between information gain and timely action is a common problem in conservation. By optimizing the trade-off between the benefits of improving mapping accuracy and the costs of delaying protection, we provide guidelines on the effective allocation of resources between habitat identification and habitat protection. Importantly, by explicitly modelling this trade-off with a range of learning curves and estimates of the rates of habitat loss or other threatening processes, we can predict the optimal time to spend learning even when relatively little is known about a species and its habitat.Abbey E. Camaclang, Iadine Chadès, Tara G. Martin, Hugh P. Possingha
Scalar perturbation spectra from warm inflation
We present a numerical integration of the cosmological scalar perturbation
equations in warm inflation. The initial conditions are provided by a
discussion of the thermal fluctuations of an inflaton field and thermal
radiation using a combination of thermal field theory and thermodynamics. The
perturbation equations include the effects of a damping coefficient
and a thermodynamic potential . We give an analytic expression for the
spectral index of scalar fluctuations in terms of a new slow-roll parameter
constructed from . A series of toy models, inspired by spontaneous
symmetry breaking and a known form of the damping coefficient, lead to a
spectrum with on large scales and on small scales.Comment: 12 pages, 5 figures, RevTeX 4, revised with extra figure
Collider signals from slow decays in supersymmetric models with an intermediate-scale solution to the mu problem
The problem of the origin of the mu parameter in the Minimal Supersymmetric
Standard Model can be solved by introducing singlet supermultiplets with
non-renormalizable couplings to the ordinary Higgs supermultiplets. The
Peccei-Quinn symmetry is broken at a scale which is the geometric mean between
the weak scale and the Planck scale, yielding a mu term of the right order of
magnitude and an invisible axion. These models also predict one or more singlet
fermions which have electroweak-scale masses and suppressed couplings to MSSM
states. I consider the case that such a singlet fermion, containing the axino
as an admixture, is the lightest supersymmetric particle. I work out the
relevant couplings in several of the simplest models of this type, and compute
the partial decay widths of the next-to-lightest supersymmetric particle
involving leptons or jets. Although these decays will have an average proper
decay length which is most likely much larger than a typical collider detector,
they can occasionally occur within the detector, providing a striking signal.
With a large sample of supersymmetric events, there will be an opportunity to
observe these decays, and so gain direct information about physics at very high
energy scales.Comment: 24 pages, LaTeX, 4 figure
The universal Glivenko-Cantelli property
Let F be a separable uniformly bounded family of measurable functions on a
standard measurable space, and let N_{[]}(F,\epsilon,\mu) be the smallest
number of \epsilon-brackets in L^1(\mu) needed to cover F. The following are
equivalent:
1. F is a universal Glivenko-Cantelli class.
2. N_{[]}(F,\epsilon,\mu)0 and every probability
measure \mu.
3. F is totally bounded in L^1(\mu) for every probability measure \mu.
4. F does not contain a Boolean \sigma-independent sequence.
It follows that universal Glivenko-Cantelli classes are uniformity classes
for general sequences of almost surely convergent random measures.Comment: 26 page
Resolutions of C^n/Z_n Orbifolds, their U(1) Bundles, and Applications to String Model Building
We describe blowups of C^n/Z_n orbifolds as complex line bundles over
CP^{n-1}. We construct some gauge bundles on these resolutions. Apart from the
standard embedding, we describe U(1) bundles and an SU(n-1) bundle. Both
blowups and their gauge bundles are given explicitly. We investigate ten
dimensional SO(32) super Yang-Mills theory coupled to supergravity on these
backgrounds. The integrated Bianchi identity implies that there are only a
finite number of U(1) bundle models. We describe how the orbifold gauge shift
vector can be read off from the gauge background. In this way we can assert
that in the blow down limit these models correspond to heterotic C^2/Z_2 and
C^3/Z_3 orbifold models. (Only the Z_3 model with unbroken gauge group SO(32)
cannot be reconstructed in blowup without torsion.) This is confirmed by
computing the charged chiral spectra on the resolutions. The construction of
these blowup models implies that the mismatch between type-I and heterotic
models on T^6/Z_3 does not signal a complication of S-duality, but rather a
problem of type-I model building itself: The standard type-I orbifold model
building only allows for a single model on this orbifold, while the blowup
models give five different models in blow down.Comment: 1+27 pages LaTeX, 2 figures, some typos correcte
Novel approach to the study of quantum effects in the early universe
We develop a theoretical frame for the study of classical and quantum
gravitational waves based on the properties of a nonlinear ordinary
differential equation for a function of the conformal time
, called the auxiliary field equation. At the classical level,
can be expressed by means of two independent solutions of the
''master equation'' to which the perturbed Einstein equations for the
gravitational waves can be reduced. At the quantum level, all the significant
physical quantities can be formulated using Bogolubov transformations and the
operator quadratic Hamiltonian corresponding to the classical version of a
damped parametrically excited oscillator where the varying mass is replaced by
the square cosmological scale factor . A quantum approach to the
generation of gravitational waves is proposed on the grounds of the previous
dependent Hamiltonian. An estimate in terms of and
of the destruction of quantum coherence due to the gravitational
evolution and an exact expression for the phase of a gravitational wave
corresponding to any value of are also obtained. We conclude by
discussing a few applications to quasi-de Sitter and standard de Sitter
scenarios.Comment: 20 pages, to appear on PRD. Already published background material has
been either settled up in a more compact form or eliminate
Relaxed MHD states of a multiple region plasma
We calculate the stability of a multiple relaxation region MHD (MRXMHD)
plasma, or stepped-Beltrami plasma, using both variational and tearing mode
treatments. The configuration studied is a periodic cylinder. In the
variational treatment, the problem reduces to an eigenvalue problem for the
interface displacements. For the tearing mode treatment, analytic expressions
for the tearing mode stability parameter , being the jump in the
logarithm in the helical flux across the resonant surface, are found. The
stability of these treatments is compared for displacements of an
illustrative RFP-like configuration, comprising two distinct plasma regions.
For pressure-less configurations, we find the marginal stability conclusions of
each treatment to be identical, confirming analytic results in the literature.
The tearing mode treatment also resolves ideal MHD unstable solutions for which
: these correspond to displacement of a resonant interface.
Wall stabilisation scans resolve the internal and external ideal kink. Scans
with increasing pressure are also performed: these indicate that both
variational and tearing mode treatments have the same stability trends with
, and show pressure stabilisation in configurations with increasing edge
pressure. Combined, our results suggest that MRXMHD configurations which are
stable to ideal perturbations plus tearing modes are automatically in a stable
state. Such configurations, and their stability properties, are of emerging
importance in the quest to find mathematically rigorous solutions of ideal MHD
force balance in 3D geometry.Comment: 11 pages, 3 figures, 22nd IAEA Fusion Energy Conference, Geneva,
Switzerland. Submitted to Nuclear Fusio
Synergistic warm inflation
We consider an alternative warm inflationary scenario in which scalar
fields coupled to a dissipative matter fluid cooperate to produce power--law
inflation. The scalar fields are driven by an exponential potential and the
bulk dissipative pressure coefficient is linear in the expansion rate. We find
that the entropy of the fluid attains its asymptotic value in a characteristic
time proportional to the square of the number of fields. This scenario remains
nearly isothermal along the inflationary stage. The perturbations in energy
density and entropy are studied in the long--wavelength regime and seen to grow
roughly as the square of the scale factor. They are shown to be compatible with
COBE measurements of the fluctuations in temperature of the CMB.Comment: 13 pages, Revtex 3 To be published in Physical Review
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