81 research outputs found
Transmission Properties of the oscillating delta-function potential
We derive an exact expression for the transmission amplitude of a particle
moving through a harmonically driven delta-function potential by using the
method of continued-fractions within the framework of Floquet theory. We prove
that the transmission through this potential as a function of the incident
energy presents at most two real zeros, that its poles occur at energies
(), and that the
poles and zeros in the transmission amplitude come in pairs with the distance
between the zeros and the poles (and their residue) decreasing with increasing
energy of the incident particle. We also show the existence of non-resonant
"bands" in the transmission amplitude as a function of the strength of the
potential and the driving frequency.Comment: 21 pages, 12 figures, 1 tabl
Knocking back invasions: variable resistance and resilience to multiple cold spells in native vs. nonnative fishes
Extreme climate events can interact synergistically with invasions to dramatically alter ecosystem structure, function, and services. Yet, the effects of extreme climate events on species invasions remain unresolved. Extreme climate events may increase resources and decrease biotic resistance by causing physiological stress and/or mortality of native taxa, resulting in invasion opportunities for nonnative species. Alternatively, extreme climate events may regulate nonnative populations, preventing them from achieving dominance. We examined whether a sequence of three cold spells had a negative or positive effect on fish invasions in the coastal Everglades. We compared resistance (initial effects) and resilience (rate of recovery) to the cold spells between native fishes and the dominant nonnative invader, the Mayan cichlid, across eight populations expanding two mangroves drainages in the southern Everglades. We tracked native fish and nonnative Mayan cichlid populations for 10 yr including 3 yr pre- and 4 yr post-cold spells. In both drainages, native fishes were more resistant to the cold spells than the nonnative species. While native fishes experienced declines at only one site, nonnative Mayan numbers were reduced by 90–100% across six sites where they were abundant pre-disturbances. Four years after the last cold spell, we saw limited resilience in the affected nonnative populations. Only one of the six affected sites fully recovered, whereas the other five sites showed no recovery in Mayan cichlid numbers. The recovered site was closest to a canal, known to act as thermal refuges for nonnative fishes. In summary, cold spells can reduce nonnative abundances, but whether cold spells can effectively knock back invasions (and range expansions) by tropical/subtropical nonnative species will depend on how the frequency and severity of cold spells are affected by climate change. We propose that these mortality-causing extreme events could provide rare management opportunities late in an invasion
Relativistic Effects in the Electromagnetic Current at GeV Energies
We employ a recent approach to the non-relativistic reduction of the
electromagnetic current operator in calculations of electronuclear reactions.
In contrast to the traditional scheme, where approximations are made for the
transferred momentum, transferred energy and initial momentum of the struck
nucleon in obtaining an on-shell inspired form for the current, we treat the
problem exactly for the transferred energy and transferred momentum. We
calculate response functions for the reaction at CEBAF (TJNAF)
energies and find large relativistic corrections. We also show that in Plane
Wave Impulse Approximation, it is always possible to use the full operator, and
we present a comparison of such a limiting case with the results incorporating
relativistic effects to the first order in the initial momentum of the struck
nucleon.Comment: 31 pages, 8 figures, Revte
Quark-hadron duality in a relativistic, confining model
Quark-hadron duality is an interesting and potentially very useful
phenomenon, as it relates the properly averaged hadronic data to a perturbative
QCD result in some kinematic regions. While duality is well established
experimentally, our current theoretical understanding is still incomplete. We
employ a simple model to qualitatively reproduce all the features of
Bloom-Gilman duality as seen in electron scattering. In particular, we address
the role of relativity, give an explicit analytic proof of the equality of the
hadronic and partonic scaling curves, and show how the transition from coherent
to incoherent scattering takes place.Comment: This paper is dedicated to the memory of our collaborator Nathan
Isgur. (34 pages, 13 figures
Model Calculations for the Two-Fragment Electro-Disintegration of He
Differential cross sections for the electro-disintegration process are calculated, using a model in which
the final state interaction is included by means of a nucleon-nucleus (3+1)
potential constructed via Marchenko inversion. The required bound-state wave
functions are calculated within the integrodifferential equation approach
(IDEA). In our model the important condition that the initial bound state and
the final scattering state are orthogonal is fulfilled. The sensitivity of the
cross section to the input interaction in certain kinematical regions
is investigated. The approach adopted could be useful in reactions involving
few cluster systems where effective interactions are not well known and exact
methods are presently unavailable. Although, our Plane-Wave Impulse
Approximation results exhibit, similarly to other calculations, a dip in the
five-fold differential cross-section around a missing momentum of , it is argued that this is an artifact of the omission of re-scattering
four-nucleon processes.Comment: 16 pages, 6 figures, accepted for publication by Phys.Rev.
Privaros: A Framework for Privacy-Compliant Delivery Drones
We present Privaros, a framework to enforce privacy policies on drones.
Privaros is designed for commercial delivery drones, such as the ones that will
likely be used by Amazon Prime Air. Such drones visit a number of host
airspaces, each of which may have different privacy requirements. Privaros
provides an information flow control framework to enforce the policies of these
hosts on the guest delivery drones. The mechanisms in Privaros are built on top
of ROS, a middleware popular in many drone platforms. This paper presents the
design and implementation of these mechanisms, describes how policies are
specified, and shows that Privaros's policy specification can be integrated
with India's Digital Sky portal. Our evaluation shows that a drone running
Privaros can robustly enforce various privacy policies specified by hosts, and
that its core mechanisms only marginally increase communication latency and
power consumption
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