6,979 research outputs found
Optimal search strategies of space-time coupled random walkers with finite lifetimes
We present a simple paradigm for detection of an immobile target by a
space-time coupled random walker with a finite lifetime. The motion of the
walker is characterized by linear displacements at a fixed speed and
exponentially distributed duration, interrupted by random changes in the
direction of motion and resumption of motion in the new direction with the same
speed. We call these walkers "mortal creepers". A mortal creeper may die at any
time during its motion according to an exponential decay law characterized by a
finite mean death rate . While still alive, the creeper has a finite
mean frequency of change of the direction of motion. In particular, we
consider the efficiency of the target search process, characterized by the
probability that the creeper will eventually detect the target. Analytic
results confirmed by numerical results show that there is an
-dependent optimal frequency that maximizes the
probability of eventual target detection. We work primarily in one-dimensional
() domains and examine the role of initial conditions and of finite domain
sizes. Numerical results in domains confirm the existence of an optimal
frequency of change of direction, thereby suggesting that the observed effects
are robust to changes in dimensionality. In the case, explicit
expressions for the probability of target detection in the long time limit are
given. In the case of an infinite domain, we compute the detection probability
for arbitrary times and study its early- and late-time behavior. We further
consider the survival probability of the target in the presence of many
independent creepers beginning their motion at the same location and at the
same time. We also consider a version of the standard "target problem" in which
many creepers start at random locations at the same time.Comment: 18 pages, 7 figures. The title has been changed with respect to the
one in the previous versio
Local habitat disturbance increases bird nest predation in the Brazilian Atlantic rainforest
La perturbación del hábitat local aumenta la depredación de nidos de aves en la pluviselva atlántica del Brasil Evaluamos los efectos de la perturbación antropógena en el bosque atlántico del Brasil. Se distribuyeron nidos artificiales en fragmentos con distintos grados de perturbación antropógena. Observamos una mayor proporción de depredación de huevos en el suelo y en los fragmentos clasificados como de perturbación alta y media que en los fragmentos con un bajo grado de perturbación. La mayor depredación de huevos está probablemente relacionada con una vegetación de complejidad estructural baja y con la elevada accesibilidad de estas zonas para depredadores oportunistas. Con vistas a mantener la biodiversidad de especies de aves, proponemos que se conserven los fragmentos forestales con vegetación de complejidad estructural elevada y escasa actividad humana.We evaluated the effect of anthropogenic disturbance on nest predation in Brazilian Atlantic forest. Artificial nests were distributed in fragments with distinct degrees of anthropogenic disturbance. We found a higher proportion of egg predation on the ground and in the fragments classified as ‘high’ and ‘medium’ disturbance than in the fragments classified as ‘low’ degree of disturbance. The higher egg predation is probably linked to low structural complexity of vegetation and high accessibility of these areas to opportunistic predators. We suggest that forest fragments with high vegetation complexity and low human activity should be preserved in order to maintain the biodiversity of bird species.La perturbación del hábitat local aumenta la depredación de nidos de aves en la pluviselva atlántica del Brasil Evaluamos los efectos de la perturbación antropógena en el bosque atlántico del Brasil. Se distribuyeron nidos artificiales en fragmentos con distintos grados de perturbación antropógena. Observamos una mayor proporción de depredación de huevos en el suelo y en los fragmentos clasificados como de perturbación alta y media que en los fragmentos con un bajo grado de perturbación. La mayor depredación de huevos está probablemente relacionada con una vegetación de complejidad estructural baja y con la elevada accesibilidad de estas zonas para depredadores oportunistas. Con vistas a mantener la biodiversidad de especies de aves, proponemos que se conserven los fragmentos forestales con vegetación de complejidad estructural elevada y escasa actividad humana
Topological insulator particles as optically induced oscillators: towards dynamical force measurements and optical rheology
We report the first experimental study upon the optical trapping and
manipulation of topological insulator (TI) particles. By virtue of the unique
TI properties, which have a conducting surface and an insulating bulk, the
particles present a peculiar behaviour in the presence of a single laser beam
optical tweezers: they oscillate in a plane perpendicular to the direction of
the laser propagation, as a result of the competition between radiation
pressure and gradient forces. In other words, TI particles behave as optically
induced oscillators, allowing dynamical measurements with unprecedented
simplicity and purely optical control. Actually, optical rheology of soft
matter interfaces and biological membranes, as well as dynamical force
measurements in macromolecules and biopolymers, may be quoted as feasible
possibilities for the near future.Comment: 6 pages, 5 figures. Correspondence and requests for Supplementary
Material should be addressed to [email protected]
Reduction of quantum noise in optical interferometers using squeezed light
We study the photon counting noise in optical interferometers used for
gravitational wave detection. In order to reduce quantum noise a squeezed
vacuum state is injected into the usually unused input port. Here, we
specifically investigate the so called `dark port case', when the beam splitter
is oriented close to 90{\deg} to the incoming laser beam, such that nearly all
photons go to one output port of the interferometer, and only a small fraction
of photons is seen in the other port (`dark port'). For this case it had been
suggested that signal amplification is possible without concurrent noise
amplification [R.Barak and Y.Ben-Aryeh, J.Opt.Soc.Am.B25(361)2008]. We show
that by injection of a squeezed vacuum state into the second input port,
counting noise is reduced for large values of the squeezing factor, however the
signal is not amplified. Signal strength only depends on the intensity of the
laser beam.Comment: 8 pages, 1 figur
Neurofibromatosis without Neurofibromas: Confirmation of a Genotype-Phenotype Correlation and Implications for Genetic Testing
Neurofibromatosis type 1 (NF1) is a multisystem disease with autosomal dominant inheritance and complete penetrance diagnosed by clinical findings. Cutaneous neurofibromas are present in almost all adult patients in the dermis, epidermis or along the peripheral nerves. Plexiform neurofibromas are subcutaneous or deep lesions involving nerve plexuses or roots. Neurofibromas can degenerate into malignant tumors, with important prognostic implications. NF1 shows a broad clinic variability even within a single family. Exceptions are cases reporting the in-frame microdeletion c.2970_2972delAAT, presenting with the typical pigmentary features of NF1, but no cutaneous or plexiform neurofibromas. We report a patient with a de novo c.2970_2972delAAT mutation who had few café-au-lait spots, only 2 of which measured >15 mm, axillary and submammary freckling, a flat angioma extending over the neck, arm and trunk, a high arched palate, micrognathia, macrocephaly, pes cavus and scoliosis. There was complete absence of observable cutaneous neurofibromas as well as external plexiform neurofibromas. She had had epileptic seizures since childhood; however, a diagnosis of NF1 had not been confirmed until she was 38, partly due to the paucity of characteristic cutaneous stigmata. We confirm the association of the c.2970_2972delAAT mutation in NF1 with a particular clinical phenotype, especially with lack of detectable neurofibromas. For an appropriate management of patients and family counseling, molecular study of the NF1 gene should be considered in patients not fulfilling NIH criteria when other features suggestive of NF1 are present. In the absence of neurofibromas, starting NF1 testing with the screening of exon 17 may be worthwhile
Anomalous Higgs Couplings
We review the effects of new effective interactions on the Higgs boson
phenomenology. New physics in the electroweak bosonic sector is expected to
induce additional interactions between the Higgs doublet field and the
electroweak gauge bosons leading to anomalous Higgs couplings as well as to
anomalous gauge-boson self-interactions. Using a linearly realized invariant effective Lagrangian to describe the bosonic sector of
the Standard Model, we review the effects of the new effective interactions on
the Higgs boson production rates and decay modes. We summarize the results from
searches for the new Higgs signatures induced by the anomalous interactions in
order to constrain the scale of new physics in particular at CERN LEP and
Fermilab Te vatron colliders.Comment: 35 pages, latex using epsfig.sty psfig.sty and axodraw.sty, 16
postscript figure
Noise Kernel in Stochastic Gravity and Stress Energy Bi-Tensor of Quantum Fields in Curved Spacetimes
The noise kernel is the vacuum expectation value of the (operator-valued)
stress-energy bi-tensor which describes the fluctuations of a quantum field in
curved spacetimes. It plays the role in stochastic semiclassical gravity based
on the Einstein-Langevin equation similar to the expectation value of the
stress-energy tensor in semiclassical gravity based on the semiclassical
Einstein equation. According to the stochastic gravity program, this two point
function (and by extension the higher order correlations in a hierarchy) of the
stress energy tensor possesses precious statistical mechanical information of
quantum fields in curved spacetime and, by the self-consistency required of
Einstein's equation, provides a probe into the coherence properties of the
gravity sector (as measured by the higher order correlation functions of
gravitons) and the quantum nature of spacetime. It reflects the low and medium
energy (referring to Planck energy as high energy) behavior of any viable
theory of quantum gravity, including string theory. It is also useful for
calculating quantum fluctuations of fields in modern theories of structure
formation and for backreaction problems in cosmological and black holes
spacetimes.
We discuss the properties of this bi-tensor with the method of
point-separation, and derive a regularized expression of the noise-kernel for a
scalar field in general curved spacetimes. One collorary of our finding is that
for a massless conformal field the trace of the noise kernel identically
vanishes. We outline how the general framework and results derived here can be
used for the calculation of noise kernels for Robertson-Walker and
Schwarzschild spacetimes.Comment: 22 Pages, RevTeX; version accepted for publication in PR
Potential control of toxic cyanobacteria blooms with Moroccan seaweed extracts
Marine macroalgae are a promising source of diverse bioactive compounds with applications in the biocontrol of harmful cyanobacteria blooms (cyanoHABs). In this work, we evaluated the potential algicidal activities of 14 species of seaweed collected from the coast of Souiria Laqdima, Morocco. Methanol extracts were screened in solid and liquid medium against the growth of the toxic cyanobacteria Microcystis aeruginosa and the microalgae Chlorella sp. used as food supplement. The results in solid medium revealed that the algicidal activity was limited to M. aeruginosa with the extract of Bornetia secundiflora showing the highest growth inhibition activity against Microcystis (27.33 ± 0.33 mm), whereas the extracts of Laminaria digitata, Halopytis incurvus, Ulva lactuca, and Sargasum muticum showed no inhibition. In liquid medium, the results indicated that all methanolic extracts of different macroalgae tested have a significant inhibitory effect on M. aeruginosa compared with that of the negative control. The maximum inhibition rates of M. aeruginosa were produced by the extracts of Bifurcaria tuberculata, Codium elongatum, and B. secundiflora. Moreover, the extracts of B. secundiflora recorded the maximum inhibition rate of Chlorella sp. Overall, the results highlight the potential of the extracts from macroalgae to control toxic cyanobacteria species.We acknowledge the projects TOXICROP (823860) funded by the H2020 program MSCA-RISE-2018 and the project VALORMAR (24517) of the 10/SI/2016-I&DT Empresarial- Programas Mobilizadores funded by the European Regional Development Fund (ERDF) and by the European Social Fund (ESF)
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