521 research outputs found
Full O(alpha) Radiative Corrections to High-Energy Compton Scattering
Using computer-algebraic methods we derive compact analytical expressions for
the virtual electroweak radiative corrections to polarized Compton scattering.
Moreover the helicity amplitudes for double Compton scattering, which prove to
be extremely simple in terms of Weyl-van der Waerden spinor products, are
presented for massless electrons. The inclusion of a finite electron mass is
described, too. Finally numerical results both for the purely photonic and the
full O(alpha) electroweak corrections, which turn out to be of the order of
5-10%, are discussed for energies ranging from 10GeV to 2TeV.Comment: 20 pages, LaTex (uuencoded figures appended on tex-file), Bi-TP 93/6
Lidar and Deep Learning Reveal Forest Structural Controls on Snowpack
Forest structure has a strong relationship with abiotic components of the environment. For example, canopy morphology controls snow depth through interception and modifies incoming thermal radiation. In turn, snow water availability affects forest growth, carbon sequestration, and nutrient cycling. We investigated how structural diversity and topography affect snow depth patterns across scales. The study site, Grand Mesa, Colorado, is representative of many areas worldwide where declining snowpack and its consequences for forest ecosystems are increasingly an environmental concern. On the basis of a convolution neural network model (R2 of 0.64; root mean squared error of 0.13 m), we found that forest structural and topographic metrics from airborne light detection and ranging (lidar) at fine scales significantly influence snow depth during the accumulation season. Moreover, complex vertically arranged foliage intercepts more snow and results in shallower snow depths below the canopy. Assessing forest structural controls on snow distribution and depth will aid efforts to improve understanding of the ecological and hydrological impacts of changing snow patterns
The Electric Charge of Neutrinos and Plasmon Decay
By using both thermal field theory and a somewhat more intuitive method, we
define the electric charge as well as the charge radius of neutrinos
propagating inside a plasma. We show that electron neutrinos acquire a charge
radius of order cm, regardless of the properties of
the medium. Then, we compute the rate of plasmon decay which such an electric
charge or a charge radius implies. Taking into account the relativistic effects
of the degenerate electron gas, we compare our results to various
approximations as well as to recent calculations and determine the regimes
where the electric charge or the charge radius does mediate the decay of
plasmons. Finally, we discuss the stellar limits on any anomalous charge radius
of neutrinos.Comment: 19pp, 4 figures (available upon request), CERN-TH-7076/9
Characterizing Genetic Diversity of Contemporary Pacific Chickens Using Mitochondrial DNA Analyses
Background\ud
Mitochondrial DNA (mtDNA) hypervariable region (HVR) sequences of prehistoric Polynesian chicken samples reflect dispersal of two haplogroups—D and E—by the settlers of the Pacific. The distribution of these chicken haplogroups has been used as an indicator of human movement. Recent analyses suggested similarities between prehistoric Pacific and South American chicken samples, perhaps reflecting prehistoric Polynesian introduction of the chicken into South America. These analyses have been heavily debated. The current distribution of the D and E lineages among contemporary chicken populations in the Western Pacific is unclear, but might ultimately help to inform debates about the movements of humans that carried them.\ud
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Objectives\ud
We sought to characterize contemporary mtDNA diversity among chickens in two of the earliest settled archipelagoes of Remote Oceania, the Marianas and Vanuatu.\ud
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Methods\ud
We generated HVR sequences for 43 chickens from four islands in Vanuatu, and for 5 chickens from Guam in the Marianas.\ud
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Results\ud
Forty samples from Vanuatu and three from Guam were assigned to haplogroup D, supporting this as a Pacific chicken haplogroup that persists in the Western Pacific. Two haplogroup E lineages were observed in Guam and two in Vanuatu. Of the E lineages in Vanuatu, one was identical to prehistoric Vanuatu and Polynesian samples and the other differed by one polymorphism. Contrary to our expectations, we observed few globally distributed domesticate lineages not associated with Pacific chicken dispersal. This might suggest less European introgression of chickens into Vanuatu than expected. If so, the E lineages might represent lineages maintained from ancient Pacific chicken introductions. The Vanuatu sample might thus provide an opportunity to distinguish between maintained ancestral Pacific chicken lineages and replacement by global domesticates through genomic analyses, which could resolve questions of contemporary haplogroup E chicken relationships and inform interpretations of debated sequences from archaeological samples
Turing patterns in a fiber laser with a nested microresonator: robust and controllable microcomb generation
Microcombs based on Turing patterns have been extensively studied in configurations that can be modelled by the Lugiato-Lefever equation. Typically, such schemes are implemented experimentally by resonant coupling of a continuous wave laser to a Kerr microcavity in order to generate highly coherent and robust waves. Here, we study the formation of such patterns in a system composed of a microresonator nested in an amplifying laser cavity, a scheme recently used to demonstrate laser cavity solitons with high optical efficiency and easy repetition rate control. Utilizing this concept, we study different regimes of Turing patterns, unveiling their formation dynamics and demonstrating their controllability and robustness. By conducting a comprehensive modulational instability study with a mean-field model of the system, we explain the pattern formation in terms of its evolution from background noise, paving the way towards complete self-starting operation. Our theoretical and experimental paper provides a clear pathway for repetition rate control of these waves over both fine (Megahertz) and large (Gigahertz) scales, featuring a fractional frequency nonuniformity better than 7 × 10−14 with a 100-ms time gate and without the need for active stabilization
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Effectiveness and safety of mechanical thrombectomy for acute ischaemic stroke in Latin American countries: A systematic review
YesTo describe the use, effectiveness, and safety outcomes of mechanical thrombectomy (MT) for acute ischemic stroke (AIS) in Latin American countries.
Studies reporting MT outcomes for AIS in Latin America were identified in CINAHL, MEDLINE, Web of Science, SciELO, EMBASE, and LILACS databases. Synthesis was conducted according to effectiveness (recanalization rates) and safety measures (mortality and functional independence at 90 days).
Seventeen studies were included, mainly from public and university hospitals. MT utilization varied from 2.6% to 50.1%, while successful recanalization ranged from 63% to 95%. Functional independence 90 days after stroke (a modified Rankin scale score of 0 to 2) was achieved in less than 40% across most studies. Mortality rates were below 30%; studies with posterior circulation strokes reported higher mortality rates. The randomized trial reported better health outcomes for functional independence among patients in the MT group (OR 2.28; 95% CI, 1.41 - 3.69), favoring MT over standard care.
The included studies had great methodological heterogeneity due to differences in study design, the MT time window, and stroke location. The only randomized trial showed improved functional independence and lower mortality rates with MT than with standard care. The rest of the studies reported similar findings to available literature. Efforts to improve stroke care are reflected in improved patient outcomes in the region. Future studies should consider standard time window criteria and reduce the risk of bias by including representative samples and comparison groups
Effortless attention as a biomarker for experienced mindfulness practitioners
Objective: The present study aimed at comparing frontal beta power between long-term (LTM) and first-time meditators (FTM), before, during and after a meditation session. We hypothesized that LTM would present lower beta power than FTM due to lower effort of attention and awareness. Methods: Twenty one participants were recruited, eleven of whom were long-term meditators. The subjects were asked to rest for 4 minutes before and after open monitoring (OM) meditation (40 minutes). Results: The two-way ANOVA revealed an interaction between the group and moment factors for the Fp1 (p<0.01), F7 (p = 0.01), F3 (p<0.01), Fz (p<0.01), F4 (p<0.01), F8 (p<0.01) electrodes. Conclusion: We found low power frontal beta activity for LTM during the task and this may be associated with the fact that OM is related to bottom-up pathways that are not present in FTM. Significance: We hypothesized that the frontal beta power pattern may be a biomarker for LTM. It may also be related to improving an attentive state and to the efficiency of cognitive functions, as well as to the long-term experience with meditation (i.e., life-time experience and frequency of practice)
Nonadiabatic Dynamics of Atoms in Nonuniform Magnetic Fields
Dynamics of neutral atoms in nonuniform magnetic fields, typical of
quadrupole magnetic traps, is considered by applying an accurate method for
solving nonlinear systems of differential equations. This method is more
general than the adiabatic approximation and, thus, permits to check the limits
of the latter and also to analyze nonadiabatic regimes of motion. An unusual
nonadiabatic regime is found when atoms are confined from one side of the
z-axis but are not confined from another side. The lifetime of atoms in a trap
in this semi-confining regime can be sufficiently long for accomplishing
experiments with a cloud of such atoms. At low temperature, the cloud is
ellipsoidal being stretched in the axial direction and moving along the z-axis.
The possibility of employing the semi-confining regime for studying the
relative motion of one component through another, in a binary mixture of gases
is discussed.Comment: 1 file, 17 pages, RevTex, 2 table
Laser cavity-soliton microcombs
Microcavity-based frequency combs, or ‘microcombs’1,2, have enabled many fundamental breakthroughs3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21 through the discovery of temporal cavity-solitons. These self-localized waves, described by the Lugiato–Lefever equation22, are sustained by a background of radiation usually containing 95% of the total power23. Simple methods for their efficient generation and control are currently being investigated to finally establish microcombs as out-of-the-lab tools24. Here, we demonstrate microcomb laser cavity-solitons. Laser cavity-solitons are intrinsically background-free and have underpinned key breakthroughs in semiconductor lasers22,25,26,27,28. By merging their properties with the physics of multimode systems29, we provide a new paradigm for soliton generation and control in microcavities. We demonstrate 50-nm-wide bright soliton combs induced at average powers more than one order of magnitude lower than the Lugiato–Lefever soliton power threshold22, measuring a mode efficiency of 75% versus the theoretical limit of 5% for bright Lugiato–Lefever solitons23. Finally, we can tune the repetition rate by well over a megahertz without any active feedback
Stability of laser cavity-solitons for metrological applications
Laser cavity-solitons can appear in systems comprised of a nonlinear microcavity nested within an amplifying fiber loop. These states are robust and self-emergent and constitute an attractive class of solitons that are highly suitable for microcomb generation. Here, we present a detailed study of the free-running stability properties of the carrier frequency and repetition rate of single solitons, which are the most suitable states for developing robust ultrafast and high repetition rate comb sources. We achieve free-running fractional stability on both optical carrier and repetition rate (i.e., 48.9 GHz) frequencies on the order of 10^-9 for a 1 s gate time. The repetition rate results compare well with the performance of state-of-the-art (externally driven) microcomb sources, and the carrier frequency stability is in the range of performance typical of modern free-running fiber lasers. Finally, we show that these quantities can be controlled by modulating the laser pump current and the cavity length, providing a path for active locking and long-term stabilization
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