707 research outputs found
The Tate conjecture for K3 surfaces over finite fields
Artin's conjecture states that supersingular K3 surfaces over finite fields
have Picard number 22. In this paper, we prove Artin's conjecture over fields
of characteristic p>3. This implies Tate's conjecture for K3 surfaces over
finite fields of characteristic p>3. Our results also yield the Tate conjecture
for divisors on certain holomorphic symplectic varieties over finite fields,
with some restrictions on the characteristic. As a consequence, we prove the
Tate conjecture for cycles of codimension 2 on cubic fourfolds over finite
fields of characteristic p>3.Comment: 20 pages, minor changes. Theorem 4 is stated in greater generality,
but proofs don't change. Comments still welcom
Trophic Dynamics of the Boreal Forests of the Kluane Region
The trophic dynamics of the Yukon boreal forest have been under investigation at the Kluane Lake Research Station since 1973. We monitored and conducted experiments on the major species in this ecosystem, except the large mammals (for logistic reasons). The central problem has been to determine the causes of the 9 – 10 year cycle of snowshoe hares, and to achieve this we carried out several large-scale experiments manipulating food supplies, predator pressure, and soil nutrient availability to test hypotheses that food, predation, or habitat quality regulate populations. The hare cycle is driven top-down by predators, and most hares die because they are killed by predators. Predators also cause stress in female hares, and the stress response seems to be responsible for the loss of reproductive potential in the decline and low phases of the hare cycle. Many of the specialist predators and some herbivores in this ecosystem fluctuate with the hare cycle. Arctic ground squirrels do, but red squirrels do not, being linked closely to white spruce seed masting years. Small rodents fluctuate in numbers in two patterns. Red-backed voles and four species of Microtus voles have a 3 – 4 year cycle that seems to be driven by food supplies and social behaviour. Deer mice, in contrast, have fluctuated dramatically in the 38 years we have monitored them, but not cyclically. White spruce seed production varies with temperature and rainfall, but was not affected by adding nutrients in fertilizer. Global warming and reduced hare browsing in the last 20 years have helped to increase the abundance of shrubs in these forests. It will be challenging to predict how this system will change as climatic warming proceeds, because even closely related species in the same trophic level respond differently to perturbations. We recommend continued monitoring of the major species in these boreal forests.La dynamique trophique de la forêt boréale du Yukon fait l’objet d’une étude à la station de recherche du lac Kluane depuis 1973. Nous avons fait des expériences et surveillé les espèces importantes de cet écosystème, sauf en ce qui a trait aux principaux mammifères (pour des raisons de logistique). Le problème central a consisté à déterminer les causes du cycle de 9 à 10 ans du lièvre d’Amérique. Pour ce faire, nous avons effectué plusieurs expériences à grande échelle dans le cadre desquelles nous avons manipulé les disponibilités alimentaires, la pression exercée par les prédateurs et la disponibilité en nutriments dans le sol afin de mettre à l’épreuve les hypothèses selon lesquelles la nourriture, la prédation ou la qualité de l’habitat régularisent les populations. Le cycle du lièvre est dicté par les prédateurs de haut en bas, et la plupart des lièvres meurent parce qu’ils sont tués par les prédateurs. Par ailleurs, les prédateurs sont une source de stress chez les lièvres femelles, et la réaction au stress semble responsable de la perte de capacité de reproduction dans la phase du déclin et la phase basse du cycle du lièvre. Grand nombre des prédateurs spécialistes et certains herbivores de cet écosystème fluctuent en fonction du cycle du lièvre. C’est le cas du spermophile arctique, mais ce n’est pas le cas de l’écureuil roux, car il est étroitement lié aux années de paisson de graines d’épinette blanche. Le nombre de petits rongeurs fluctue en fonction de deux modèles. Le campagnol à dos roux et quatre espèces de campagnols Microtus ont un cycle de trois à quatre ans qui semble dicté par les disponibilités alimentaires et le comportement social, tandis que la souris sylvestre a connu d’énormes fluctuations pendant les 38 années qui ont fait l’objet d’une surveillance, sans toutefois afficher de cycles. La production de graines d’épinette blanche varie en fonction des températures et des chutes de pluie, mais n’a pas été influencée par l’ajout de nutriments au fertilisant. Le réchauffement planétaire et le broutage réduit des lièvres ces 20 dernières années ont aidé à accroître l’abondance d’arbustes dans ces forêts. Il sera difficile de prévoir comment ce système changera au fur et à mesure du réchauffement climatique, car même les espèces étroitement liées du même niveau trophique réagissent aux perturbations de manière différente. Nous recommandons la surveillance continue des principales espèces de ces forêts boréales
Compile-Time Analysis and Specialization of Clocks in Concurrent Programs
Clocks are a mechanism for providing synchronization barriers in concurrent programming languages. They are usually implemented using primitive communication mechanisms and thus spare the programmer from reasoning about low-level implementation details such as remote procedure calls and error conditions. Clocks provide flexibility, but programs often use them in specific ways that do not require their full implementation. In this paper, we describe a tool that mitigates the overhead of general-purpose clocks by statically analyzing how programs use them and choosing optimized implementations when available. We tackle the clock implementation in the standard library of the X10 programming language—a parallel, distributed object-oriented language. We report our findings for a small set of analyses and benchmarks. Our tool only adds a few seconds to analysis time, making it practical to use as part of a compilation chain
Charmless Decays Based on the six-quark Effective Hamiltonian with Strong Phase Effects II
We provide a systematic study of charmless decays (
and denote pseudoscalar and vector mesons, respectively) based on an
approximate six-quark operator effective Hamiltonian from QCD. The calculation
of the relevant hard-scattering kernels is carried out, the resulting
transition form factors are consistent with the results of QCD sum rule
calculations. By taking into account important classes of power corrections
involving "chirally-enhanced" terms and the vertex corrections as well as weak
annihilation contributions with non-trivial strong phase, we present
predictions for the branching ratios and CP asymmetries of decays into
PP, PV and VV final states, and also for the corresponding polarization
observables in VV final states. It is found that the weak annihilation
contributions with non-trivial strong phase have remarkable effects on the
observables in the color-suppressed and penguin-dominated decay modes. In
addition, we discuss the SU(3) flavor symmetry and show that the symmetry
relations are generally respected
NMR and NQR Fluctuation Effects in Layered Superconductors
We study the effect of thermal fluctuations of the s-wave order parameter of
a quasi two dimensional superconductor on the nuclear spin relaxation rate near
the transition temperature Tc. We consider both the effects of the amplitude
fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations
in weakly coupled layered superconductors. In the treatment of the amplitude
fluctuations we employ the Gaussian approximation and evaluate the longitudinal
relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair
breaking effects, using the static pair fluctuation propagator D. The increase
in 1/T1 due to pair breaking in D is overcompensated by the decrease arising
from the single particle Green's functions. The result is a strong effect on
1/T1 for even a small amount of pair breaking. The phase fluctuations are
described in terms of dynamical BKT excitations in the form of pancake
vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field
fluctuations caused by the translational motion of VA excitations on 1/T1 and
on the transverse relaxation rate 1/T2 on both sides of the BKT transitation
temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly
on the diffusion constant that governs the motion of free and bound vortices as
well as the annihilation of VA pairs. We discuss the relaxation rates for real
multilayer systems where the diffusion constant can be small and thus increase
the lifetime of a VA pair, leading to an enhancement of the rates. We also
discuss in some detail the experimental feasibility of observing the effects of
amplitude fluctuations in layered s-wave superconductors such as the
dichalcogenides and the effects of phase fluctuations in s- or d-wave
superconductors such as the layered cuprates.Comment: 38 pages, 12 figure
Analysis of the Three-Body Decay
The decay process is an interesting channel for the
investigation of CP violating effects in the sector. We write down a decay
amplitude constrained by a low-energy theorem, which also includes the
contribution of resonant and wave beauty and charmed mesons, and we
determine the relevant matrix elements in the infinite heavy quark mass limit,
assuming the factorization ansatz. We estimate the rate of the decay: . We also analyze the
time-independent and time-dependent differential decay distributions,
concluding that a signal for this process should be observed at the
B-factories. Finally, we give an estimate of the decay rate of the
Cabibbo-favoured process .Comment: LaTex, 20 pages, 4 figure
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
Measurement of the p-pbar -> Wgamma + X cross section at sqrt(s) = 1.96 TeV and WWgamma anomalous coupling limits
The WWgamma triple gauge boson coupling parameters are studied using p-pbar
-> l nu gamma + X (l = e,mu) events at sqrt(s) = 1.96 TeV. The data were
collected with the DO detector from an integrated luminosity of 162 pb^{-1}
delivered by the Fermilab Tevatron Collider. The cross section times branching
fraction for p-pbar -> W(gamma) + X -> l nu gamma + X with E_T^{gamma} > 8 GeV
and Delta R_{l gamma} > 0.7 is 14.8 +/- 1.6 (stat) +/- 1.0 (syst) +/- 1.0 (lum)
pb. The one-dimensional 95% confidence level limits on anomalous couplings are
-0.88 < Delta kappa_{gamma} < 0.96 and -0.20 < lambda_{gamma} < 0.20.Comment: Submitted to Phys. Rev. D Rapid Communication
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt{s} = 1.96 TeV using Kinematic Characteristics of Lepton + Jets Events
We present a measurement of the top quark pair ttbar production cross section
in ppbar collisions at a center-of-mass energy of 1.96 TeV using 230 pb**{-1}
of data collected by the DO detector at the Fermilab Tevatron Collider. We
select events with one charged lepton (electron or muon), large missing
transverse energy, and at least four jets, and extract the ttbar content of the
sample based on the kinematic characteristics of the events. For a top quark
mass of 175 GeV, we measure sigma(ttbar) = 6.7 {+1.4-1.3} (stat) {+1.6- 1.1}
(syst) +/-0.4 (lumi) pb, in good agreement with the standard model prediction.Comment: submitted to Phys.Rev.Let
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