1,783 research outputs found
Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system
The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very
short flash of Cherenkov radiation generated by the cascade of relativistic
charged particles produced when a TeV gamma ray strikes the atmosphere. This
energetic air shower is initiated at an altitude of 10-30 km depending on the
energy and the arrival direction of the primary gamma ray. Whether the best
image of the shower is obtained by focusing the telescope at infinity and
measuring the Cherenkov photon angles or focusing on the central region of the
shower is a not obvious question. This is particularly true for large size IACT
for which the depth of the field is much smaller. We address this issue in
particular with the fifth telescope (CT5) of the High Energy Stereoscopic
System (H.E.S.S.); a 28 m dish large size telescope recently entered in
operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped
with a focus system, its working principle and the expected effect of focusing
depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil
A new look at the cosmic ray positron fraction
The positron fraction in cosmic rays was found to be a steadily increasing in
function of energy, above 10 GeV. This behaviour contradicts standard
astrophysical mechanisms, in which positrons are secondary particles, produced
in the interactions of primary cosmic rays during the propagation in the
interstellar medium. The observed anomaly in the positron fraction triggered a
lot of excitement, as it could be interpreted as an indirect signature of the
presence of dark matter species in the Galaxy. Alternatively, it could be
produced by nearby astrophysical sources, such as pulsars. Both hypotheses are
probed in this work in light of the latest AMS-02 positron fraction
measurements. The transport of the primary and secondary positrons in the
Galaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used to
model the positron flux generated by dark matter species. The description of
the positron fraction from astrophysical sources is based on the pulsar
observations included in the ATNF catalogue. We find that the mass of the
favoured dark matter candidates is always larger than 500 GeV. The only dark
matter species that fulfils the numerous gamma ray and cosmic microwave
background bounds is a particle annihilating into four leptons through a light
scalar or vector mediator, with a mixture of tau (75%) and electron (25%)
channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also be
explained by a single astrophysical source and a list of five pulsars from the
ATNF catalogue is given. Those results are obtained with the cosmic ray
transport parameters that best fit the B/C ratio. Uncertainties in the
propagation parameters turn out to be very significant. In the WIMP
annihilation cross section to mass plane for instance, they overshadow the
error contours derived from the positron data.Comment: 20 pages, 16 figures, accepted for publication in A&A, corresponds to
published versio
Continuous Uniform Finite Time Stabilization of Planar Controllable Systems
Continuous homogeneous controllers are utilized in a full state feedback setting for the uniform finite time stabilization of a perturbed double integrator in the presence of uniformly decaying piecewise continuous disturbances. Semiglobal strong Lyapunov functions are identified to establish uniform asymptotic stability of the closed-loop planar system. Uniform finite time stability is then proved by extending the homogeneity principle of discontinuous systems to the continuous case with uniformly decaying piecewise continuous nonhomogeneous disturbances. A finite upper bound on the settling time is also computed. The results extend the existing literature on homogeneity and finite time stability by both presenting uniform finite time stabilization and dealing with a broader class of nonhomogeneous disturbances for planar controllable systems while also proposing a new class of homogeneous continuous controllers
Modeling Antarctic tides in response to ice shelf thinning and retreat
Tides play an important role in ice sheet dynamics by modulating ice stream velocity, fracturing, and moving ice shelves and mixing water beneath them. Any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability. Here, we show that removal or reduction in extent and/or thickness of the Ross and Ronne-Filchner ice shelves would have a significant impact on the tides around Antarctica. The Ronne-Filchner appears particularly vulnerable, with an increase in M2 amplitude of over 0.5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf/sheet dynamics. These results highlight the importance of understanding tidal feedbacks on ice shelves/streams due to their influence on ice sheet dynamics
Avaliação do potencial de cinco cultivares de videiras americanas para sucos de uva no sul de Minas Gerais.
Objetivou-se, no presente trabalho realizar uma caracterização dos sucos de uva de cinco cultivares de videira americana Vitis labrusca (Folha de Figo, Alwood, Concord, BRS-RĂșbea e Isabel) segundo suas caracterĂsticas visuais, olfativas e gustativas, utilizando um mĂ©todo descritivo por um painel de degustadores da Embrapa Uva e Vinho (EMBRAPA/CNPUV) de Bento Gonçalves e da Associação Brasileira de Enologia (ABE), constituĂdo por doze degustadores. De acordo com os resultados, os sucos das cultivares Isabel e Folha de Figo foram superiores aos demais, segundo a anĂĄlise de componentes principais (ACP), principalmente pelas variĂĄveis limpidez, equilĂbrio olfativo, intensidade e corpo gustativo, persistĂȘncia olfato-gustativa e julgamento geral (notas). O suco Folha de Figo foi superior ao Isabel quanto Ă tonalidade, sendo o primeiro caracterizado como vermelho-violĂĄceo. A cultivar Folha de Figo Ă© a mais utilizada na regiĂŁo para a produção de suco, podendo os produtores utilizarem tambĂ©m a cultivar Isabel, que mostrou potencial nas condiçÔes edafo-climĂĄticas do sul de Minas Gerais.DisponĂvel tambĂ©m on-line
Postoperative urinary retention:Risk factors, bladder filling rate and time to catheterization: an observational study as part of a randomized controlled trial
Background: Knowledge of risk factors for postoperative urinary retention may guide appropriate and timely urinary catheterization. We aimed to determine independent risk factors for postoperative urinary catheterization in general surgical patients. In addition, we calculated bladder filling rate and assessed the time to spontaneous voiding or catheterization. We used the patients previously determined individual maximum bladder capacity as threshold for urinary catheterization.Methods: Risk factors for urinary catheterization were prospectively determined in 936 general surgical patients. Patients were at least 18 years of age and operated under general or spinal anesthesia without the need for an indwelling urinary catheter. Patients measured their maximum bladder capacity preoperatively at home, by voiding in a calibrated bowl after a strong urge that could no longer be ignored. Postoperatively, bladder volumes were assessed hourly with ultrasound. When patients reached their maximum bladder capacity and were unable to void, they were catheterized by the nursing staff. Bladder filling rate and time to catheterization were determined.Results: Spinal anesthesia was the main independent modifiable risk factor for urinary catheterization (hyperbaric bupivacaine, relative risk 8.1, articaine RR 3.1). Unmodifiable risk factors were a maximum bladder capacity <500 mL (RR 6.7), duration of surgery >= 60 min (RR 5.5), first scanned bladder volume at the Post Anesthesia Care Unit >= 250mL (RR 2.1), and age >= 60 years (RR 2.0). Urine production varied from 100 to 200 mL/h. Catheterization or spontaneous voiding took place approximately 4 h postoperatively.Conclusion: Spinal anesthesia, longer surgery time, and older age are the main risk factors for urinary retention catheterization. Awareness of these risk factors, regularly bladder volume scanning (at least every 3 h) and using the individual maximum bladder capacity as volume threshold for urinary catheterization may avoid unnecessary urinary catheterization and will prevent bladder overdistention with the attendant risk of lower urinary tract injury.</p
Spin physics at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)
We outline the opportunities for spin physics which are offered by a next
generation and multi-purpose fixed-target experiment exploiting the proton LHC
beam extracted by a bent crystal. In particular, we focus on the study of
single transverse spin asymetries with the polarisation of the target.Comment: Contributed to the 20th International Spin Physics Symposium,
SPIN2012, 17-22 September 2012, Dubna, Russia, 4 pages, LaTe
Thermodynamics of self-gravitating systems
Self-gravitating systems are expected to reach a statistical equilibrium
state either through collisional relaxation or violent collisionless
relaxation. However, a maximum entropy state does not always exist and the
system may undergo a ``gravothermal catastrophe'': it can achieve ever
increasing values of entropy by developing a dense and hot ``core'' surrounded
by a low density ``halo''. In this paper, we study the phase transition between
``equilibrium'' states and ``collapsed'' states with the aid of a simple
relaxation equation [Chavanis, Sommeria and Robert, Astrophys. J. 471, 385
(1996)] constructed so as to increase entropy with an optimal rate while
conserving mass and energy. With this numerical algorithm, we can cover the
whole bifurcation diagram in parameter space and check, by an independent
method, the stability limits of Katz [Mon. Not. R. astr. Soc. 183, 765 (1978)]
and Padmanabhan [Astrophys. J. Supp. 71, 651 (1989)]. When no equilibrium state
exists, our relaxation equation develops a self-similar collapse leading to a
finite time singularity.Comment: 54 pages. 25 figures. Submitted to Phys. Rev.
The HPS electromagnetic calorimeter
The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called âheavy photon.â Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015â2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier
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