30,938 research outputs found
Biases on cosmological parameters by general relativity effects
General relativistic corrections to the galaxy power spectrum appearing at
the horizon scale, if neglected, may induce biases on the measured values of
the cosmological parameters. In this paper, we study the impact of general
relativistic effects on non standard cosmologies such as scenarios with a time
dependent dark energy equation of state, with a coupling between the dark
energy and the dark matter fluids or with non-Gaussianities. We then explore
whether general relativistic corrections affect future constraints on
cosmological parameters in the case of a constant dark energy equation of state
and of non-Gaussianities. We find that relativistic corrections on the power
spectrum are not expected to affect the foreseen errors on the cosmological
parameters nor to induce large biases on them.Comment: 17 pages, 5 figures, one added figure, results of Tab. I revised,
version accepted for publication in PR
Single-photon signals at LEP in supersymmetric models with a light gravitino
We study the single-photon signals expected at LEP in models with a very
light gravitino. The dominant process is neutralino-gravitino production (e+e-
-> chi+ G) with subsequent neutralino decay via chi->gamma+G, giving a
gamma+E_miss signal. We first calculate the cross section at arbitrary
center-of-mass energies and provide new analytic expressions for the
differential cross section valid for general neutralino compositions. We then
consider the constraints on the gravitino mass from LEP 1 and LEP161
single-photon searches, and possible such searches at the Tevatron. We show
that it is possible to evade the stringent LEP 1 limits and still obtain an
observable rate at LEP 2, in particular in the region of parameter space that
may explain the CDF e+e+gamma+gamma+E_T,miss event. As diphoton events from
neutralino pair-production would not be kinematically accessible in this
scenario, the observation of whichever photonic signal will discriminate among
the various light-gravitino scenarios in the literature. We also perform a
Monte Carlo simulation of the expected energy and angular distributions of the
emitted photon, and of the missing invariant mass expected in the events.
Finally we specialize the results to the case of a recently proposed
one-parameter no-scale supergravity model.Comment: 31 pages, LaTeX, 14 figures (included
Damage-induced phosphorylation of Sld3 is important to block late origin firing.
Origins of replication are activated throughout the S phase of the cell cycle such that some origins fire early and others fire late to ensure that each chromosome is completely replicated in a timely fashion. However, in response to DNA damage or replication fork stalling, eukaryotic cells block activation of unfired origins. Human cells derived from patients with ataxia telangiectasia are deficient in this process due to the lack of a functional ataxia telangiectasia mutated (ATM) kinase and elicit radioresistant DNA synthesis after γ-irradiation(2). This effect is conserved in budding yeast, as yeast cells lacking the related kinase Mec1 (ATM and Rad3-related (ATR in humans)) also fail to inhibit DNA synthesis in the presence of DNA damage. This intra-S-phase checkpoint actively regulates DNA synthesis by inhibiting the firing of late replicating origins, and this inhibition requires both Mec1 and the downstream checkpoint kinase Rad53 (Chk2 in humans). However, the Rad53 substrate(s) whose phosphorylation is required to mediate this function has remained unknown. Here we show that the replication initiation protein Sld3 is phosphorylated by Rad53, and that this phosphorylation, along with phosphorylation of the Cdc7 kinase regulatory subunit Dbf4, blocks late origin firing in Saccharomyces cerevisiae. Upon exposure to DNA-damaging agents, cells expressing non-phosphorylatable alleles of SLD3 and DBF4 (SLD3-m25 and dbf4-m25, respectively) proceed through the S phase faster than wild-type cells by inappropriately firing late origins of replication. SLD3-m25 dbf4-m25 cells grow poorly in the presence of the replication inhibitor hydroxyurea and accumulate multiple Rad52 foci. Moreover, SLD3-m25 dbf4-m25 cells are delayed in recovering from transient blocks to replication and subsequently arrest at the DNA damage checkpoint. These data indicate that the intra-S-phase checkpoint functions to block late origin firing in adverse conditions to prevent genomic instability and maximize cell survival
High speed single photon detection in the near-infrared
InGaAs avalanche photodiodes (APDs) are convenient for single photon
detection in the near-infrared (NIR) including the fibre communication bands
(1.31/1.55 m). However, to suppress afterpulse noise due to trapped
avalanche charge, they must be gated with MHz repetition frequencies, thereby
severely limiting the count rate in NIR applications. Here we show gating
frequencies for InGaAs-APDs well beyond 1 GHz. Using a self-differencing
technique to sense much weaker avalanches, we reduce drastically afterpulse
noise. At 1.25 GHz, we obtain a detection efficiency of 10.8% with an
afterpulse probability of 6.16%. In addition, the detector features low jitter
(55 ps) and a count rate of 100 MHz
Wind tunnel and analytical investigation of over-the-wing propulsion/air frame interferences for a short-haul aircraft at Mach numbers from 0.6 to 0.78
Results of analytical calculations and wind tunnel tests at cruise speeds of a representative four engine short haul aircraft employing upper surface blowing (USB) with a supercritical wing are discussed. Wind tunnel tests covered a range of Mach number M from 0.6 to 0.78. Tests explored the use of three USB nozzle configurations. Results are shown for the isolated wing body and for each of the three nozzle types installed. Experimental results indicate that a low angle nacelle and streamline contoured nacelle yielded the same interference drag at the design Mach number. A high angle powered lift nacelle had higher interference drag primarily because of nacelle boattail low pressures and flow separation. Results of varying the spacing between the nacelles and the use of trailing edge flap deflections, wing upper surface contouring, and a convergent-divergent nozzle to reduce potential adverse jet effects were also discussed. Analytical comparisons with experimental data, made for selected cases, indicate favorable agreement
Jets, knots and tails in planetary nebulae: NGC 3918, K 1-2 and Wray 17-1
We analyze optical images and high-resolution, long-slit spectra of three
planetary nebulae which possess collimated, low-ionization features. NGC 3918
is composed of an inner, spindle-shaped shell mildly inclined with respect to
the plane of the sky. Departing from the polar regions of this shell, we find a
two-sided jet expanding with velocities which increase linearly with distance
from 50 to 100 km/s. The jet is probably coeval with the inner shell (with the
age of approximately 1000 D yr, where D is the distance in kpc), suggesting
that its formation should be ascribed to the same dynamical processes which
also shaped the main nebula, and not to a more recent mass loss episode. We
discuss the formation of the aspherical shell and jet in the light of current
hydrodynamical and magnetohydrodynamical theories. K 1-2 is a planetary nebula
with a close binary nucleus which shows a collimated string of knots embedded
in a diffuse, elliptical shell. The knots expand with a velocity similar to
that of the elliptical nebula (25 km/s), except for an extended tail located
out of the main nebula, which linearly accelerates up to 45 km/s. We estimate
an inclination on the line of the sight of 40 degres for the string of knots;
once the orientation of the orbit is also determined, this information will
allow us to test the prediction of current theories of the occurrence of polar
jets from close binary systems. Wray 17-1 has a complex morphology, showing two
pairs of low-ionization structures located in almost perpendicular directions
from the central star, and embedded in a large, diffuse nebula. The two pairs
show notable similarities and differences, and their origin is very puzzling.Comment: 20 pages plus 10 figures. ApJ recently published (ApJ 523, 721
(1999)
Electric field inversion asymmetry: Rashba and Stark effects for holes in resonant tunneling devices
We report experimental evidence of excitonic spin-splitting, in addition to
the conventional Zeeman effect, produced by a combination of the Rashba
spin-orbit interaction, Stark shift and charge screening. The
electric-field-induced modulation of the spin-splitting are studied during the
charging and discharging processes of p-type GaAs/AlAs double barrier resonant
tunneling diodes (RTD) under applied bias and magnetic field. The abrupt
changes in the photoluminescence, with the applied bias, provide information of
the charge accumulation effects on the device.Comment: 4 pages, 2 figure
Modulation of Immune Response in Lambs
Experiments using different types of antigen-adjuvant preparations were conducted in outbred sheep to compare effects of adjuvants on immune responses. Trinitrophenyl-ovalbumin (TNP-ovalbumin) incorporated in a preparation with nonionic block copolymers elicited high antibody titers to both ovalbumin and TNP. Different humoral immune responses were observed when Pasteurella haemolytica lipopolysaccharide (LPS) was added to the preparations. Responses to ovalbumin and TNP were reduced when Pasteurella haemolytica LPS was added to copolymer L121. The antibody titers to ovalbumin or TNP were not affected by the addition of LPS to the preparation containing copolymer L180.5. Lymphocyte proliferation assays demonstrated high stimulation indices at day 17 to ovalbumin by lymphocytes from lambs receiving preparations containing copolymers without Pasteurella haemolytica LPS
Quintessence and cosmic acceleration
A cosmological model with perfect fluid and self-interacting quintessence
field is considered in the framework of the spatially flat
Friedmann-Robertson-Walker (FRW) geometry. By assuming that all physical
quantities depend on the volume scale factor of the Universe, the general
solution of the gravitational field equations can be expressed in an exact
parametric form. The quintessence field is a free parameter. With an
appropriate choice of the scalar field a class of exact solutions is obtained,
with an exponential type scalar field potential fixed via the gravitational
field equations. The general physical behavior of the model is consistent with
the recent cosmological scenario favored by supernova Type Ia observations,
indicating an accelerated expansion of the Universe.Comment: 6 pages, 3 figures, to appear in Int. J. Mod. Phys.
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