25,748 research outputs found
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
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
Pulsar Prospects for the Cherenkov Telescope Array
In the last few years, the Fermi-LAT telescope has discovered over a 100
pulsars at energies above 100 MeV, increasing the number of known gamma-ray
pulsars by an order of magnitude. In parallel, imaging Cherenkov telescopes,
such as MAGIC and VERITAS, have detected for the first time VHE pulsed
gamma-rays from the Crab pulsar. Such detections have revealed that the Crab
VHE spectrum follows a power-law up to at least 400 GeV, challenging most
theoretical models, and opening wide possibilities of detecting more pulsars
from the ground with the future Cherenkov Telescope Array (CTA). In this
contribution, we study the capabilities of CTA for detecting Fermi pulsars. For
this, we extrapolate their spectra with "Crab-like" power-law tails in the VHE
range, as suggested by the latest MAGIC and VERITAS results.Comment: 4 pages, 3 figures. In Proceedings of the 2012 Heidelberg Symposium
on High Energy Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
New Constraints on Neutralino Dark Matter in the Supersymmetric Standard Model
We investigate the prospects for neutralino dark matter within the
Supersymmetric Standard Model (SSM) including the constraints from universal
soft supersymmetry breaking and radiative breaking of the electroweak symmetry.
The latter is enforced by using the one-loop Higgs effective potential which
automatically gives the one-loop corrected Higgs boson masses. We perform an
exhaustive search of the allowed five-dimensional parameter space and find that
the neutralino relic abundance depends most strongly on the
ratio . For the relic abundance is almost
always much too large, whereas for the opposite occurs. For
there are wide ranges of the remaining parameters for which
. We also determine that m_{\tilde q}\gsim250\GeV and
m_{\tilde l}\gsim100\GeV are necessary in order to possibly achieve
. These lower bounds are much weaker than the corresponding
ones derived previously when radiative breaking was {\it not} enforced.Comment: 12 pages plus 6 figures (not included), CERN-TH.6584/92,
CTP-TAMU-56/92, UAHEP921
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