1,877 research outputs found
Viscous-Inviscid Interactions in a Boundary-Layer Flow Induced by a Vortex Array
In this paper we investigate the asymptotic validity of boundary layer
theory. For a flow induced by a periodic row of point-vortices, we compare
Prandtl's solution to Navier-Stokes solutions at different numbers. We
show how Prandtl's solution develops a finite time separation singularity. On
the other hand Navier-Stokes solution is characterized by the presence of two
kinds of viscous-inviscid interactions between the boundary layer and the outer
flow. These interactions can be detected by the analysis of the enstrophy and
of the pressure gradient on the wall. Moreover we apply the complex singularity
tracking method to Prandtl and Navier-Stokes solutions and analyze the previous
interactions from a different perspective
Naturalness and Fine Tuning in the NMSSM: Implications of Early LHC Results
We study the fine tuning in the parameter space of the semi-constrained
NMSSM, where most soft Susy breaking parameters are universal at the GUT scale.
We discuss the dependence of the fine tuning on the soft Susy breaking
parameters M_1/2 and m0, and on the Higgs masses in NMSSM specific scenarios
involving large singlet-doublet Higgs mixing or dominant Higgs-to-Higgs decays.
Whereas these latter scenarios allow a priori for considerably less fine tuning
than the constrained MSSM, the early LHC results rule out a large part of the
parameter space of the semi-constrained NMSSM corresponding to low values of
the fine tuning.Comment: 19 pages, 10 figures, bounds from Susy searches with ~1/fb include
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Gate Drive For High Speed, High Power IGBTs
A new gate drive for high-voltage, high-power IGBTs has been developed for the SLAC NLC (Next Linear Collider) Solid State Induction Modulator. This paper describes the design and implementation of a driver that allows an IGBT module rated at 800A/3300V to switch up to 3000A at 2200V in 3{micro}S with a rate of current rise of more than 10000A/{micro}S, while still being short circuit protected. Issues regarding fast turn on, high de-saturation voltage detection, and low short circuit peak current will be presented. A novel approach is also used to counter the effect of unequal current sharing between parallel chips inside most high-power IGBT modules. It effectively reduces the collector-emitter peak current, and thus protects the IGBT from being destroyed during soft short circuit conditions at high di/dt
Sommerfeld Enhancement from Multiple Mediators
We study the Sommerfeld enhancement experienced by a scattering object that
couples to a tower of mediators. This can occur in, e.g., models of secluded
dark matter when the mediator scale is generated naturally by hidden-sector
confinement. Specializing to the case of a confining CFT, we show that
off-resonant values of the enhancement can be increased by ~ 20% for cases of
interest when (i) the (strongly-coupled) CFT admits a weakly-coupled dual
description and (ii) the conformal symmetry holds up to the Planck scale.
Larger enhancements are possible for lower UV scales due to an increase in the
coupling strength of the tower.Comment: 17p, 2 figures; v2 JHEP version (inconsequential typo fixed,
references added
Solid-state Marx based two-switch voltage modulator for the On-Line Isotope Mass Separator accelerator at the European Organization for Nuclear Research
A new circuit topology is proposed to replace the actual pulse transformer and thyratron based resonant modulator that supplies the 60 kV target potential for the ion acceleration of the On-Line Isotope Mass Separator accelerator, the stability of which is critical for the mass resolution downstream separator, at the European Organization for Nuclear Research. The improved modulator uses two solid-state switches working together, each one based on the Marx generator concept, operating as series and parallel switches, reducing the stress on the series stacked semiconductors, and also as auxiliary pulse generator in order to fulfill the target requirements. Preliminary results of a 10 kV prototype, using 1200 V insulated gate bipolar transistors and capacitors in the solid-state Marx circuits, ten stages each, with an electrical equivalent circuit of the target, are presented, demonstrating both the improved voltage stability and pulse flexibility potential wanted for this new modulator
Updated Measurement of the Strong Phase in D0 --> K+pi- Decay Using Quantum Correlations in e+e- --> D0 D0bar at CLEO
We analyze a sample of 3 million quantum-correlated D0 D0bar pairs from 818
pb^-1 of e+e- collision data collected with the CLEO-c detector at E_cm = 3.77
GeV, to give an updated measurement of \cos\delta and a first determination of
\sin\delta, where \delta is the relative strong phase between doubly
Cabibbo-suppressed D0 --> K+pi- and Cabibbo-favored D0bar --> K+pi- decay
amplitudes. With no inputs from other experiments, we find \cos\delta = 0.81
+0.22+0.07 -0.18-0.05, \sin\delta = -0.01 +- 0.41 +- 0.04, and |\delta| = 10
+28+13 -53-0 degrees. By including external measurements of mixing parameters,
we find alternative values of \cos\delta = 1.15 +0.19+0.00 -0.17-0.08,
\sin\delta = 0.56 +0.32+0.21 -0.31-0.20, and \delta = (18 +11-17) degrees. Our
results can be used to improve the world average uncertainty on the mixing
parameter y by approximately 10%.Comment: Minor revisions, version accepted by PR
Studies of the decays D^0 \rightarrow K_S^0K^-\pi^+ and D^0 \rightarrow K_S^0K^+\pi^-
The first measurements of the coherence factor R_{K_S^0K\pi} and the average
strong--phase difference \delta^{K_S^0K\pi} in D^0 \to K_S^0 K^\mp\pi^\pm
decays are reported. These parameters can be used to improve the determination
of the unitary triangle angle \gamma\ in B^- \rightarrow
decays, where is either a D^0 or a D^0-bar meson decaying to
the same final state, and also in studies of charm mixing. The measurements of
the coherence factor and strong-phase difference are made using
quantum-correlated, fully-reconstructed D^0D^0-bar pairs produced in e^+e^-
collisions at the \psi(3770) resonance. The measured values are R_{K_S^0K\pi} =
0.70 \pm 0.08 and \delta^{K_S^0K\pi} = (0.1 \pm 15.7) for an
unrestricted kinematic region and R_{K*K} = 0.94 \pm 0.12 and \delta^{K*K} =
(-16.6 \pm 18.4) for a region where the combined K_S^0 \pi^\pm
invariant mass is within 100 MeV/c^2 of the K^{*}(892)^\pm mass. These results
indicate a significant level of coherence in the decay. In addition, isobar
models are presented for the two decays, which show the dominance of the
K^*(892)^\pm resonance. The branching ratio {B}(D^0 \rightarrow
K_S^0K^+\pi^-)/{B}(D^0 \rightarrow K_S^0K^-\pi^+) is determined to be 0.592 \pm
0.044 (stat.) \pm 0.018 (syst.), which is more precise than previous
measurements.Comment: 38 pages. Version 3 updated to include the erratum information.
Errors corrected in Eqs (25), (26), 28). Fit results updated accordingly, and
external inputs updated to latest best known values. Typo corrected in Eq(3)-
no other consequence
Measurement of B(Ds+ -->ell+ nu) and the Decay Constant fDs From 600/pb of e+e- Annihilation Data Near 4170 MeV
We examine e+e- --> Ds^-D_s^{*+} and Ds^{*-}Ds^{+} interactions at 4170 MeV
using the CLEO-c detector in order to measure the decay constant fDs with good
precision. Previously our measurements were substantially higher than the most
precise lattice based QCD calculation of (241 +/- 3) MeV. Here we use the D_s^+
--> ell^+ nu channel, where the ell^+ designates either a mu^+ or a tau^+, when
the tau^+ --> pi^+ anti-nu. Analyzing both modes independently, we determine
B(D_s^+ --> mu^+ nu)= 0.565 +/- 0.045 +/- 0.017)%, and B(D_s^+ --> mu^+ nu)=
(6.42 +/- 0.81 +/- 0.18)%. We also analyze them simultaneously to find an
effective value of B^{eff}(D_s^+ --> mu^+ nu)= (0.591 +/- 0.037 +/- 0.018)% and
fDs=(263.3 +/- 8.2 +/- 3.9) MeV. Combining with the CLEO-c value determined
independently using D_s^+ --> tau^+ nu, tau^+ --> e^+ nu anti-nu decays, we
extract fDs=(259.5 +/- 6.6 +/- 3.1) MeV. Combining with our previous
determination of B(D^+ --> mu^+ nu), we extract the ratio fDs/fD+=1.26 +/- 0.06
+/- 0.02. No evidence is found for a CP asymmetry between Gamma(D_s^+ -->
mu^+\nu) and \Gamma(D_s^- --> mu^- nu); specifically the fractional difference
in rates is measured to be (4.8 +/- 6.1)%. Finally, we find B(D_s^+ --> e^+ nu)
< 1.2x10^{-4} at 90% confidence level.Comment: 26 pages, 16 figure
Observation of the Dalitz Decay
Using 586 of collision data acquired at
GeV with the CLEO-c detector at the Cornell Electron Storage
Ring, we report the first observation of
with a significance of . The ratio of branching fractions
\calB(D_{s}^{*+} \to D_{s}^{+} e^{+} e^{-}) / \calB(D_{s}^{*+} \to D_{s}^{+}
\gamma) is measured to be , which is consistent with theoretical expectations
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