443 research outputs found
An efficiency upper bound for inverse covariance estimation
We derive an upper bound for the efficiency of estimating entries in the
inverse covariance matrix of a high dimensional distribution. We show that in
order to approximate an off-diagonal entry of the density matrix of a
-dimensional Gaussian random vector, one needs at least a number of samples
proportional to . Furthermore, we show that with samples, the
hypothesis that two given coordinates are fully correlated, when all other
coordinates are conditioned to be zero, cannot be told apart from the
hypothesis that the two are uncorrelated.Comment: 7 Page
Aerothermodynamic Characteristics of Boundary Layer Transition and Trip Effectiveness of the HIFiRE Flight 5 Vehicle
An experimental wind tunnel test was conducted in the NASA Langley Research Center s 20-Inch Mach 6 Tunnel in support of the Hypersonic International Flight Research Experimentation Program. The information in this report is focused on the Flight 5 configuration, one in a series of flight experiments. This report documents experimental measurements made over a range of Reynolds numbers and angles of attack on several scaled ceramic heat transfer models of the Flight 5 vehicle. The heat transfer rate was measured using global phosphor thermography and the resulting images and heat transfer rate distributions were used to infer the state of the boundary layer on the windside, leeside and side surfaces. Boundary layer trips were used to force the boundary layer turbulent, and a study was conducted to determine the effectiveness of the trips with various heights. The experimental data highlighted in this test report were used determine the allowable roughness height for both the windside and side surfaces of the vehicle as well as provide for future tunnel-to-tunnel comparisons
Muon Catalyzed Fusion in 3 K Solid Deuterium
Muon catalyzed fusion in deuterium has traditionally been studied in gaseous
and liquid targets. The TRIUMF solid-hydrogen-layer target system has been used
to study the fusion reaction rates in the solid phase of D_2 at a target
temperature of 3 K. Products of two distinct branches of the reaction were
observed; neutrons by a liquid organic scintillator, and protons by a silicon
detector located inside the target system. The effective molecular formation
rate from the upper hyperfine state of and the hyperfine transition
rate have been measured: , and .
The molecular formation rate is consistent with other recent measurements, but
not with the theory for isolated molecules. The discrepancy may be due to
incomplete thermalization, an effect which was investigated by Monte Carlo
calculations. Information on branching ratio parameters for the s and p wave
d+d nuclear interaction has been extracted.Comment: 19 pages, 11 figures, submitted to PRA Feb 20, 199
Measurement of the Resonant Molecular Formation Rate in Solid HD
Measurements of muon-catalyzed dt fusion () in solid
HD have been performed. The theory describing the energy dependent resonant
molecular formation rate for the reaction + HD is
compared to experimental results in a pure solid HD target. Constraints on the
rates are inferred through the use of a Monte Carlo model developed
specifically for the experiment. From the time-of- flight analysis of fusion
events in 16 and 37 targets, an average formation rate
consistent with 0.897(0.046) (0.166) times the
theoretical prediction was obtained.Comment: 4 pages, 5 figure
Chandra grating spectroscopy of three hot white dwarfs
High-resolution soft X-ray spectroscopic observations of single hot white
dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have
observed two white dwarfs, one is of spectral type DA (LB 1919) and the other
is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars
are analyzed, together with an archival Chandra spectrum of another DA white
dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are
investigated in order to study the effect of gravitational settling and
radiative levitation of metals in their photospheres. LB 1919 is of interest
because it has a significantly lower metallicity than DAs with otherwise
similar atmospheric parameters. GD 246 is the only white dwarf known that shows
identifiable individual iron lines in the soft X-ray range. For the PG1159
star, a precise effective temperature determination is performed in order to
confine the position of the blue edge of the GW Vir instability region in the
HRD. (abridged)Comment: A&A, in pres
Discrete-time quantum walks on one-dimensional lattices
In this paper, we study discrete-time quantum walks on one-dimensional
lattices. We find that the coherent dynamics depends on the initial states and
coin parameters. For infinite size of lattice, we derive an explicit expression
for the return probability, which shows scaling behavior
and does not depends on the initial states of the walk. In the long-time limit,
the probability distribution shows various patterns, depending on the initial
states, coin parameters and the lattice size. The average mixing time
closes to the limiting probability in linear (size of the
lattice) for large values of thresholds . Finally, we introduce
another kind of quantum walk on infinite or even-numbered size of lattices, and
show that the walk is equivalent to the traditional quantum walk with
symmetrical initial state and coin parameter.Comment: 17 pages research not
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