153,266 research outputs found
Observation of meteors by MST radar
The observation of meteor trails by a vertical mesosphere - stratosphere - troposphere (MST) radar beam has the advantage of good height resolution and an approximate knowledge of the zenith angle since the trails are horizontal or near-horizontal. An extension of the ablation theory of meteors was developed for near horizontal trails which takes into account the curvature of the earth. Observations of the Geminid meteor shower by MST radar reveal the 'diffusion heights' to be in fair agreement with the true height, but with some discrepancies that can amount to 4 km. The true heights are almost entirely confined to the range 87-91 km, although the upper limit is attributed to the coherent integration time of the existing MST radar processing
Development of Auditory Selective Attention: Why Children Struggle to Hear in Noisy Environments
Children’s hearing deteriorates markedly in the presence of unpredictable noise. To explore why, 187 school-age children (4–11 years) and 15 adults performed a tone-in-noise detection task, in which the masking noise varied randomly between every presentation. Selective attention was evaluated by measuring the degree to which listeners were influenced by (i.e., gave weight to) each spectral region of the stimulus. Psychometric fits were also used to estimate levels of internal noise and bias. Levels of masking were found to decrease with age, becoming adult-like by 9–11 years. This change was explained by improvements in selective attention alone, with older listeners better able to ignore noise similar in frequency to the target. Consistent with this, age-related differences in masking were abolished when the noise was made more distant in frequency to the target. This work offers novel evidence that improvements in selective attention are critical for the normal development of auditory judgments
Keck Spectroscopy of Faint 3 < z < 7 Lyman Break Galaxies: III. The Mean Ultraviolet Spectrum at z=4
We present and discuss the mean rest-frame ultraviolet spectrum for a sample
of 81 Lyman Break Galaxies (LBGs) selected to be B-band dropouts with a mean
redshift of z=3.9 and apparent magnitudes z_AB<26. Most of the individual
spectra are drawn from our ongoing survey in the GOODS fields with the Keck
DEIMOS spectrograph, and we have augmented our sample with published data taken
with FORS2 on the VLT. In general we find similar trends in the spectral
diagnostics to those found in the earlier, more extensive survey of LBGs at z=3
undertaken by Shapley et al (2003). Specifically, we find low-ionization
absorption lines which trace the presence of neutral outflowing gas are weaker
in galaxies with stronger Lyman-alpha emission, bluer UV spectral slopes, lower
stellar masses, lower UV luminosities and smaller half-light radii. This is
consistent with a physical picture whereby star formation drives outflows of
neutral gas which scatters Lyman-alpha and gives rise to strong low-ionization
absorption lines, while increasing the stellar mass, size, metallicity, and
dust content of galaxies. Typical galaxies are thus expected to have stronger
Lyman-alpha emission and weaker low-ionization absorption at earlier times
(higher redshifts). Indeed, our mean spectrum at z=4 shows somewhat weaker
low-ionization absorption lines than at z=3 and available data at higher
redshift indicates a rapid decrease in low-ionization absorption strength with
redshift. We argue that the reduced low-ionization absorption is likely caused
by a decrease in the covering fraction and/or velocity range of outflowing
neutral gas at earlier epochs. Our continuing survey will enable us to extend
these diagnostics more reliably to higher redshift and determine the
implications for the escape fraction of ionizing photons which governs the role
of early galaxies in cosmic reionization. [Abridged]Comment: 17 pages, 12 figures, submitted to ApJ. Comments welcom
Development of pigments for thermal control coatings Final report, 17 Jun. - 16 Dec. 1965
Powdered metal oxide pigments by nucleation for temperature control coating
Leptogenesis in the two right-handed neutrino model revisited
We revisit leptogenesis in the minimal non-supersymmetric type I see-saw
mechanism with two right-handed (RH) neutrinos, including flavour effects and
allowing both RH neutrinos N_1 and N_2 to contribute, rather than just the
lightest RH neutrino N_1 that has hitherto been considered. By performing scans
over parameter space in terms of the single complex angle z of the orthogonal
matrix R, for a range of PMNS parameters, we find that in regions around z \sim
\pm \pi/2, for the case of a normal mass hierarchy, the N_2 contribution can
dominate the contribution to leptogenesis, allowing the lightest RH neutrino
mass to be decreased by about an order of magnitude in these regions, down to
M_1 \sim 1.3*10^11 GeV for vanishing initial N_2-abundance, with the numerical
results supported by analytic estimates. We show that the regions around z \sim
\pm \pi /2 correspond to light sequential dominance, so the new results in this
paper may be relevant to unified model building.Comment: 41 pages, 10 figures; v2 matches published version in PR
NNLO predictions for Z-boson pair production at the LHC
We present a calculation of the NNLO QCD corrections to Z-boson pair
production at hadron colliders, based on the N-jettiness method for the real
radiation parts. We discuss the size and shape of the perturbative corrections
along with their associated scale uncertainties and compare our results to
recent LHC data at TeV.Comment: 19 pages, 2 Tables, 4 figures. Version to appear in JHE
Hydrogen maser oscillation at 10 K
A low temperature atomic hydrogen maser was developed using frozen atomic neon as the storage surface. The maser has been operated in the pulsed mode at temperatures from 6 K to 11 K and as a self-excited oscillator from 9 K to 10.5 K
Numerical evaluation of two-loop integrals with pySecDec
We describe the program pySecDec, which factorises endpoint singularities
from multi-dimensional parameter integrals and can serve to calculate integrals
occurring in higher order perturbative calculations numerically. We focus on
the new features and on frequently asked questions about the usage of the
program.Comment: 11 pages, to appear in the proceedings of the HiggsTools Final
Meeting, IPPP, University of Durham, UK, September 201
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