253 research outputs found
Final-State Spectrum of He after Decay of Tritium Anions T
The final-state spectrum of decaying tritium anions T was
calculated. The wavefunctions describing the initial T ground state and the
final He states were obtained by the full configuration-interaction method.
The transition probability was calculated within the sudden approximation. The
transition probability into the electronic continuum is extracted from the
complex-scaled resolvent and is shown to converge for very high-energies to an
approximate analytical model probability distribution.Comment: 7 pages, 2 figure
Variational calculations for the hydrogen-antihydrogen system with a mass-scaled Born-Oppenheimer potential
The problem of proton-antiproton motion in the --
system is investigated by means of the variational method. We introduce a
modified nuclear interaction through mass-scaling of the Born-Oppenheimer
potential. This improved treatment of the interaction includes the nondivergent
part of the otherwise divergent adiabatic correction and shows the correct
threshold behavior.
Using this potential we calculate the vibrational energy levels with angular
momentum 0 and 1 and the corresponding nuclear wave functions, as well as the
S-wave scattering length. We obtain a full set of all bound states together
with a large number of discretized continuum states that might be utilized in
variational four-body calculations. The results of our calculations gives an
indication of resonance states in the hydrogen-antihydrogen system
Stellar atmosphere parameters with MAx, a MAssive compression of x^2 for spectral fitting
MAx is a new tool to estimate parameters from stellar spectra. It is based on
the maximum likelihood method, with the likelihood compressed in a way that the
information stored in the spectral fluxes is conserved. The compressed data are
given by the size of the number of parameters, rather than by the number of
flux points. The optimum speed-up reached by the compression is the ratio of
the data set to the number of parameters. The method has been tested on a
sample of low-resolution spectra from the Sloan Extension for Galactic
Understanding and Exploration (SEGUE) survey for the estimate of metallicity,
effective temperature and surface gravity, with accuracies of 0.24 dex, 130K
and 0.5 dex, respectively. Our stellar parameters and those recovered by the
SEGUE Stellar Parameter Pipeline agree reasonably well. A small sample of
high-resolution VLT-UVES spectra were also used to test the method and the
results have been compared to a more classical approach. The speed and
multi-resolution capability of MAx combined with its performance compared with
other methods indicates that it will be a useful tool for the analysis of
upcoming spectral surveys.Comment: 17 pages, 10 figures, minor changes after the chief language editor.
A&A, in pres
Intraspecfic variation in cold-temperature metabolic phenotypes of Arabidopsis lyrata ssp petraea
Atmospheric temperature is a key factor in determining the distribution of a plant species. Alongside this, plant populations growing at the margin of their range may exhibit traits that indicate genetic differentiation and adaptation to their local abiotic environment. We investigated whether geographically separated marginal populations of Arabidopsis lyrata ssp. petraea have distinct metabolic phenotypes associated with exposure to cold temperatures. Seeds of A. petraea were obtained from populations along a latitudinal gradient, namely Wales, Sweden and Iceland and grown in a controlled cabinet environment. Mannose, glucose, fructose, sucrose and raffinose concentrations were different between cold treatments and populations, especially in the Welsh population, but polyhydric alcohol concentrations were not. The free amino acid compositions were population specific, with fold differences in most amino acids, especially in the Icelandic populations, with gross changes in amino acids, particularly those associated with glutamine metabolism. Metabolic fingerprints and profiles were obtained. Principal component analysis (PCA) of metabolite fingerprints revealed metabolic characteristic phenotypes for each population and temperature. It is suggested that amino acids and carbohydrates were responsible for discriminating populations within the PCA. Metabolite fingerprinting and profiling has proved to be sufficiently sensitive to identify metabolic differences between plant populations at different atmospheric temperatures. These findings show that there is significant natural variation in cold metabolism among populations of A. l. petraea which may signify plant adaptation to local climates
Development of a PbWO4 Detector for Single-Shot Positron Annihilation Lifetime Spectroscopy at the GBAR Experiment
We have developed a PbWO4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross-section for the (anti)hydrogen formation by (anti)proton-positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment
Centrifugal separation and equilibration dynamics in an electron-antiproton plasma
Charges in cold, multiple-species, non-neutral plasmas separate radially by
mass, forming centrifugally-separated states. Here, we report the first
detailed measurements of such states in an electron-antiproton plasma, and the
first observations of the separation dynamics in any centrifugally-separated
system. While the observed equilibrium states are expected and in agreement
with theory, the equilibration time is approximately constant over a wide range
of parameters, a surprising and as yet unexplained result. Electron-antiproton
plasmas play a crucial role in antihydrogen trapping experiments
CS 30322-023: an ultra metal-poor TP-AGB star?
With [Fe/H] = -3.5, CS 30322-023 is the most metal-poor star to exhibit a
clear s-process signature and the most metal-poor ``lead star'' known. CS
30322-023 is also remarkable in having the lowest surface gravity (log g <=
-0.3) among the metal-poor stars studied to date. The available evidence
indicates that this star is presently a thermally-pulsing asymptotic giant
branch (TP-AGB) star, with no strong indication of binarity thus far (although
a signal of period 192 d is clearly present in the radial-velocity data, this
is likely due to pulsation of the stellar envelope). We show that low-mass
TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited
sample such as the HK survey, because their high luminosities make it possible
to sample them over a very large volume. The strong N overabundance and the low
12C/13C ratio (4) in this star is typical of the operation of the CN cycle.
Coupled with a Na overabundance and the absence of a strong C overabundance,
this pattern seems to imply that hot-bottom burning operated in this star,
which should then have a mass of at least 2 Msun. However, the luminosity
associated with this mass would put the star at a distance of about 50 kpc, in
the outskirts of the galactic halo. We explore alternative scenarios in which
the observed abundance pattern results from some mixing mechanism yet to be
identified occurring in a single low-metallicity 0.8 Msun AGB star, or from
pollution by matter from an intermediate-mass AGB companion which has undergone
hot-bottom burning. We stress, however, that our abundances may be subject to
uncertainties due to NLTE or 3D granulation effects which were not taken into
consideration.Comment: 17 pages, Astronomy & Astrophysics, in press; also available at
http://www.astro.ulb.ac.be/Html/ps.html#PR
Antihydrogen and mirror-trapped antiproton discrimination: Discriminating between antihydrogen and mirror-trapped antiprotons in a minimum-B trap
Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum
(minimum-B) trap formed by superconducting octupole and mirror magnet coils.
The trapped antiatoms were detected by rapidly turning off these magnets,
thereby eliminating the magnetic minimum and releasing any antiatoms contained
in the trap. Once released, these antiatoms quickly hit the trap wall,
whereupon the positrons and antiprotons in the antiatoms annihilated. The
antiproton annihilations produce easily detected signals; we used these signals
to prove that we trapped antihydrogen. However, our technique could be
confounded by mirror-trapped antiprotons, which would produce
seemingly-identical annihilation signals upon hitting the trap wall. In this
paper, we discuss possible sources of mirror-trapped antiprotons and show that
antihydrogen and antiprotons can be readily distinguished, often with the aid
of applied electric fields, by analyzing the annihilation locations and times.
We further discuss the general properties of antiproton and antihydrogen
trajectories in this magnetic geometry, and reconstruct the antihydrogen energy
distribution from the measured annihilation time history.Comment: 17 figure
Alpha Antihydrogen Experiment
ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise
test of CPT symmetry with trapped antihydrogen atoms. After reviewing the
motivations, we discuss our recent progress toward the initial goal of stable
trapping of antihydrogen, with some emphasis on particle detection techniques.Comment: Invited talk presented at the Fifth Meeting on CPT and Lorentz
Symmetry, Bloomington, Indiana, June 28-July 2, 201
Surface Covering of Downed Logs: Drivers of a Neglected Process in Dead Wood Ecology
Many species use coarse woody debris (CWD) and are disadvantaged by the forestry-induced loss of this resource. A neglected process affecting CWD is the covering of the surfaces of downed logs caused by sinking into the ground (increasing soil contact, mostly covering the underside of the log), and dense overgrowth by ground vegetation. Such cover is likely to profoundly influence the quality and accessibility of CWD for wood-inhabiting organisms, but the factors affecting covering are largely unknown. In a five-year experiment we determined predictors of covering rate of fresh logs in boreal forests and clear-cuts. Logs with branches were little covered because they had low longitudinal ground contact. For branchless logs, longitudinal ground contact was most strongly related to estimated peat depth (positive relation). The strongest predictor for total cover of branchless logs was longitudinal ground contact. To evaluate the effect on cover of factors other than longitudinal ground contact, we separately analyzed data from only those log sections that were in contact with the ground. Four factors were prominent predictors of percentage cover of such log sections: estimated peat depth, canopy shade (both increasing cover), potential solar radiation calculated from slope and slope aspect, and diameter of the log (both reducing cover). Peat increased cover directly through its low resistance, which allowed logs to sink and soil contact to increase. High moisture and low temperatures in pole-ward facing slopes and under a canopy favor peat formation through lowered decomposition and enhanced growth of peat-forming mosses, which also proved to rapidly overgrow logs. We found that in some boreal forests, peat and fast-growing mosses can rapidly cover logs lying on the ground. When actively introducing CWD for conservation purposes, we recommend that such rapid covering is avoided, thereby most likely improving the CWD's longevity as habitat for many species
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