257 research outputs found
The role of antihydrogen formation in the radial transport of antiprotons in positron plasmas
Simulations of the behaviour of antiprotons in positron clouds during antihydrogen formation.The work added to our understanding of the underlying processes, including the dominant role played by antihydrogen formation itself
Resonant Phenomena in Antihydrogen-Hydrogen Scattering
We present a treatment of cold hydrogen-antihydrogen collisions based on the
asymptotic properties of atom-antiatom interactions. We derive general formulas
for the elastic and inelastic cross sections and for the scattering lengths and
analyze their sensitivity to the parameters characterizing the inelasticity of
the collision process. Given the inelasticity, we obtain bounds for the complex
scattering length. We investigate the influence of strong nuclear forces and
the isotope effects in and collisions
and demonstrate enhancement of these effects due to the presence of the
near-threshold narrow () states. The
values of the elastic and inelastic cross-sections with simultaneous account of
rearrangement and strong forces are presented. General expressions for the
(complex) energies of the near-threshold states are
obtained.Comment: 26 pages 7 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
Beryllium abundances in metal-poor stars
We have determined beryllium abundances for 25 metal-poor stars based on the
high resolution and high signal-to-noise ratio spectra from the VLT/UVES
database. Our results confirm that Be abundances increase with Fe, supporting
the global enrichment of Be in the Galaxy. Oxygen abundances based on [O I]
forbidden line implies a linear relation with a slope close to one for the Be
vs. O trend, which indicates that Be is probably produced in a primary process.
Some strong evidences are found for the intrinsic dispersion of Be abundances
at a given metallicity. The deviation of HD132475 and HD126681 from the general
Be vs. Fe and Be vs. O trend favours the predictions of the superbubble model,
though the possibility that such dispersion originates from the inhomogeneous
enrichment in Fe and O of the protogalactic gas cannot be excluded.Comment: 12 pages with 9 figures, to be published in MNRA
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
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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