515 research outputs found
Metabolism of a tropical rainforest stream
Gradients in photosynthesis (P) and respiration (R) were measured on an unperturbed portion of the Rio Mameyes, a tropical stream in the Luquillo Experimental Forest, northeastern Puerto Rico. Rates of P, which were similar to those of streams in temperate-deciduous forests, were low in the heavily canopied headwaters (\u3c70 g O2 mâ2 yâ1) and were higher (453â634 g O2 mâ2 yâ1) in middle and lower reaches. Periphyton biomass did not show the expected increase as the canopy opened downstream, probably because of increasing herbivory in downstream reaches. Rates of R, which were much higher than in most temperate streams, also were lower in the headwaters (767 g O2 mâ2 yâ1) than in the middle and lower reaches (1550â1660 g O2 mâ2 yâ1). High rates of R and suppressed periphyton abundance caused annual P/R to be \u3c\u3c1 from headwaters to lower reaches. Results for the Rio Mameyes suggest that intense herbivory, which is favored by the presence of large herbivores and consistently high temperatures, may be more typical of tropical than temperate streams. Results also show that the tropical rainforest provides the stream with sufficient amounts of labile organic C to support high rates of respiration over long distances across the basin
Recent results from the canfranc dark matter search with germanium detectors
Two germanium detectors are currently operating in the Canfranc Underground
Laboratory at 2450 m.w.e looking for WIMP dark matter. One is a 2 kg 76Ge IGEX
detector (RG-2) which has an energy threshold of 4 keV and a low-energy
background rate of about 0.3 c/keV/kg/day. The other is a small (234 g) natural
abundance Ge detector (COSME), of low energy threshold (2.5 keV) and an energy
resolution of 0.4 keV at 10 keV which is looking for WIMPs and for solar
axions. The analysis of 73 kg-days of data taken by COSME in a search for solar
axions via their photon Primakoff conversion and Bragg scattering in the Ge
crystal yields a 95% C.L. limit for the axion-photon coupling g < 2.8 10^-9
GeV^-1. These data, analyzed for WIMP searches provide an exclusion plot for
WIMP-nucleon spin-independent interaction which improves previous plots in the
low mass region. On the other hand, the exclusion plot derived from the 60
kg-days of data from the RG-2 IGEX detector improves the exclusion limits
derived from other ionization (non thermal) germanium detector experiments in
the region of WIMP masses from 30 to 100 GeV recently singled out by the
reported DAMA annual modulation effect.Comment: 6 pages, talk given at IDM2000, York, September 200
Vortices in Bogomol'nyi Limit of Einstein Maxwell Higgs Theory with or without External Sources
The Abelian Higgs model with or without external particles is considered in
curved space. Using the dual transformation, we rewrite the model in terms of
dual gauge fields and derive the Bogomol'nyi-type bound. We examine
cylindrically symmetric solutions to Einstein equations and the first-order
Bogomol'nyi equations, and find vortex solutions and vortex-particle composites
which lie on the spatial manifold with global geometry described by a cylinder
asymptotically or a two sphere in addition to the well-known cone.Comment: LaTeX, 23 pages, 10 LaTeX figures included, KHTP-93-05, SNUTP-93-100,
DPNU-93-46. (A note and several references added
A Wormhole at the core of an infinite cosmic string
We study a solution of Einstein's equations that describes a straight cosmic
string with a variable angular deficit, starting with a deficit at the
core. We show that the coordinate singularity associated to this defect can be
interpreted as a traversible wormhole lodging at the the core of the string. A
negative energy density gradually decreases the angular deficit as the distance
from the core increases, ending, at radial infinity, in a Minkowski spacetime.
The negative energy density can be confined to a small transversal section of
the string by gluing to it an exterior Gott's like solution, that freezes the
angular deficit existing at the matching border. The equation of state of the
string is such that any massive particle may stay at rest anywhere in this
spacetime. In this sense this is 2+1 spacetime solution.Comment: 1 tex file and 5 eps files. To be Published in Nov. in Phys.Rev.
Dilatonic global strings
We examine the field equations of a self-gravitating global string in low
energy superstring gravity, allowing for an arbitrary coupling of the global
string to the dilaton. Massive and massless dilatons are considered. For the
massive dilaton the spacetime is similar to the recently discovered
non-singular time-dependent Einstein self-gravitating global string, but the
massless dilaton generically gives a singular spacetime, even allowing for
time-dependence. We also demonstrate a time-dependent non-singular
string/anti-string configuration, in which the string pair causes a
compactification of two of the spatial dimensions, albeit on a very large
scale.Comment: 18 pages RevTeX, 3 figures, references amende
Status of IGEX dark matter search at Canfranc Underground Laboratory
One IGEX 76Ge double-beta decay detector is currently operating in the
Canfranc Underground Laboratory in a search for dark matter WIMPs, through the
Ge nuclear recoil produced by the WIMP elastic scattering. In this talk we
report on the on-going efforts to understand and eventually reject the
background at low energy. These efforts have led to the improvement of the
neutron shielding and to partial reduction of the background, but still the
remaining events are not totally identified. A tritium contamination or
muon-induced neutrons are considered as possible sources, simulations and
experimental test being still under progress. According to the success of this
study we comment the prospects of the experiment as well as those of its future
extension, the GEDEON dark matter experiment.Comment: 6 pages, 3 figures, talk given at 4th International Workshop on the
Identification of Dark Matter, York, September 200
Gravitating Model Solitons
We study axially symmetric static solitons of O(3) nonlinear model
coupled to (2+1)-dimensional anti-de Sitter gravity. The obtained solutions are
not self-dual under static metric. The usual regular topological lump solution
cannot form a black hole even though the scale of symmetry breaking is
increased. There exist nontopological solitons of half integral winding in a
given model, and the corresponding spacetimes involve charged Baados-Teitelboim-Zanelli black holes without non-Abelian scalar hair.Comment: 35 pages, RevTe
The dynamical Green's function and an exact optical potential for electron-molecule scattering including nuclear dynamics
We derive a rigorous optical potential for electron-molecule scattering
including the effects of nuclear dynamics by extending the common many-body
Green's function approach to optical potentials beyond the fixed-nuclei limit
for molecular targets. Our formalism treats the projectile electron and the
nuclear motion of the target molecule on the same footing whereby the dynamical
optical potential rigorously accounts for the complex many-body nature of the
scattering target. One central result of the present work is that the common
fixed-nuclei optical potential is a valid adiabatic approximation to the
dynamical optical potential even when projectile and nuclear motion are
(nonadiabatically) coupled as long as the scattering energy is well below the
electronic excitation thresholds of the target. For extremely low projectile
velocities, however, when the cross sections are most sensitive to the
scattering potential, we expect the influences of the nuclear dynamics on the
optical potential to become relevant. For these cases, a systematic way to
improve the adiabatic approximation to the dynamical optical potential is
presented that yields non-local operators with respect to the nuclear
coordinates.Comment: 22 pages, no figures, accepted for publ., Phys. Rev.
A Decommissioned LHC Model Magnet as an Axion Telescope
The 8.4 Tesla, 10 m long transverse magnetic field of a twin aperture LHC
bending magnet can be utilized as a macroscopic coherent solar axion-to-photon
converter. Numerical calculations show that the integrated time of alignment
with the Sun would be 33 days per year with the magnet on a tracking table
capable of in the vertical direction and in the horizontal
direction. The existing lower bound on the axion-to-photon coupling constant
can be improved by a factor between 50 and 100 in 3 years, i.e.,
for axion masses
1 eV. This value falls within the existing open axion mass window.
The same set-up can simultaneously search for low- and high-energy celestial
axions, or axion-like particles, scanning the sky as the Earth rotates and
orbits the Sun.Comment: Final version, accepted for publication in Nucl. Instr. Meth. A. More
information can be found at http://wwwinfo.cern.ch/~collar/SATAN/alvaro.htm
- âŠ