1,084 research outputs found
Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna
Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of longâterm, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C3 vs. C4) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixedâfeeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8 m2/ha), and woody canopy cover (31%), translating to a ~8.5 t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5 t/ha) in total soil carbon to 30âcm depth, with ~71% of this derived from C4 grasses (vs. ~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system
Do we know the mass of a black hole? Mass of some cosmological black hole models
Using a cosmological black hole model proposed recently, we have calculated
the quasi-local mass of a collapsing structure within a cosmological setting
due to different definitions put forward in the last decades to see how similar
or different they are. It has been shown that the mass within the horizon
follows the familiar Brown-York behavior. It increases, however, outside the
horizon again after a short decrease, in contrast to the Schwarzschild case.
Further away, near the void, outside the collapsed region, and where the
density reaches the background minimum, all the mass definitions roughly
coincide. They differ, however, substantially far from it. Generically, we are
faced with three different Brown-York mass maxima: near the horizon, around the
void between the overdensity region and the background, and another at
cosmological distances corresponding to the cosmological horizon. While the
latter two maxima are always present, the horizon mass maxima is absent before
the onset of the central singularity.Comment: 11 pages, 8 figures, revised version, accepted in General Relativity
and Gravitatio
Back-to-back emission of the electrons in double photoionization of helium
We calculate the double differential distributions and distributions in
recoil momenta for the high energy non-relativistic double photoionization of
helium. We show that the results of recent experiments is the pioneering
experimental manifestation of the quasifree mechanism for the double
photoionization, predicted long ago in our papers. This mechanism provides a
surplus in distribution over the recoil momenta at small values of the latter,
corresponding to nearly "back-to-back" emission of the electrons. Also in
agreement with previous analysis the surplus is due to the quadrupole terms of
the photon-electron interaction. We present the characteristic angular
distribution for the "back-to-back" electron emission. The confirmation of the
quasifree mechanism opens a new area of exiting experiments, which are expected
to increase our understanding of the electron dynamics and of the bound states
structure. The results of this Letter along with the recent experiments open a
new field for studies of two-electron ionization not only by photons but by
other projectiles, e.g. by fast electrons or heavy ions.Comment: 10 pages, 2 figure
Selected Topics in High Energy Semi-Exclusive Electro-Nuclear Reactions
We review the present status of the theory of high energy reactions with
semi-exclusive nucleon electro-production from nuclear targets. We demonstrate
how the increase of transferred energies in these reactions opens a complete
new window in studying the microscopic nuclear structure at small distances.
The simplifications in theoretical descriptions associated with the increase of
the energies are discussed. The theoretical framework for calculation of high
energy nuclear reactions based on the effective Feynman diagram rules is
described in details. The result of this approach is the generalized eikonal
approximation (GEA), which is reduced to Glauber approximation when nucleon
recoil is neglected. The method of GEA is demonstrated in the calculation of
high energy electro-disintegration of the deuteron and A=3 targets.
Subsequently we generalize the obtained formulae for A>3 nuclei. The relation
of GEA to the Glauber theory is analyzed. Then based on the GEA framework we
discuss some of the phenomena which can be studied in exclusive reactions,
these are: nuclear transparency and short-range correlations in nuclei. We
illustrate how light-cone dynamics of high-energy scattering emerge naturally
in high energy electro-nuclear reactions.Comment: LaTex file with 51 pages and 23 eps figure
Sur la p-dimension des corps
Let A be an excellent integral henselian local noetherian ring, k its residue
field of characteristic p>0 and K its fraction field. Using an algebraization
technique introduced by the first named author, and the one-dimension case
already proved by Kazuya KATO, we prove the following formula: cd_p(K) = dim(A)
+ p-rank(k), if k is separably closed and K of characteristic zero. A similar
statement is valid without those assumptions on k and K
Feynman Graphs and Generalized Eikonal Approach to High Energy Knock-Out Processes
The cross section of hard semi-exclusive reactions for fixed
missing energy and momentum is calculated within the eikonal approximation.
Relativistic dynamics and kinematics of high energy processes are unambiguously
accounted for by using the analysis of appropriate Feynman diagrams. A
significant dependence of the final state interactions on the missing energy is
found, which is important for interpretation of forthcoming color transparency
experiments. A new, more stringent kinematic restriction on the region where
the contribution of short-range nucleon correlations is enhanced in
semi-exclusive knock-out processes is derived. It is also demonstrated that the
use of light-cone variables leads to a considerable simplification of the
description of high-energy knock-out reactions.Comment: 24 pages, LaTex, two Latex and two ps figures, uses FEYNMAN.tex and
psfig.sty. Revisied version to appear in Phys. Rev.
Model Calculations for the Two-Fragment Electro-Disintegration of He
Differential cross sections for the electro-disintegration process are calculated, using a model in which
the final state interaction is included by means of a nucleon-nucleus (3+1)
potential constructed via Marchenko inversion. The required bound-state wave
functions are calculated within the integrodifferential equation approach
(IDEA). In our model the important condition that the initial bound state and
the final scattering state are orthogonal is fulfilled. The sensitivity of the
cross section to the input interaction in certain kinematical regions
is investigated. The approach adopted could be useful in reactions involving
few cluster systems where effective interactions are not well known and exact
methods are presently unavailable. Although, our Plane-Wave Impulse
Approximation results exhibit, similarly to other calculations, a dip in the
five-fold differential cross-section around a missing momentum of , it is argued that this is an artifact of the omission of re-scattering
four-nucleon processes.Comment: 16 pages, 6 figures, accepted for publication by Phys.Rev.
International graduate training program in one health at the university of Saskatchewan, Saskatoon, Canada: A two year assessment
Imaging Molecular Structure through Femtosecond Photoelectron Diffraction on Aligned and Oriented Gas-Phase Molecules
This paper gives an account of our progress towards performing femtosecond
time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe
setup combining optical lasers and an X-ray Free-Electron Laser. We present
results of two experiments aimed at measuring photoelectron angular
distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and
dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss
them in the larger context of photoelectron diffraction on gas-phase molecules.
We also show how the strong nanosecond laser pulse used for adiabatically
laser-aligning the molecules influences the measured electron and ion spectra
and angular distributions, and discuss how this may affect the outcome of
future time-resolved photoelectron diffraction experiments.Comment: 24 pages, 10 figures, Faraday Discussions 17
QED on Curved Background and on Manifolds with Boundaries: Unitarity versus Covariance
Some recent results show that the covariant path integral and the integral
over physical degrees of freedom give contradicting results on curved
background and on manifolds with boundaries. This looks like a conflict between
unitarity and covariance. We argue that this effect is due to the use of
non-covariant measure on the space of physical degrees of freedom. Starting
with the reduced phase space path integral and using covariant measure
throughout computations we recover standard path integral in the Lorentz gauge
and the Moss and Poletti BRST-invariant boundary conditions. We also
demonstrate by direct calculations that in the approach based on Gaussian path
integral on the space of physical degrees of freedom some basic symmetries are
broken.Comment: 29 pages, LaTEX, no figure
- âŠ