113,185 research outputs found
Effect of habituation on the susceptibility of the rat to restraint ulcers
The frequency and gravity of restraint ulcers were found to significantly diminish in rats previously exposed to brief periods of immobilization. The rats' becoming habituated to restraint conditions probably explains this phenomenon
Current dependence of grain boundary magnetoresistance in La_0.67Ca_0.33MnO_3 films
We prepared epitaxial ferromagnetic manganite films on bicrystal substrates
by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was
measured as a function of magnetic field, temperature and current. At low
temperatures hysteretic changes in resistivity up to 70% due to switching of
magnetic domains at the coercitive field are observed. The strongly non-ohmic
behavior of the current-voltage leads to a complete suppression of the MR
effect at high bias currents with the identical current dependence at low and
high magnetic fields. We discuss the data in view of tunneling and mesoscale
magnetic transport models and propose an explicit dependence of the spin
polarization on the applied current in the grain boundary region.Comment: 5 pages, to appear in J. Appl. Phy
A comparison of two magnetic ultra-cold neutron trapping concepts using a Halbach-octupole array
This paper describes a new magnetic trap for ultra-cold neutrons (UCNs) made
from a 1.2 m long Halbach-octupole array of permanent magnets with an inner
bore radius of 47 mm combined with an assembly of superconducting end coils and
bias field solenoid. The use of the trap in a vertical, magneto-gravitational
and a horizontal setup are compared in terms of the effective volume and
ability to control key systematic effects that need to be addressed in high
precision neutron lifetime measurements
Classical simulatability, entanglement breaking, and quantum computation thresholds
We investigate the amount of noise required to turn a universal quantum gate
set into one that can be efficiently modelled classically. This question is
useful for providing upper bounds on fault tolerant thresholds, and for
understanding the nature of the quantum/classical computational transition. We
refine some previously known upper bounds using two different strategies. The
first one involves the introduction of bi-entangling operations, a class of
classically simulatable machines that can generate at most bipartite
entanglement. Using this class we show that it is possible to sharpen
previously obtained upper bounds in certain cases. As an example, we show that
under depolarizing noise on the controlled-not gate, the previously known upper
bound of 74% can be sharpened to around 67%. Another interesting consequence is
that measurement based schemes cannot work using only 2-qubit non-degenerate
projections. In the second strand of the work we utilize the Gottesman-Knill
theorem on the classically efficient simulation of Clifford group operations.
The bounds attained for the pi/8 gate using this approach can be as low as 15%
for general single gate noise, and 30% for dephasing noise.Comment: 12 pages, 3 figures. v2: small typos changed, no change to result
Magnetic Flux Loss and Flux Transport in a Decaying Active Region
We estimate the temporal change of magnetic flux perpendicular to the solar
surface in a decaying active region by using a time series of the spatial
distribution of vector magnetic fields in the photosphere. The vector magnetic
fields are derived from full spectropolarimetric measurements with the Solar
Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux
transport rate in a decaying sunspot and its surrounding moat region. The
amount of magnetic flux that decreases in the sunspot and moat region is very
similar to magnetic flux transported to the outer boundary of the moat region.
The flux loss rates [] of magnetic elements with positive and
negative polarities are balanced each other around the outer boundary of the
moat region. These results suggest that most of the magnetic flux in the
sunspot is transported to the outer boundary of the moat region as moving
magnetic features, and then removed from the photosphere by flux cancellation
around the outer boundary of the moat region.Comment: 16 pages, 7 figures, Accepted for publication in Ap
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Modelling the inorganic nitrogen behaviour in a small Mediterranean forested catchment, Fuirosos (Catalonia)
The aim of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4-N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil biological processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. Our simulation highlighted the riparian zone as a possible source of nitrate, especially after the summer drought period, but it can also act as an important sink of nitrate due to denitrification, in particular during the wettest period of the year. The riparian zone was a key element to simulate the catchment nitrate behaviour. The lumped LU4-N model (which does not include the riparian zone) could not be validated, while both the semi-distributed LU4-R-N and SD4-R-N model (which include the riparian zone) gave satisfactory results for the calibration process and acceptable results for the temporal validation process
Special Supersymmetric features of large invariant mass unpolarized and polarized top-antitop production at LHC
We consider the top-antitop invariant mass distributions for production of
unpolarized and polarized top quark pairs at LHC, in the theoretical framework
of the MSSM. Assuming a "moderately" light SUSY scenario, we derive the leading
logarithmic electroweak contributions at one loop in a region of large
invariant mass, TeV, for the unpolarized differential
cross section and for the differential longitudinal top
polarization asymmetry . We perform a realistic evaluation of
the expected uncertainties of the two quantities, both from a theoretical and
from an experimental point of view, and discuss the possibility of obtaining,
from accurate measurements of the two mass distributions, stringent consistency
tests of the model, in particular identifications of large effects.Comment: 23 pages, 9 eps figure
Extracting the Proton ubar content from pp->Direct Photon plus Jet Cross Sections
An analysis procedure is proposed to measure the antiquark distributions in
the proton over the region 0.01 < x < 0.1. The procedure involves the
measurement of high p_t asymmetric direct photon and jet final states in pp
interactions. This measurement can be made at the RHIC collider running in pp
mode at an energy of sqrt(s)=500 GeV/c. This analysis identifies a region of
phase space where the contribution from quark-antiquark annihilation
uncharacteristically approaches the magnitude of the contribution from the
leading process, quark-gluon Compton scattering. The forward-backward angular
asymmetry in the parton center of mass is sensitive to the antiquark content of
the proton and the ubar parton density function can be extracted.Comment: 21 pages, 7 figure
The pion-pion scattering amplitude
We obtain reliable scattering amplitudes consistent with
experimental data, both at low and high energies, and fulfilling appropriate
analyticity properties. We do this by first fitting experimental low energy
() phase shifts and inelasticities with expressions
that incorporate analyticity and unitarity. In particular, for the S wave with
isospin~0, we discuss in detail several sets of experimental data. This
provides low energy partial wave amplitudes that summarize the known
experimental information. Then, we impose Regge behaviour as follows from
factorization and experimental data for the imaginary parts of the scattering
amplitudes at higher energy, and check fulfillment of dispersion relations up
to 0.925 GeV. This allows us to improve our fits. The ensuing
scattering amplitudes are then shown to verify dispersion relations up to 1.42
GeV, as well as crossing sum rules and other consistency
conditions. The improved parametrizations therefore provide a reliable
representation of pion-pion amplitudes with which one can test chiral
perturbation theory calculations, pionium decays, or use as input for
CP-violating decays. In this respect, we find
and
.Comment: Version to be published in Phys. Rev. D. Plain TeX file. (minor
changes). 16 figures (some multiple
Unintegrated parton distributions in nuclei
We study how unintegrated parton distributions in nuclei can be calculated
from the corresponding integrated partons using the EPS09 parametrization. The
role of nuclear effects is presented in terms of the ratio
for both large and small domains.Comment: 9 pages, 4 figure
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