9,428 research outputs found
A Statistical Model for Simultaneous Template Estimation, Bias Correction, and Registration of 3D Brain Images
Template estimation plays a crucial role in computational anatomy since it
provides reference frames for performing statistical analysis of the underlying
anatomical population variability. While building models for template
estimation, variability in sites and image acquisition protocols need to be
accounted for. To account for such variability, we propose a generative
template estimation model that makes simultaneous inference of both bias fields
in individual images, deformations for image registration, and variance
hyperparameters. In contrast, existing maximum a posterori based methods need
to rely on either bias-invariant similarity measures or robust image
normalization. Results on synthetic and real brain MRI images demonstrate the
capability of the model to capture heterogeneity in intensities and provide a
reliable template estimation from registration
Three-Quark Potential in SU(3) Lattice QCD
The static three-quark (3Q) potential is measured in the SU(3) lattice QCD
with and at the quenched level. From the 3Q Wilson
loop, the 3Q ground-state potential is extracted using the
smearing technique for the ground-state enhancement. With accuracy better than
a few %, is well described by a sum of a constant, the two-body
Coulomb term and the three-body linear confinement term , where denotes the minimal length of the color flux tube
linking the three quarks. By comparing with the Q- potential, we
find a universal feature of the string tension, , as well as the one-gluon-exchange result for the
Coulomb coefficient, .Comment: 7 pages, 3 figur
The flux distribution of the three quark system in SU(3)
We study the abelian color-flux distribution of the three quark system in the
maximally abelian gauge on SU(3) lattices. The distribution of the color
electric field suggests , which might be interpreted through the dual
superconductor picture as the result of the vacuum pressure in the confined
phase. In order to clarify the flux structure, we investigate the color
electric field in the three quark system also in the monopole part and in the
photon part.Comment: 3pages, 5figures, Lattice2002(topology
Cold guided beams of water isotopologs
Electrostatic velocity filtering and guiding is an established technique to
produce high fluxes of cold polar molecules. In this paper we clarify different
aspects of this technique by comparing experiments to detailed calculations. In
the experiment, we produce cold guided beams of the three water isotopologs
H2O, D2O and HDO. Their different rotational constants and orientations of
electric dipole moments lead to remarkably different Stark shift properties,
despite the molecules being very similar in a chemical sense. Therefore, the
signals of the guided water isotopologs differ on an absolute scale and also
exhibit characteristic electrode voltage dependencies. We find excellent
agreement between the relative guided fractions and voltage dependencies of the
investigated isotopologs and predictions made by our theoretical model of
electrostatic velocity filtering.Comment: 14 pages, 13 figures; small changes to the text, updated reference
Velocity-selected molecular pulses produced by an electric guide
Electrostatic velocity filtering is a technique for the production of
continuous guided beams of slow polar molecules from a thermal gas. We extended
this technique to produce pulses of slow molecules with a narrow velocity
distribution around a tunable velocity. The pulses are generated by
sequentially switching the voltages on adjacent segments of an electric
quadrupole guide synchronously with the molecules propagating at the desired
velocity. This technique is demonstrated for deuterated ammonia (ND),
delivering pulses with a velocity in the range of and a
relative velocity spread of at FWHM. At velocities around
, the pulses contain up to molecules each. The data are
well reproduced by Monte-Carlo simulations, which provide useful insight into
the mechanisms of velocity selection.Comment: 8 pages, 6 figure
Monte Carlo simulations of random copolymers at a selective interface
We investigate numerically using the bond--fluctuation model the adsorption
of a random AB--copolymer at the interface between two solvents. From our
results we infer several scaling relations: the radius of gyration of the
copolymer in the direction perpendicular to the interface () scales
with , the interfacial selectivity strength, as
where is the usual Flory exponent and
is the copolymer's length; furthermore the monomer density at the interface
scales as for small . We also determine numerically the
monomer densities in the two solvents and discuss their dependence on the
distance from the interface.Comment: Latex text file appended with figures.tar.g
Quenching Effects in the Hadron Spectrum
Lattice QCD has generated a wealth of data in hadronic physics over the last
two decades. Until relatively recently, most of this information has been
within the "quenched approximation" where virtual quark--anti-quark pairs are
neglected. This review presents a descriptive discussion of the effects of
removing this approximation in the calculation of hadronic masses.Comment: To appear in "Lattice Hadron Physics", ed. A.C. Kalloniatis, D.B.
Leinweber and A.G. William
Electrostatic extraction of cold molecules from a cryogenic reservoir
We present a method which delivers a continuous, high-density beam of slow
and internally cold polar molecules. In our source, warm molecules are first
cooled by collisions with a cryogenic helium buffer gas. Cold molecules are
then extracted by means of an electrostatic quadrupole guide. For ND the
source produces fluxes up to molecules/s with
peak densities up to molecules/cm. For
HCO the population of rovibrational states is monitored by depletion
spectroscopy, resulting in single-state populations up to .Comment: 4 pages, 4 figures, changes to the text, updated figures and
reference
Algal nutrient competition in continuous culture
Natural phytoplankton from Lake Constance was exposed to competition experiments in continuous culture at 6 different molar rations of PratioSi as potentially limiting nutrients. As predicted by theory there was competitive exclusion of all species but one at single nutrient limitation, and coexistence of two species in steady state when P was limiting for one and Si for the other species. In one case coexistence of three species occurred. Equilibrium species composition, which stabilized after 3 to 6 weeks of cultivation, was independent of the species composition of the inoculum and only controlled by the nutrient regime. Comparability of experimental results and field observations is discussed
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