1,547 research outputs found
A Cost-Effective Design for a Neutrino Factory
There have been active efforts in the U.S., Europe, and Japan on the design
of a Neutrino Factory. This type of facility produces intense beams of
neutrinos from the decay of muons in a high energy storage ring. In the U.S., a
second detailed Feasibility Study (FS2) for a Neutrino Factory was completed in
2001. Since that report was published, new ideas in bunching, cooling and
acceleration of muon beams have been developed. We have incorporated these
ideas into a new facility design, which we designate as Study 2B (ST2B), that
should lead to significant cost savings over the FS2 design.Comment: 46 pages, 38 figures; to be submitted to Physical Review Special
Topics: Accelerators and Beam
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Automatic procedure for realistic 3D finite element modelling of human brain for bioelectromagnetic computations
Realistic computer modelling of biological objects requires building of very accurate and realistic computer models based on geometric and material data, type, and accuracy of numerical analyses. This paper presents some of the automatic tools and algorithms that were used to build accurate and realistic 3D finite element (FE) model of whole-brain. These models were used to solve the forward problem in magnetic field tomography (MFT) based on Magnetoencephalography (MEG). The forward problem involves modelling and computation of magnetic fields produced by human brain during cognitive processing. The geometric parameters of the model were obtained from accurate Magnetic Resonance Imaging (MRI) data and the material properties – from those obtained from Diffusion Tensor MRI (DTMRI). The 3D FE models of the brain built using this approach has been shown to be very accurate in terms of both geometric and material properties. The model is stored on the computer in Computer-Aided Parametrical Design (CAD) format. This allows the model to be used in a wide a range of methods of analysis, such as finite element method (FEM), Boundary Element Method (BEM), Monte-Carlo Simulations, etc. The generic model building approach presented here could be used for accurate and realistic modelling of human brain and many other biological objects
Effect of the Equivalence Between Topological and Electric Charge on the Magnetization of the Hall Ferromagnet
The dependence on temperature of the spin magnetization of a two-dimensional
electron gas at filling factor unity is studied. Using classical Monte Carlo
simulations we analyze the effect that the equivalence between topological and
electrical charge has on the the behavior of the magnetization. We find that at
intermediate temperatures the spin polarization increases in a thirty per cent
due to the Hartree interaction between charge fluctuations.Comment: 4 pages. Submitted to Phys.Rev.
Fermi surface instabilities at finite Temperature
We present a new method to detect Fermi surface instabilities for interacting
systems at finite temperature. We first apply it to a list of cases studied
previously, recovering already known results in a very economic way, and
obtaining most of the information on the phase diagram analytically. As an
example, in the continuum limit we obtain the critical temperature as an
implicit function of the magnetic field and the chemical potential
. By applying the method to a model proposed to describe reentrant
behavior in , we reproduce the phase diagram obtained
experimentally and show the presence of a non-Fermi Liquid region at
temperatures above the nematic phase.Comment: 10 pages, 10 figure
Proximity of Iron Pnictide Superconductors to a Quantum Tricritical Point
We determine the nature of the magnetic quantum critical point in the doped
LaFeAsO using a set of constrained density functional calculations that provide
ab initio coefficients for a Landau order parameter analysis. The system turns
out to be remarkably close to a quantum tricritical point, where the nature of
the phase transition changes from first to second order. We compare with the
effective field theory and discuss the experimental consequences.Comment: 4 pages, 4 figure
N-flation
The presence of many axion fields in four-dimensional string vacua can lead
to a simple, radiatively stable realization of chaotic inflation.Comment: 16 pages, 0 figures, latex; v2: added refs; v3: more refs, correction
to \S2.
An Inflaton Mass Problem in String Inflation from Threshold Corrections to Volume Stabilization
Inflationary models whose vacuum energy arises from a D-term are believed not
to suffer from the supergravity eta problem of F-term inflation. That is,
D-term models have the desirable property that the inflaton mass can naturally
remain much smaller than the Hubble scale. We observe that this advantage is
lost in models based on string compactifications whose volume is stabilized by
a nonperturbative superpotential: the F-term energy associated with volume
stabilization causes the eta problem to reappear. Moreover, any shift
symmetries introduced to protect the inflaton mass will typically be lifted by
threshold corrections to the volume-stabilizing superpotential. Using threshold
corrections computed by Berg, Haack, and Kors, we illustrate this point in the
example of the D3-D7 inflationary model, and conclude that inflation is
possible, but only for fine-tuned values of the stabilized moduli. More
generally, we conclude that inflationary models in stable string
compactifications, even D-term models with shift symmetries, will require a
certain amount of fine-tuning to avoid this new contribution to the eta
problem.Comment: 25 page
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Enabling HCCI modeling: The RIOT/CMCS Web Service for Automatic Reaction Mechanism Reduction
New approaches are being developed to facilitate multidisciplinary collaborative research of Homogeneous Charge Compression Ignition (HCCI) combustion processes. In this paper, collaborative sharing of the Range Identification and Optimization Toolkit (RIOT) and related data and models is discussed. RIOT is a developmental approach to reduce the computational complexity of detailed chemical kinetic mechanisms, enabling their use in modeling kinetically-controlled combustion applications such as HCCI. These approaches are being developed and piloted as a part of the Collaboratory for Multiscale Chemical Sciences (CMCS) project. The capabilities of the RIOT code are shared through a portlet in the CMCS portal that allows easy specification and processing of RIOT inputs, remote execution of RIOT, tracking of data pedigree and translation of RIOT outputs (such as the reduced model) to a table view and to the commonly-used CHEMKIN mechanism format. The reduced model is thus immediately ready to be used for more efficient simulation of the chemically reacting system of interest. This effort is motivated by the need to improve computational efficiency in modeling HCCI systems. Preliminary use of the web service to obtain reduced models for this application has yielded computational speedup factors of up to 20 as presented in this paper
Threat-sensitive anti-predator defence in precocial wader, the northern lapwing Vanellus vanellus
Birds exhibit various forms of anti-predator behaviours to avoid reproductive failure, with mobbing—observation, approach and usually harassment of a predator—being one of the most commonly observed. Here, we investigate patterns of temporal variation in the mobbing response exhibited by a precocial species, the northern lapwing (Vanellus vanellus). We test whether brood age and self-reliance, or the perceived risk posed by various predators, affect mobbing response of lapwings. We quantified aggressive interactions between lapwings and their natural avian predators and used generalized additive models to test how timing and predator species identity are related to the mobbing response of lapwings. Lapwings diversified mobbing response within the breeding season and depending on predator species. Raven Corvus corax, hooded crow Corvus cornix and harriers evoked the strongest response, while common buzzard Buteo buteo, white stork Ciconia ciconia, black-headed gull Chroicocephalus ridibundus and rook Corvus frugilegus were less frequently attacked. Lapwings increased their mobbing response against raven, common buzzard, white stork and rook throughout the breeding season, while defence against hooded crow, harriers and black-headed gull did not exhibit clear temporal patterns. Mobbing behaviour of lapwings apparently constitutes a flexible anti-predator strategy. The anti-predator response depends on predator species, which may suggest that lapwings distinguish between predator types and match mobbing response to the perceived hazard at different stages of the breeding cycle. We conclude that a single species may exhibit various patterns of temporal variation in anti-predator defence, which may correspond with various hypotheses derived from parental investment theory
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