2,261 research outputs found
Calculation of the pion electromagnetic form factor from lattice QCD
We present a lattice calculation of the vector form factor of the pion for
two flavours of non-perturbatively O(a) improved Wilson fermions. For the
measurements we utilise the CLS ensembles which include various lattice
spacings and pion masses down to about 250 MeV. To obtain a fine momentum
resolution near zero momentum transfer (q^2) partially twisted boundary
conditions are employed using several twist angles. Due to the fine resolution
around q^2=0 we are able to determine the slope of the form factor and, in
turn, extract the charge radius of the pion without any model dependence. The
results for the form factor and the charge radius are then compared to chiral
perturbation theory and phenomenological models which are used to extrapolate
the results to the physical point.Comment: 4 pages, 4 figures, talk presented at Hadron 2011: 14th International
Conference on Hadron Spectroscopy, Munich, German
Testing Navigation in Real Space: Contributions to Understanding the Physiology and Pathology of Human Navigation Control
Successful navigation relies on the flexible and appropriate use of metric representations of space or topological knowledge of the environment. Spatial dimensions (2D vs. 3D), spatial scales (vista-scale vs. large-scale environments) and the abundance of visual landmarks critically affect navigation performance and behavior in healthy human subjects. Virtual reality (VR)-based navigation paradigms in stationary position have given insight into the major navigational strategies, namely egocentric (body-centered) and allocentric (world-centered), and the cerebral control of navigation. However, VR approaches are biased towards optic flow and visual landmark processing. This major limitation can be overcome to some extent by increasingly immersive and realistic VR set-ups (including large-screen projections, eye tracking and use of head-mounted camera systems). However, the highly immersive VR settings are difficult to apply particularly to older subjects and patients with neurological disorders because of cybersickness and difficulties with learning and conducting the tasks. Therefore, a need for the development of novel spatial tasks in real space exists, which allows a synchronous analysis of navigational behavior, strategy, visual explorations and navigation-induced brain activation patterns. This review summarizes recent findings from real space navigation studies in healthy subjects and patients with different cognitive and sensory neurological disorders. Advantages and limitations of real space navigation testing and different VR-based navigation paradigms are discussed in view of potential future applications in clinical neurology
The pion vector form factor from lattice QCD and NNLO chiral perturbation theory
We present a comprehensive study of the electromagnetic form factor, the decay constant and the mass of the pion computed in lattice QCD with two degenerate O(a)-improved Wilson quarks at three different lattice spacings in the range 0.05-0.08fm and pion masses between 280 and 630MeV at mπ L ≥ 4. Using partially twisted boundary conditions and stochastic estimators, we obtain a dense set of precise data points for the form factor at very small momentum transfers, allowing for a model-independent extraction of the charge radius. Chiral Perturbation Theory (ChPT) augmented by terms which model lattice artefacts is then compared to the data. At next-to-leading order the effective theory fails to produce a consistent description of the full set of pion observables but describes the data well when only the decay constant and mass are considered. By contrast, using the next-to-next-to-leading order expressions to perform global fits result in a consistent description of all data. We obtain ⟨r2π⟩ = 0.481(33)(13)fm2 as our final result for the charge radius at the physical point. Our calculation also yields estimates for the pion decay constant in the chiral limit, Fπ/F = 1.080(16)(6), the quark condensate, Σ1/3MSbar (2GeV) = 261(13)(1)MeV and several low-energy constants of SU(2) ChPT
Evaporating Rayleigh-B\'enard convection: prediction of interface temperature and global heat transfer modulation
We propose an analytical model to estimate the interface temperature
and the Nusselt number for an evaporating two-layer
Rayleigh-B\'enard configuration in statistically stationary conditions. The
model is based on three assumptions: (i) the Oberbeck-Boussinesq approximation
can be applied to the liquid phase, while the gas thermophysical properties are
generic functions of thermodynamic pressure, local temperature, and vapour
composition, (ii) the Grossmann-Lohse theory for thermal convection can be
applied to the liquid and gas layers separately, (iii) the vapour content in
the gas can be taken as the mean value at the gas-liquid interface. We validate
this setting using direct numerical simulations (DNS) in a parameter space
composed of the Rayleigh number () and the temperature
differential (), which modulates the variation of
state variables in the gas layer. To better disentangle the variable property
effects on and , simulations are performed in two
conditions. First, we consider the case of uniform gas properties except for
the gas density and gas-liquid diffusion coefficient. Second, we include the
variation of specific heat capacity, dynamic viscosity, and thermal
conductivity using realistic equations of state. Irrespective of the employed
setting, the proposed model agrees very well with the numerical simulations
over the entire range of investigated
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