797 research outputs found
A Lindenstrauss theorem for some classes of multilinear mappings
Under some natural hypotheses, we show that if a multilinear mapping belongs
to some Banach multlinear ideal, then it can be approximated by multilinear
mappings belonging to the same ideal whose Arens extensions simultaneously
attain their norms. We also consider the class of symmetric multilinear
mappings.Comment: 11 page
Towards a mesoscopic model of water-like fluids with hydrodynamic interactions
We present a mesoscopic lattice model for non-ideal fluid flows with
directional interactions, mimicking the effects of hydrogen-bonds in water. The
model supports a rich and complex structural dynamics of the orientational
order parameter, and exhibits the formation of disordered domains whose size
and shape depend on the relative strength of directional order and thermal
diffusivity. By letting the directional forces carry an inverse density
dependence, the model is able to display a correlation between ordered domains
and low density regions, reflecting the idea of water as a denser liquid in the
disordered state than in the ordered one
Negative ion Time Projection Chamber operation with SF at nearly atmospheric pressure
We present measurements of drift velocities and mobilities of some innovative
negative ion gas mixtures at nearly atmospheric pressure based on SF as
electronegative capture agent and of pure SF at various pressures,
performed with the NITEC detector. NITEC is a Time Projection Chamber with 5 cm
drift distance readout by a GEMPix, a triple thin GEMs coupled to a
Quad-Timepix chip, directly sensitive to the deposited charge on each of the 55
55 m pixel. Our results contribute to expanding the knowledge
on the innovative use of SF as negative ion gas and extend to triple thin
GEMs the possibility of negative ion operation for the first time. Above all,
our findings show the feasibility of negative ion operation with
He:CF:SF at 610 Torr, opening extremely interesting possibility for
next generation directional Dark Matter detectors at 1 bar
An accurate and efficient Lagrangian sub-grid model
A computationally efficient model is introduced to account for the sub-grid
scale velocities of tracer particles dispersed in statistically homogeneous and
isotropic turbulent flows. The model embeds the multi-scale nature of turbulent
temporal and spatial correlations, that are essential to reproduce
multi-particle dispersion. It is capable to describe the Lagrangian diffusion
and dispersion of temporally and spatially correlated clouds of particles.
Although the model neglects intermittent corrections, we show that pair and
tetrad dispersion results nicely compare with Direct Numerical Simulations of
statistically isotropic and homogeneous turbulence. This is in agreement
with recent observations that deviations from self-similar pair dispersion
statistics are rare events
Scalar Turbulence in Convective Boundary Layers by Changing the Entrainment Flux
Abstract
A large-eddy simulation model is adopted to investigate the evolution of scalars transported by atmospheric cloud-free convective boundary layer flows. Temperature fluctuations due to the ground release of sensible heat and concentration fluctuations of a trace gas emitted at the homogeneous surface are mixed by turbulence within the unstable boundary layer. On the top, the entrainment zone is varied to obtain two distinct situations: (i) the temperature inversion is strong and the trace gas increment across the entrainment region is small, yielding to a small top flux with respect to the surface emission; (ii) the temperature inversion at the top of the convective boundary layer is weak, and the scalar increment large enough to achieve a concentration flux toward the free atmosphere that overwhelms the surface flux. In both cases, an estimation of the entrainment flux is obtained within a simple model, and it is tested against numerical data. The evolution of the scalar profiles is discussed in terms of the different entrainment–surface flux ratios.
Results show that, when entrainment at the top of the boundary layer is weak, temperature and trace gas scalar fields are strongly correlated, particularly in the lower part of the boundary layer. This means that they exhibit similar behavior from the largest down to the smallest spatial scales. However, when entrainment is strong, as moving from the surface, differences in the transport of the two scalars arise.
Finally, it is shown that, independently of the scalar regime, the temperature field exhibits more intermittent fluctuations than the trace gas
Approximate (NLO)-L-3 Higgs-boson production cross section using physical-kernel constraints
The single-logarithmic enhancement of the physical kernel for Higgs
production by gluon-gluon fusion in the heavy top-quark limit is employed to
derive the leading so far unknown contributions, ln^{5,4,3}(1-z), to the N^3LO
coefficient function in the threshold expansion. Also using knowledge from
Higgs-exchange DIS to estimate the remaining terms not vanishing for z =
m_H^2/s^hat -> 1, these results are combined with the recently completed soft +
virtual contributions to provide an uncertainty band for the complete N^3LO
correction. For the 2008 MSTW parton distributions these N^3LO contributions
increase the cross section at 14 TeV by (10 +- 2)% and (3 +- 2.5)% for the
standard choices mu_R = m_H and mu_R = m_H/2 of the renormalization scale. The
remaining uncertainty arising from the hard-scattering cross sections can be
quantified as no more than 5%, which is smaller than that due to the strong
coupling and the parton distributions.Comment: 24 pages, 6 figures. arXiv-dysfunctional LaTeX macro for beta_0
replaced. Minor text changes with 3 new references. Version to appear in JHE
Vacuum polarization near cosmic string in RS2 brane world
Gravitational field of cosmic strings in theories with extra spatial
dimensions must differ significantly from that in the Einstein's theory. This
means that all gravity induced properties of cosmic strings need to be revised
too. Here we consider the effect of vacuum polarization outside a straight
infinitely thin cosmic string embedded in a RS2 brane world. Perturbation
technique combined with the method of dimensional regularization is used to
calculate for a massless scalar field.Comment: 8 pages, RevTeX
Casimir energies with finite-width mirrors
We use a functional approach to the Casimir effect in order to evaluate the
exact vacuum energy for a real scalar field in dimensions, in the
presence of backgrounds that, in a particular limit, impose Dirichlet boundary
conditions on one or two parallel surfaces. Outside of that limit, the
background may be thought of as describing finite-width mirrors with
frequency-dependent transmission and reflection coefficients. We provide new
explicit results for the Casimir energy in some particular backgroundsComment: 18 pages, no figures. Version to appear in Phys. Rev.
The proximity force approximation for the Casimir energy as a derivative expansion
The proximity force approximation (PFA) has been widely used as a tool to
evaluate the Casimir force between smooth objects at small distances. In spite
of being intuitively easy to grasp, it is generally believed to be an
uncontrolled approximation. Indeed, its validity has only been tested in
particular examples, by confronting its predictions with the next to leading
order (NTLO) correction extracted from numerical or analytical solutions
obtained without using the PFA. In this article we show that the PFA and its
NTLO correction may be derived within a single framework, as the first two
terms in a derivative expansion. To that effect, we consider the Casimir energy
for a vacuum scalar field with Dirichlet conditions on a smooth curved surface
described by a function in front of a plane. By regarding the Casimir
energy as a functional of , we show that the PFA is the leading term in a
derivative expansion of this functional. We also obtain the general form of
corresponding NTLO correction, which involves two derivatives of . We
show, by evaluating this correction term for particular geometries, that it
properly reproduces the known corrections to PFA obtained from exact
evaluations of the energy.Comment: Minor changes. Version to appear in Phys. Rev.
Exact zero-point interaction energy between cylinders
We calculate the exact Casimir interaction energy between two perfectly
conducting, very long, eccentric cylindrical shells using a mode summation
technique. Several limiting cases of the exact formula for the Casimir energy
corresponding to this configuration are studied both analytically and
numerically. These include concentric cylinders, cylinder-plane, and eccentric
cylinders, for small and large separations between the surfaces. For small
separations we recover the proximity approximation, while for large separations
we find a weak logarithmic decay of the Casimir interaction energy, typical of
cylindrical geometries.Comment: 20 pages, 7 figure
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