2,905 research outputs found
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Lower Energy Consequences of an Anomalous High-Energy Neutrino Cross-Section
A new strong-interaction has been postulated for neutrinos above ~10^{19} eV
to explain the production of highest-energy cosmic ray events. We derive a
dispersion relation relating the hypothesized high-energy cross-section to the
lower-energy neutrino-nucleon elastic amplitude. Remarkably, we find that the
real forward amplitude becomes anomalous seven orders of magnitude lower in
energy than does the total cross-section. We discuss possible measurable
consequences of this early onset of new neutrino physics, and conclude that a
significantly enhanced elastic \nu-N scattering rate may occur for the neutrino
beams available at Fermilab and CERN.Comment: 13 pages, LaTe
Critical Casimir effect in classical binary liquid mixtures
If a fluctuating medium is confined, the ensuing perturbation of its
fluctuation spectrum generates Casimir-like effective forces acting on its
confining surfaces. Near a continuous phase transition of such a medium the
corresponding order parameter fluctuations occur on all length scales and
therefore close to the critical point this effect acquires a universal
character, i.e., to a large extent it is independent of the microscopic details
of the actual system. Accordingly it can be calculated theoretically by
studying suitable representative model systems.
We report on the direct measurement of critical Casimir forces by total
internal reflection microscopy (TIRM), with femto-Newton resolution. The
corresponding potentials are determined for individual colloidal particles
floating above a substrate under the action of the critical thermal noise in
the solvent medium, constituted by a binary liquid mixture of water and
2,6-lutidine near its lower consolute point. Depending on the relative
adsorption preferences of the colloid and substrate surfaces with respect to
the two components of the binary liquid mixture, we observe that, upon
approaching the critical point of the solvent, attractive or repulsive forces
emerge and supersede those prevailing away from it. Based on the knowledge of
the critical Casimir forces acting in film geometries within the Ising
universality class and with equal or opposing boundary conditions, we provide
the corresponding theoretical predictions for the sphere-planar wall geometry
of the experiment. The experimental data for the effective potential can be
interpreted consistently in terms of these predictions and a remarkable
quantitative agreement is observed.Comment: 30 pages, 17 figure
Critical Casimir forces and adsorption profiles in the presence of a chemically structured substrate
Motivated by recent experiments with confined binary liquid mixtures near
demixing, we study the universal critical properties of a system, which belongs
to the Ising universality class, in the film geometry. We employ periodic
boundary conditions in the two lateral directions and fixed boundary conditions
on the two confining surfaces, such that one of them has a spatially
homogeneous adsorption preference while the other one exhibits a laterally
alternating adsorption preference, resembling locally a single chemical step.
By means of Monte Carlo simulations of an improved Hamiltonian, so that the
leading scaling corrections are suppressed, numerical integration, and
finite-size scaling analysis we determine the critical Casimir force and its
universal scaling function for various values of the aspect ratio of the film.
In the limit of a vanishing aspect ratio the critical Casimir force of this
system reduces to the mean value of the critical Casimir force for laterally
homogeneous ++ and +- boundary conditions, corresponding to the surface spins
on the two surfaces being fixed to equal and opposite values, respectively. We
show that the universal scaling function of the critical Casimir force for
small but finite aspect ratios displays a linear dependence on the aspect ratio
which is solely due to the presence of the lateral inhomogeneity. We also
analyze the order-parameter profiles at criticality and their universal scaling
function which allows us to probe theoretical predictions and to compare with
experimental data.Comment: revised version, section 5.2 expanded; 53 pages, 12 figures, iopart
clas
Neutrino production through hadronic cascades in AGN accretion disks
We consider the production of neutrinos in active galactic nuclei (AGN)
through hadronic cascades. The initial, high energy nucleons are accelerated in
a source above the accretion disk around the central black hole. From the
source, the particles diffuse back to the disk and initiate hadronic cascades.
The observable output from the cascade are electromagnetic radiation and
neutrinos. We use the observed diffuse background X-ray luminosity, which
presumably results {}from this process, to predict the diffuse neutrino flux
close to existing limits from the Frejus experiment. The resulting neutrino
spectrum is down to the \GeV region. We discuss modifications of
this scenario which reduce the predicted neutrino flux.Comment: 12 Pages, LaTeX, TK 92 0
Which blazars are neutrino loud?
Protons accelerated in the cores of active galactic nuclei can effectively
produce neutrinos only if the soft radiation background in the core is
sufficiently high. We find restrictions on the spectral properties and
luminosity of blazars under which they can be strong neutrino sources. We
analyze the possibility that neutrino flux is highly beamed along the rotation
axis of the central black hole. The enhancement of neutrino flux compared to
GeV gamma-ray flux from a given source makes the detection of neutrino point
sources more probable. At the same time the smaller open angle reduces the
number of possible neutrino-loud blazars compared to the number of gamma-ray
loud ones. We present the table of 15 blazars which are the most likely
candidates for the detection by future neutrino telescopes.Comment: 9 pages, 5 figures, version to be published in PR
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