3,960 research outputs found
An upper bound from helioseismology on the stochastic background of gravitational waves
The universe is expected to be permeated by a stochastic background of
gravitational radiation of astrophysical and cosmological origin. This
background is capable of exciting oscillations in solar-like stars. Here we
show that solar-like oscillators can be employed as giant hydrodynamical
detectors for such a background in the muHz to mHz frequency range, which has
remained essentially unexplored until today. We demonstrate this approach by
using high-precision radial velocity data for the Sun to constrain the
normalized energy density of the stochastic gravitational-wave background
around 0.11 mHz. These results open up the possibility for asteroseismic
missions like CoRoT and Kepler to probe fundamental physics.Comment: 6 pages, 2 figures. Updated to match published versio
Hypernuclear No-Core Shell Model
We extend the No-Core Shell Model (NCSM) methodology to incorporate
strangeness degrees of freedom and apply it to single- hypernuclei.
After discussing the transformation of the hyperon-nucleon (YN) interaction
into Harmonic-Oscillator (HO) basis and the Similarity Renormalization Group
transformation applied to it to improve model-space convergence, we present two
complementary formulations of the NCSM, one that uses relative Jacobi
coordinates and symmetry-adapted basis states to fully exploit the symmetries
of the hypernuclear Hamiltonian, and one working in a Slater determinant basis
of HO states where antisymmetrization and computation of matrix elements is
simple and to which an importance-truncation scheme can be applied. For the
Jacobi-coordinate formulation, we give an iterative procedure for the
construction of the antisymmetric basis for arbitrary particle number and
present the formulae used to embed two- and three-baryon interactions into the
many-body space. For the Slater-determinant formulation, we discuss the
conversion of the YN interaction matrix elements from relative to
single-particle coordinates, the importance-truncation scheme that tailors the
model space to the description of the low-lying spectrum, and the role of the
redundant center-of-mass degrees of freedom. We conclude with a validation of
both formulations in the four-body system, giving converged ground-state
energies for a chiral Hamiltonian, and present a short survey of the
hyper-helium isotopes.Comment: 17 pages, 8 figures; accepted versio
Characterizing gas flow from aerosol particle injectors
A novel methodology for measuring gas flow from small orifices or nozzles
into vacuum is presented. It utilizes a high-intensity femtosecond laser pulse
to create a plasma within the gas plume produced by the nozzle, which is imaged
by a microscope. Calibration of the imaging system allows for the extraction of
absolute number densities. We show detection down to helium densities of
~cm with a spatial resolution of a few micrometer. The
technique is used to characterize the gas flow from a convergent-nozzle aerosol
injector [Struct.\ Dyn.~2, 041717 (2015)] as used in single-particle
diffractive imaging experiments at free-electron laser sources. Based on the
measured gas-density profile we estimate the scattering background signal under
typical operating conditions of single-particle imaging experiments and
estimate that fewer than 50 photons per shot can be expected on the detector
Bulk dynamics of Brownian hard disks: Dynamical density functional theory versus experiments on two-dimensional colloidal hard spheres
Using dynamical density functional theory (DDFT), we theoretically study
Brownian self-diffusion and structural relaxation of hard disks and compare to
experimental results on quasi two-dimensional colloidal hard spheres. To this
end, we calculate the self and distinct van Hove correlation functions by
extending a recently proposed DDFT-approach for three-dimensional systems to
two dimensions. We find that the theoretical results for both self- and
distinct part of the van Hove function are in very good quantitative agreement
with the experiments up to relatively high fluid packing fractions of roughly
0.60. However, at even higher densities, deviations between experiment and the
theoretical approach become clearly visible. Upon increasing packing fraction,
in experiments the short-time self diffusive behavior is strongly affected by
hydrodynamic effects and leads to a significant decrease in the respective
mean-squared displacement. In contrast, and in accordance with previous
simulation studies, the present DDFT which neglects hydrodynamic effects, shows
no dependence on the particle density for this quantity
Optimizing aerodynamic lenses for single-particle imaging
A numerical simulation infrastructure capable of calculating the flow of gas
and the trajectories of particles through an aerodynamic lens injector is
presented. The simulations increase the fundamental understanding and predict
optimized injection geometries and parameters. Our simulation results were
compared to previous reports and also validated against experimental data for
500 nm polystyrene spheres from an aerosol-beam- characterization setup. The
simulations yielded a detailed understanding of the radial phase-space
distribution and highlighted weaknesses of current aerosol injectors for
single-particle diffractive imaging. With the aid of these simulations we
developed new experimental implementations to overcome current limitations
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