200 research outputs found
Peculiar Velocities of Galaxy Clusters
We investigate the peculiar velocities predicted for galaxy clusters by
theories in the cold dark matter family. A widely used hypothesis identifies
rich clusters with high peaks of a suitably smoothed version of the linear
density fluctuation field. Their peculiar velocities are then obtained by
extrapolating the similarly smoothed linear peculiar velocities at the
positions of these peaks. We test these ideas using large high resolution
N-body simulations carried out within the Virgo supercomputing consortium. We
find that at early times the barycentre of the material which ends up in a rich
cluster is generally very close to a high peak of the initial density field.
Furthermore the mean peculiar velocity of this material agrees well with the
linear value at the peak. The late-time growth of peculiar velocities is,
however, systematically underestimated by linear theory. At the time clusters
are identified we find their rms peculiar velocity to be about 40% larger than
predicted. Nonlinear effects are particularly important in superclusters. These
systematics must be borne in mind when using cluster peculiar velocities to
estimate the parameter combination .Comment: 8 pages, 4 figures; submitted to MNRA
The Debye-Waller Factor in solid 3He and 4He
The Debye-Waller factor and the mean-squared displacement from lattice sites
for solid 3He and 4He were calculated with Path Integral Monte Carlo at
temperatures between 5 K and 35 K, and densities between 38 nm^(-3) and 67
nm^(-3). It was found that the mean-squared displacement exhibits finite-size
scaling consistent with a crossover between the quantum and classical limits of
N^(-2/3) and N^(-1/3), respectively. The temperature dependence appears to be
T^3, different than expected from harmonic theory. An anisotropic k^4 term was
also observed in the Debye-Waller factor, indicating the presence of
non-Gaussian corrections to the density distribution around lattice sites. Our
results, extrapolated to the thermodynamic limit, agree well with recent values
from scattering experiments.Comment: 5 figure
Long-lived neutral-kaon flux measurement for the KOTO experiment
The KOTO ( at Tokai) experiment aims to observe the CP-violating rare
decay by using a long-lived neutral-kaon
beam produced by the 30 GeV proton beam at the Japan Proton Accelerator
Research Complex. The flux is an essential parameter for the measurement
of the branching fraction. Three neutral decay modes, , , and were used to
measure the flux in the beam line in the 2013 KOTO engineering run. A
Monte Carlo simulation was used to estimate the detector acceptance for these
decays. Agreement was found between the simulation model and the experimental
data, and the remaining systematic uncertainty was estimated at the 1.4\%
level. The flux was measured as per protons on a
66-mm-long Au target.Comment: 27 pages, 16 figures. To be appeared in Progress of Theoretical and
Experimental Physic
Gathering in the City: An Annotated Bibliography and Review of the Literature About Human-Plant Interactions in Urban Ecosystems
The past decade has seen resurgence in interest in gathering wild plants and fungi in cities. In addition to gathering by individuals, dozens of groups have emerged in U.S., Canadian, and European cities to facilitate access to nontimber forest products (NTFPs), particularly fruits and nuts, in public and private spaces. Recent efforts within cities to encourage public orchards and food forests, and to incorporate more fruit and nut trees into street tree planting programs indicate a growing recognition among planners that gathering is an important urban activity. Yet the academic literature has little to say about urban gathering practices or the people who engage in them. This annotated bibliography and literature review is a step toward filling the gap in knowledge about the socioecological roles of NTFPs in urban ecosystems in the United States. Our objectives are to demonstrate that gathering—the collecting of food and raw materials—is a type of human-plant interaction that warrants greater attention in urban green space management, and to provide an overview of the literature on human-plant interactions—including gathering—in urban environments. Our review found that very few studies of urban gathering have been done. Consequently, we included gathering field guides, Web sites, and articles from the popular media in our search. These sources, together with the small number of scientific studies of urban gathering, indicated that people derive numerous benefits from gathering plants and fungi in U.S. cities. Gathering provides useful products, encourages physical activity, offers opportunities to connect with and learn about nature, helps strengthen social ties and cultural identities, and, in some contexts, can serve as a strategic tool for ecological restoration. These benefits parallel those identified in environmental psychology and cultural ecology studies of the effects of gardening and being in nature. The literature on human-plant interactions also emphasizes that humans need to be treated as endogenous factors in dynamic, socially and spatially heterogeneous urban ecosystems. Spatially explicit analyses of human-plant interactions show that the distribution of wealth and power within societies affects the composition, species distribution, and structure of urban ecologies. Our review also indicates that tensions exist between NTFP gatherers and land managers, as well as between gatherers and other citizens over gathering, particularly in public spaces. This tension likely is related to perceptions about the impact these practices have on cherished species and spaces. We conclude that gathering is an important urban activity and deserves a greater role in urban management given its social and potential ecological benefits. Research on urban gathering will require sensitivity to existing power imbalances and the use of theoretical frameworks and methodologies that assume humans are integral and not always negative components of ecosystems
Short time evolved wave functions for solving quantum many-body problems
The exact ground state of a strongly interacting quantum many-body system can
be obtained by evolving a trial state with finite overlap with the ground state
to infinite imaginary time. In this work, we use a newly discovered fourth
order positive factorization scheme which requires knowing both the potential
and its gradients. We show that the resultaing fourth order wave function
alone, without further iterations, gives an excellent description of strongly
interacting quantum systems such as liquid 4He, comparable to the best
variational results in the literature.Comment: 5 pages, 3 figures, 1 tabl
Variational Monte Carlo study of the ground state properties and vacancy formation energy of solid para-H2 using a shadow wave function
A Shadow Wave Function (SWF) is employed along with Variational Monte Carlo
techniques to describe the ground state properties of solid molecular
para-hydrogen. The study has been extended to densities below the equilibrium
value, to obtain a parameterization of the SWF useful for the description of
inhomogeneous phases. We also present an estimate of the vacancy formation
energy as a function of the density, and discuss the importance of relaxation
effects near the vacant site
A Monte Carlo study of the three-dimensional Coulomb frustrated Ising ferromagnet
We have investigated by Monte-Carlo simulation the phase diagram of a
three-dimensional Ising model with nearest-neighbor ferromagnetic interactions
and small, but long-range (Coulombic) antiferromagnetic interactions. We have
developed an efficient cluster algorithm and used different lattice sizes and
geometries, which allows us to obtain the main characteristics of the
temperature-frustration phase diagram. Our finite-size scaling analysis
confirms that the melting of the lamellar phases into the paramgnetic phase is
driven first-order by the fluctuations. Transitions between ordered phases with
different modulation patterns is observed in some regions of the diagram, in
agreement with a recent mean-field analysis.Comment: 14 pages, 10 figures, submitted to Phys. Rev.
Zero-point vacancies in quantum solids
A Jastrow wave function (JWF) and a shadow wave function (SWF) describe a
quantum solid with Bose--Einstein condensate; i.e. a supersolid. It is known
that both JWF and SWF describe a quantum solid with also a finite equilibrium
concentration of vacancies x_v. We outline a route for estimating x_v by
exploiting the existing formal equivalence between the absolute square of the
ground state wave function and the Boltzmann weight of a classical solid. We
compute x_v for the quantum solids described by JWF and SWF employing very
accurate numerical techniques. For JWF we find a very small value for the zero
point vacancy concentration, x_v=(1.4\pm0.1) x 10^-6. For SWF, which presently
gives the best variational description of solid 4He, we find the significantly
larger value x_v=(1.4\pm0.1) x 10^-3 at a density close to melting. We also
study two and three vacancies. We find that there is a strong short range
attraction but the vacancies do not form a bound state.Comment: 19 pages, submitted to J. Low Temp. Phy
Vortex Dynamics in Superfluid Systems: Cyclotron Type Motion
Vortex dynamics in superfluids is investigated in the framework of the
nonlinear Schr\"{o}dinger equation. The natural motion of the vortex is of
cyclotron type, whose frequency is found to be on the order of phonon velocity
divided by the coherence length, and may be heavily damped due to phonon
radiation. Trapping foreign particles into the vortex core can reduce the
cyclotron frequency and make the cyclotron motion underdamped. The density
fluctuations can follow the vortex motion adiabatically within the phonon wave
length at the cyclotron frequency, which results in a further downward
renormalization of the cyclotron frequency. We have also discussed applications
on the dynamics of vortices in superconducting films.Comment: 21 pages, 1 figure include
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