22,292 research outputs found
Dynamical Supersymmetry Breaking and Low Energy Gauge Mediation
Dynamical breaking of supersymmetry was long thought to be an exceptional
phenomenon, but recent developments have altered this view. A question of great
interest in the current framework is the value of the underlying scale of
supersymmetry breaking. The "little hierarchy" problem suggests that
supersymmetry should be broken at low energies. Within one class of models, low
energy breaking be achieved as a consequence of symmetries, without requiring
odd coincidences. The low energy theories are distinguished by the presence or
absence of symmetries; in either case, and especially the latter one often
finds modifications of the minimal gauge-mediated spectrum which can further
ameliorate problems of fine tuning. Various natural mechanisms exist to solve
the problem in this framework.Comment: 20 pages (minor change in referencing
One- and two-particle microrheology
We study the dynamics of rigid spheres embedded in viscoelastic media and
address two questions of importance to microrheology. First we calculate the
complete response to an external force of a single bead in a homogeneous
elastic network viscously coupled to an incompressible fluid. From this
response function we find the frequency range where the standard assumptions of
microrheology are valid. Second we study fluctuations when embedded spheres
perturb the media around them and show that mutual fluctuations of two
separated spheres provide a more accurate determination of the complex shear
modulus than do the fluctuations of a single sphere.Comment: 4 pages, 1 figur
Predictive validity of the HCR-20 for inpatient aggression:the effect of intellectual disability on accuracy
BackgroundPeople with intellectual disability (ID) account for a large proportion of aggressive incidents in secure and forensic psychiatric services. Although the Historical, Clinical, Risk Management 20 (HCR-20) has good predictive validity in inpatient settings, it does not perform equally in all groups and there is little evidence for its efficacy in those with ID.MethodA pseudo-prospective cohort study of the predictive efficacy of the HCR-20 for those with ID (n = 109) was conducted in a UK secure mental health setting using routinely collected risk data. Performance of the HCR-20 in the ID group was compared with a comparison group of adult inpatients without an ID (n = 504). Analysis controlled for potential covariates including security level, length of stay, gender and diagnosis.ResultsThe HCR-20 total score was a significant predictor of any aggression and of physical aggression for both groups, although the area under the curve values did not reach the threshold for a large effect size. The clinical subscale performed significantly better in those without an ID compared with those with. The ID group had a greater number of relevant historical and risk management items. The clinicians' summary judgment significantly predicted both types of aggressive outcomes in the ID group, but did not predict either in those without an ID.ConclusionsThis study demonstrates that, after controlling for a range of potential covariates, the HCR-20 is a significant predictor of inpatient aggression in people with an ID and performs as well as for a comparison group of mentally disordered individuals without ID. The potency of HCR-20 subscales and items varied between the ID and comparison groups suggesting important target areas for improved prediction and risk management interventions in those with ID
Temporal variations of the anomalous oxygen component, 1977 - 1984
A survey is of the long term temporal variations of 6.6 to 12 MeV/nucleon anomalous oxygen at 1 AU covering the period 1977 to 1984. This time interval included the recent solar maximum, with the recovery at neutron monitor energies beginning in 1982. During this time interval, 6.6 to 12 MeV/nucleon 0 fluxes decreased by at least a factor of 50, and indeed remained below the instrumental detection threshold after 1979. By late 1984, neutron monitors had recovered to roughly 1979 levels from the 1982 solar maximum, and anomalous O still remained below the detection threshold
Entropy and weak solutions in the thermal model for the compressible Euler equations
Among the existing models for compressible fluids, the one by Kataoka and
Tsutahara (KT model, Phys. Rev. E 69, 056702, 2004) has a simple and rigorous
theoretical background. The drawback of this KT model is that it can cause
numerical instability if the local Mach number exceeds 1. The precise mechanism
of this instability has not yet been clarified. In this paper, we derive
entropy functions whose local equilibria are suitable to recover the Euler-like
equations in the framework of the lattice Boltzmann method for the KT model.
Numerical examples are also given, which are consistent with the above
theoretical arguments, and show that the entropy condition is not fully
guaranteed in KT model. The negative entropy may be the inherent cause for the
non-physical oscillations in the vicinity of the shock. In contrast to these
Karlin's microscopic entropy approach, the corresponding subsidiary entropy
condition in the LBM calculation could also be deduced explicitly from the
macroscopic version, which provides some insights on the numerical instability
of the lattice Boltzmann model for shock calculation.Comment: 27 pages,6 figure
Localized Breathing Modes in Granular Crystals with Defects
We investigate nonlinear localized modes at light-mass impurities in a
one-dimensional, strongly-compressed chain of beads under Hertzian contacts.
Focusing on the case of one or two such "defects", we analyze the problem's
linear limit to identify the system eigenfrequencies and the linear defect
modes. We then examine the bifurcation of nonlinear defect modes from their
linear counterparts and study their linear stability in detail. We identify
intriguing differences between the case of impurities in contact and ones that
are not in contact. We find that the former bears similarities to the single
defect case, whereas the latter features symmetry-breaking bifurcations with
interesting static and dynamic implications
The response function of a sphere in a viscoelastic two-fluid medium
In order to address basic questions of importance to microrheology, we study
the dynamics of a rigid sphere embedded in a model viscoelastic medium
consisting of an elastic network permeated by a viscous fluid. We calculate the
complete response of a single bead in this medium to an external force and
compare the result to the commonly-accepted, generalized Stokes-Einstein
relation (GSER). We find that our response function is well approximated by the
GSER only within a particular frequency range determined by the material
parameters of both the bead and the network. We then discuss the relevance of
this result to recent experiments. Finally we discuss the approximations made
in our solution of the response function by comparing our results to the exact
solution for the response function of a bead in a viscous (Newtonian) fluid.Comment: 12 pages, 2 figure
The Deficit of Distant Galaxy Clusters in the RIXOS X-ray Survey
Clusters of galaxies are the largest gravitationally bound systems and
therefore provide an important way of studying the formation and evolution of
the large scale structure of the Universe. Cluster evolution can be inferred
from observations of the X-ray emission of the gas in distant clusters, but
interpreting these data is not straightforward. In a simplified view, clusters
grow from perturbations in the matter distribution: their intracluster gas is
compressed and shock-heated by the gravitational collapse. The resulting
X-ray emission is determined by the hydrostatic equilibrium of the gas in the
changing gravitational potential. However, if processes such as radiative
cooling or pre-collapse heating of the gas are important, then the X-ray
evolution will be strongly influenced by the thermal history of the gas. Here
we present the first results from a faint flux-limited sample of X-ray selected
clusters compiled as part of the ROSAT International X-ray and Optical Survey
(RIXOS). Very few distant clusters have been identified. Most importantly,
their redshift distribution appears to be inconsistent with simple models based
on the evolution of the gravitational potential. Our results suggest that
radiative cooling or non-gravitational heating of the intracluster gas must
play an important role in the evolution of clusters.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Intrinsic Energy Localization through Discrete Gap Breathers in One-Dimensional Diatomic Granular Crystals
We present a systematic study of the existence and stability of discrete
breathers that are spatially localized in the bulk of a one-dimensional chain
of compressed elastic beads that interact via Hertzian contact. The chain is
diatomic, consisting of a periodic arrangement of heavy and light spherical
particles. We examine two families of discrete gap breathers: (1) an unstable
discrete gap breather that is centered on a heavy particle and characterized by
a symmetric spatial energy profile and (2) a potentially stable discrete gap
breather that is centered on a light particle and is characterized by an
asymmetric spatial energy profile. We investigate their existence, structure,
and stability throughout the band gap of the linear spectrum and classify them
into four regimes: a regime near the lower optical band edge of the linear
spectrum, a moderately discrete regime, a strongly discrete regime that lies
deep within the band gap of the linearized version of the system, and a regime
near the upper acoustic band edge. We contrast discrete breathers in anharmonic
FPU-type diatomic chains with those in diatomic granular crystals, which have a
tensionless interaction potential between adjacent particles, and highlight in
that the asymmetric nature of the latter interaction potential may lead to a
form of hybrid bulk-surface localized solutions
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