26,807 research outputs found
String Evolution with Friction
We study the effects of friction on the scaling evolution of string networks
in condensed matter and cosmological contexts. We derive a generalized
`one-scale' model with the string correlation length and velocity as
dynamical variables. In non-relativistic systems, we obtain a well-known
law, showing that loop production is important. For
electroweak cosmic strings, we show transient damped epoch scaling with
(or, in the matter era, ). A low initial
density implies an earlier period with . For GUT strings, the
approach to linear scaling is faster than previously estimated.Comment: 8 pages, uuencoded gziped .ps file. Paper submitted to Phys. Rev.
Let
Contribution of domain wall networks to the CMB power spectrum
We use three domain wall simulations from the radiation era to the late time
dark energy domination era based on the PRS algorithm to calculate the
energy-momentum tensor components of domain wall networks in an expanding
universe. Unequal time correlators in the radiation, matter and cosmological
constant epochs are calculated using the scaling regime of each of the
simulations. The CMB power spectrum of a network of domain walls is determined.
The first ever quantitative constraint for the domain wall surface tension is
obtained using a Markov chain Monte Carlo method; an energy scale of domain
walls of 0.93 MeV, which is close but below the Zel'dovich bound, is
determined.Comment: Submitted to Physics Letters
Effects of Inflation on a Cosmic String Loop Population
We study the evolution of simple cosmic string loop solutions in an
inflationary universe. We show, for the particular case of circular loops, that
periodic solutions do exist in a de Sitter universe, below a critical loop
radius . On the other hand, larger loops freeze in comoving
coordinates, and we explicitly show that they can survive more -foldings of
inflation than point-like objects. We discuss the implications of these
findings for the survival of realistic cosmic string loops during inflation,
and for the general characteristics of post-inflationary cosmic string
networks. We also consider the analogous solutions for domain walls, in which
case the critical radius is .Comment: 5 pages, 5 figures, accepted for publication in Physical Review
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DEveloping a Complex Intervention for DEteriorating Patients using Theoretical Modelling (DECIDE study): study protocol
AIM: To develop a theory-based complex intervention (targeting nursing staff), to enhance enablers and overcome barriers to enacting expected behaviour when monitoring patients and responding to abnormal vital signs that signal deterioration.
DESIGN: A mixed method design including structured observations on hospital wards, field notes, brief, un-recorded interviews and semi-structured interviews to inform the development of an intervention to enhance practice.
METHODS: Semi-structured interviews will be conducted with nursing staff using a topic guide informed by the Theoretical Domains Framework. Semi-structured interviews will be transcribed verbatim and coded deductively into the 14 Theoretical Domains Framework domains and then inductively into 'belief statements'. Priority domains will be identified and mapped to appropriate behaviour change techniques. Intervention content and mode of delivery (how behaviour change techniques are operationalised) will be developed using nominal groups, during which participants (clinicians) will rank behaviour change techniques /mode of delivery combinations according to acceptability and feasibility. Findings will be synthesised to develop an intervention manual.
DISCUSSION: Despite being a priority for clinicians, researchers and policymakers for two decades, 'sub-optimal care' of the deteriorating ward patient persists. Existing interventions have been largely educational (i.e., targeting assumed knowledge deficits) with limited evidence that they change staff behaviour. Staff behaviour when monitoring and responding to abnormal vital signs is likely influenced by a range of mediators that includes barriers and enablers.
IMPACT: Systematically applying theory and evidence-based methods, will result in the specification of an intervention which is more likely to result in behaviour change and can be tested empirically in future research. This article is protected by copyright. All rights reserved
Mapping the train model for earthquakes onto the stochastic sandpile model
We perform a computational study of a variant of the ``train'' model for
earthquakes [PRA 46, 6288 (1992)], where we assume a static friction that is a
stochastic function of position rather than being velocity dependent. The model
consists of an array of blocks coupled by springs, with the forces between
neighbouring blocks balanced by static friction. We calculate the probability,
P(s), of the occurrence of avalanches with a size s or greater, finding that
our results are consistent with the phenomenology and also with previous models
which exhibit a power law over a wide range. We show that the train model may
be mapped onto a stochastic sandpile model and study a variant of the latter
for non-spherical grains. We show that, in this case, the model has critical
behaviour only for grains with large aspect ratio, as was already shown in
experiments with real ricepiles. We also demonstrate a way to introduce
randomness in a physically motivated manner into the model.Comment: 14 pages and 6 figures. Accepted in European Physical Journal
Minu, Startu and All That: Pitfalls in Estimating the Sensitivity of a Worker's Wage to Aggregate Unemployment
In this paper we show that panel estimates of tenure specific sensitivity to the business cycle of wages is subject to serious pitfalls. Three canonical variates used in the literature â the minimum unemployment rate during a workerâs time at the firm (min u), the unemployment rate at the start of her tenure (Su) and the current unemployment rate interacted with a new hire dummy (δu) â can all be significant and "correctly" signed even when each worker in the firm receives the same wage, regardless of tenure (equal treatment). In matched data the problem can be resolved by the inclusion in the panel of firm-year interaction dummies. In unmatched data where this is not possible, we propose a solution for min u and Su based on Solon, Barsky and Parker's (1994) two step method. Our proposed solution method is however suboptimal because it removes a lot of potentially informative variation in average wages. Unfortunately δu cannot be identified in unmatched data because a differential wage response to unemployment of new hires and incumbents will appear under both equal treatment and unequal treatment.wage cyclicality, unemployment
Probing the gluon density of the proton in the exclusive photoproduction of vector mesons at the LHC: A phenomenological analysis
The current uncertainty on the gluon density extracted from the global parton
analysis is large in the kinematical range of small values of the Bjorken -
variable and low values of the hard scale . An alternative to reduces this
uncertainty is the analysis of the exclusive vector meson photoproduction in
photon - hadron and hadron - hadron collisions. This process offers a unique
opportunity to constrain the gluon density of the proton, since its cross
section is proportional to the gluon density squared. In this paper we consider
current parametrizations for the gluon distribution and estimate the exclusive
vector meson photoproduction cross section at HERA and LHC using the leading
logarithmic formalism. We perform a fit of the normalization of the
cross section and the value of the hard scale for the process and demonstrate
that the current LHCb experimental data are better described by models that
assume a slow increasing of the gluon distribution at small - and low
.Comment: 8 pages, 6 figures, 1 table. Version published in European Physical
Journal
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