4,542 research outputs found
Experimental investigation of cut-off phenomena in non-linear photonic crystal fibers
The modal cut-off is investigated experimentally in a series of high quality
non-linear photonic crystal fibers. We demonstrate a suitable measurement
technique to determine the cut-off wavelength and verify it by inspecting the
near field of the modes that may be excited below and above the cut-off. We
observe a double peak structure in the cut-off spectra, which is attributed to
a splitting of the higher order modes. The cut-off is measured for seven
different fiber geometries with different pitches and relative hole size, and a
very good agreement with recent theoretical work is found.Comment: 3 pages including 1 table and 4 figures. Accepted for Optics Letter
Permissive Controller Synthesis for Probabilistic Systems
We propose novel controller synthesis techniques for probabilistic systems
modelled using stochastic two-player games: one player acts as a controller,
the second represents its environment, and probability is used to capture
uncertainty arising due to, for example, unreliable sensors or faulty system
components. Our aim is to generate robust controllers that are resilient to
unexpected system changes at runtime, and flexible enough to be adapted if
additional constraints need to be imposed. We develop a permissive controller
synthesis framework, which generates multi-strategies for the controller,
offering a choice of control actions to take at each time step. We formalise
the notion of permissivity using penalties, which are incurred each time a
possible control action is disallowed by a multi-strategy. Permissive
controller synthesis aims to generate a multi-strategy that minimises these
penalties, whilst guaranteeing the satisfaction of a specified system property.
We establish several key results about the optimality of multi-strategies and
the complexity of synthesising them. Then, we develop methods to perform
permissive controller synthesis using mixed integer linear programming and
illustrate their effectiveness on a selection of case studies
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Effective elements of cognitive behaviour therapy for psychosis: results of a novel type of subgroup analysis based on principal stratification
Background. Meta-analyses show that cognitive behaviour therapy for psychosis (CBT-P) improves distressing positive symptoms. However, it is a complex intervention involving a range of techniques. No previous study has assessed the delivery of the different elements of treatment and their effect on outcome. Our aim was to assess the differential effect of type of treatment delivered on the effectiveness of CBT-P, using novel statistical methodology.
Method. The Psychological Prevention of Relapse in Psychosis (PRP) trial was a multi-centre randomized controlled trial (RCT) that compared CBT-P with treatment as usual (TAU). Therapy was manualized, and detailed evaluations of therapy delivery and client engagement were made. Follow-up assessments were made at 12 and 24 months. In a planned analysis, we applied principal stratification (involving structural equation modelling with finite mixtures) to estimate intention-to-treat (ITT) effects for subgroups of participants, defined by qualitative and quantitative differences in receipt of therapy, while maintaining the constraints of randomization.
Results. Consistent delivery of full therapy, including specific cognitive and behavioural techniques, was associated with clinically and statistically significant increases in months in remission, and decreases in psychotic and affective symptoms. Delivery of partial therapy involving engagement and assessment was not effective.
Conclusions. Our analyses suggest that CBT-P is of significant benefit on multiple outcomes to patients able to engage in the full range of therapy procedures. The novel statistical methods illustrated in this report have general application to the evaluation of heterogeneity in the effects of treatment
Polarization dependence of four-wave mixing in a degenerate two-level system
Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate
two-level atomic transition is theoretically and experimentally examined. Using
the model presented by A. Lezama et al [Phys. Rev. A 61, 013801 (2000)] the
NDFWM spectra corresponding to different pump and probe polarization cases are
calculated and discussed. The calculated spectra are compared to the
observation of NDFWM within the transition of
cesium in a phase conjugation experiment using magneto optically cooled atomsComment: 10 pages, 13 figures; submitted to Phys. Rev.
Generic Model Refactorings
Many modeling languages share some common concepts and principles. For example, Java, MOF, and UML share some aspects of the concepts\ud
of classes, methods, attributes, and inheritance. However, model\ud
transformations such as refactorings specified for a given language\ud
cannot be readily reused for another language because their related\ud
metamodels may be structurally different. Our aim is to enable a\ud
flexible reuse of model transformations across various metamodels.\ud
Thus, in this paper, we present an approach allowing the specification\ud
of generic model transformations, in particular refactorings, so\ud
that they can be applied to different metamodels. Our approach relies\ud
on two mechanisms: (1) an adaptation based mainly on the weaving\ud
of aspects; (2) the notion of model typing, an extension of object\ud
typing in the model-oriented context. We validated our approach by\ud
performing some experiments that consisted of specifying three well\ud
known refactorings (Encapsulate Field, Move Method, and Pull Up Method)\ud
and applying each of them onto three different metamodels (Java,\ud
MOF, and UML)
Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies
We use spectroscopic observations of a sample of 82 HII regions in 76 blue
compact galaxies to determine the primordial helium abundance Yp and the slope
dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ
measurement, we have included new spectrophotometric observations of 33 HII
regions which span a large metallicity range, with oxygen abundance 12+log(O/H)
varying between 7.43 and 8.30 (Zsun/30<Z<Zsun/4). For a subsample of 7 HII
regions, we derive the He mass fraction taking into account known systematic
effects, including collisional and fluorescent enhancements of HeI emission
lines, collisional excitation of hydrogen emission, underlying stellar HeI
absorption and the difference between the temperatures Te(HeII) in the He^+
zone and Te(OIII) derived from the collisionally excited [OIII] lines. We find
that the net result of all the systematic effects combined is small, changing
the He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linear
regression to O/H=0 for 7 HII regions of this subsample, we obtain
Yp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds to dY/dZ=3.7+/-1.2,
assuming the oxygen mass fraction to be O=0.66Z. In the framework of the
standard Big Bang nucleosynthesis theory, this Yp corresponds to Omega_b h^2 =
0.012^+0.003_-0.002, where h is the Hubble constant in units of 100 km/s/Mpc.
This is smaller at the 2sigma level than the value obtained from recent
deuterium abundance and microwave background radiation measurements. The linear
regression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for the
whole sample of 82 HII regions is similar to that derived for the subsample of
7 HII regions, although it has a considerably smaller uncertainty.Comment: 53 pages, 3 Postscript figures, accepted for publication in the
Astrophysical Journa
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