34,246 research outputs found
Statistics of statisticians: Critical mass of statistics and operational research groups in the UK
Using a recently developed model, inspired by mean field theory in
statistical physics, and data from the UK's Research Assessment Exercise, we
analyse the relationship between the quality of statistics and operational
research groups and the quantity researchers in them. Similar to other academic
disciplines, we provide evidence for a linear dependency of quality on quantity
up to an upper critical mass, which is interpreted as the average maximum
number of colleagues with whom a researcher can communicate meaningfully within
a research group. The model also predicts a lower critical mass, which research
groups should strive to achieve to avoid extinction. For statistics and
operational research, the lower critical mass is estimated to be 9 3. The
upper critical mass, beyond which research quality does not significantly
depend on group size, is about twice this value
Analytical mode normalization and resonant state expansion for optical fibers - an efficient tool to model transverse disorder
We adapt the resonant state expansion to optical fibers such as capillary and
photonic crystal fibers. As a key requirement of the resonant state expansion
and any related perturbative approach, we derive the correct analytical
normalization for all modes of these fiber structures, including leaky modes
that radiate energy perpendicular to the direction of propagation and have
fields that grow with distance from the fiber core. Based on the normalized
fiber modes, an eigenvalue equation is derived that allows for calculating the
influence of small and large perturbations such as structural disorder on the
guiding properties. This is demonstrated for two test systems: a capillary
fiber and an endlessly single mode fiber.Comment: 10 pages, 4 figure
High-precision measurements of seawater Pb isotope compositions by double spike thermal ionization mass spectrometry
A new method for the determination of seawater Pb isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. For isotopic analysis, the procedure involves initial separation of Pb from 1 to 2 L of seawater by co-precipitation with Mg hydroxide and further purification by a two stage anion exchange procedure. The Pb isotope measurements are subsequently carried out by thermal ionization mass spectrometry using a Pb-207-Pb-204 double spike for correction of instrumental mass fractionation. These methods are associated with a total procedural Pb blank of 28 +/- 21 pg(1sd) and typical Pb recoveries of 40-60%. The Pb concentrations are determined by isotope dilution (ID) on 50 mL of seawater, using a simplified version of above methods. Analyses of multiple aliquots of six seawater samples yield a reproducibility of about +/- 1to +/- 10%(1sd) for Pb concentrations of between 7 and 50 pmol/kg, where precision was primarily limited by the uncertainty of the blank correction (12 +/- 4 pg; 1sd). For the Pb isotope analyses, typical reproducibilities (+/- 2sd) of 700-1500 ppm and 1000-2000ppm were achieved for Pb-207/Pb-206, Pb-208/Pb-206 and Pb-206/Pb-204, Pb-207/Pb-204, Pb-208/Pb-204, respectively. These results are superior to literature data that were obtained using plasma source mass spectrometry and they are at least a factor of five more precise for ratios involving the minor Pb-204 isotope. Both Pb concentration and isotope data, furthermore, show good agreement with published results for two seawater intercomparison samples of the GEOTRACES program. Finally, the new methods were applied to a seawater depth profile from the eastern South Atlantic. Both Pb contents and isotope compositions display a smooth evolution with depth, and no obvious outliers. Compared to previous Pb isotope data for seawater, the Pb-206/Pb-204 ratios are well correlated with Pb-207/Pb-206, underlining the significant improvement achieved in the measurement of the minor Pb-204 isotope
Pulsation models for the roAp star HD 134214
Precise time-series photometry with the MOST satellite has led to
identification of 10 pulsation frequencies in the rapidly oscillating Ap (roAp)
star HD 134214. We have fitted the observed frequencies with theoretical
frequencies of axisymmetric modes in a grid of stellar models with dipole
magnetic fields. We find that, among models with a standard composition of
and with suppressed convection, eigenfrequencies of a
model with and a polar
magnetic field strength of 4.1kG agree best with the observed frequencies. We
identify the observed pulsation frequency with the largest amplitude as a
deformed dipole () mode, and the four next-largest-amplitude
frequencies as deformed modes. These modes have a radial quasi-node
in the outermost atmospheric layers (). Although the model
frequencies agree roughly with observed ones, they are all above the acoustic
cut-off frequency for the model atmosphere and hence are predicted to be
damped. The excitation mechanism for the pulsations of HD 134214 is not clear,
but further investigation of these modes may be a probe of the atmospheric
structure in this magnetic chemically peculiar star.Comment: 9 pages, 6 figures; accepted for publication in MNRA
Violation of the Leggett-Garg Inequality in Neutrino Oscillations
The Leggett-Garg inequality, an analogue of Bell's inequality involving
correlations of measurements on a system at different times, stands as one of
the hallmark tests of quantum mechanics against classical predictions. The
phenomenon of neutrino oscillations should adhere to quantum-mechanical
predictions and provide an observable violation of the Leggett-Garg inequality.
We demonstrate how oscillation phenomena can be used to test for violations of
the classical bound by performing measurements on an ensemble of neutrinos at
distinct energies, as opposed to a single neutrino at distinct times. A study
of the MINOS experiment's data shows a greater than violation over
a distance of 735 km, representing the longest distance over which either the
Leggett-Garg inequality or Bell's inequality has been tested.Comment: Updated to match published version. 6 pages, 2 figure
On the detection of Lorentzian profiles in a power spectrum: A Bayesian approach using ignorance priors
Aims. Deriving accurate frequencies, amplitudes, and mode lifetimes from
stochastically driven pulsation is challenging, more so, if one demands that
realistic error estimates be given for all model fitting parameters. As has
been shown by other authors, the traditional method of fitting Lorentzian
profiles to the power spectrum of time-resolved photometric or spectroscopic
data via the Maximum Likelihood Estimation (MLE) procedure delivers good
approximations for these quantities. We, however, show that a conservative
Bayesian approach allows one to treat the detection of modes with minimal
assumptions (i.e., about the existence and identity of the modes).
Methods. We derive a conservative Bayesian treatment for the probability of
Lorentzian profiles being present in a power spectrum and describe an efficient
implementation that evaluates the probability density distribution of
parameters by using a Markov-Chain Monte Carlo (MCMC) technique.
Results. Potentially superior to "best-fit" procedure like MLE, which only
provides formal uncertainties, our method samples and approximates the actual
probability distributions for all parameters involved. Moreover, it avoids
shortcomings that make the MLE treatment susceptible to the built-in
assumptions of a model that is fitted to the data. This is especially relevant
when analyzing solar-type pulsation in stars other than the Sun where the
observations are of lower quality and can be over-interpreted. As an example,
we apply our technique to CoRoT observations of the solar-type pulsator HD
49933.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Einsatz von gecufften Endotrachealtuben bei Kindern in der Notfall- und Rettungsmedizin
Zusammenfassung: Moderne PĂ€diatrietuben mit Cuff kommen in der KinderanĂ€sthesie und Kinderintensivmedizin vermehrt zur Anwendung. Hohe Treffsicherheit bei der TubusgröĂenwahl sowie die zuverlĂ€ssige Abdichtung des Atemwegssystems ohne den Einsatz ĂŒbergroĂer Tuben sind entscheidende Vorteile gecuffter Tuben auch bei SĂ€uglingen und kleinen Kindern in der Notfall- und Rettungsmedizin. FĂŒr einen sicheren Einsatz gecuffter Tuben in dieser Altersgruppe ist die Auswahl eines PĂ€diatrietubus mit korrekt platziertem Hochvolumen-Niederdruck-Cuff und Intubationstiefenmarkierung sowie eine geprĂŒfte TubusgröĂenauswahlliste die Vorraussetzung. Die ĂberprĂŒfung des Tubus auf eine Luftleckage nach der Intubation bei noch entleertem Cuff, das FĂŒllen des Cuffs unter Cuffdruckkontrolle sowie die Cuffdruckbegrenzung auf maximal 20cmH2O erlauben den sicheren Einsatz von gecufften Tuben bei SĂ€uglingen und Kindern. Fehlen gute Kindertuben mit Cuff, AltersgröĂenauswahlliste und Cuffmanometer, so dĂŒrfen gecuffte Tuben bei SĂ€uglingen und Kindern in der Rettungs- und Notfallmedizin nicht eingesetzt werden. Dann sollten weiterhin ungecuffte Tuben zum Einsatz gelange
Friedel oscillations in disordered quantum wires: Influence of e-e interactions on the localization length
The Friedel oscillations caused due to an impurity located at one edge of a
disordered interacting quantum wire are calculated numerically. The electron
density in the system's ground state is determined using the DMRG method, and
the Friedel oscillations data is extracted using the density difference between
the case in which the wire is coupled to an impurity and the case where the
impurity is uncoupled. We show that the power law decay of the oscillations
occurring for an interacting clean 1D samples described by Luttinger liquid
theory, is multiplied by an exponential decay term due to the disorder. Scaling
of the average Friedel oscillations by this exponential term collapses the
disordered samples data on the clean results. We show that the length scale
governing the exponential decay may be associated with the Anderson
localization length and thus be used as a convenient way to determine the
dependence of the localization length on disorder and interactions. The
localization length decreases as a function of the interaction strength, in
accordance with previous predictions.Comment: 7 pages, 7 figure
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