20,602 research outputs found
Experimental Issues for Precision Electroweak Physics at a High-Luminosity Z Factory
We discuss the ultimate precision for ALR, and therefore for the weak mixing
angle, at a high-luminosity Linear Collider. Drawing on our experience at the
SLC, and considering various machine parameter sets for the NLC and for TESLA,
it emerges that a compromise between peak luminosity and precision will be a
likely outcome. This arises due to the severe requirements on the uncertainty
in the luminosity weighted collision energy (Ecm). We consider the cases with
and without a polarized positron beam.Comment: Submitted to LCWS2000 (Linear Collider Workshop 20000), Fermilab,
10-24-200
Multivariate emulation of computer simulators: model selection and diagnostics with application to a humanitarian relief model
We present a common framework for Bayesian emulation methodologies for multivariate-output simulators, or computer models, that employ either parametric linear models or nonparametric Gaussian processes. Novel diagnostics suitable for multivariate covariance-separable emulators are developed and techniques to improve the adequacy of an emulator are discussed and implemented. A variety of emulators are compared for a humanitarian relief simulator, modelling aid missions to Sicily after a volcanic eruption and earthquake, and a sensitivity analysis is conducted to determine the sensitivity of the simulator output to changes in the input variables. The results from parametric and nonparametric emulators are compared in terms of prediction accuracy, uncertainty quantification and scientific interpretability
The needs of people with dementia living at home from user, caregiver and professional perspectives: a cross-sectional survey
Few reports have been published about differences in perspectives on perceived needs among community-residing people with dementia, their family caregivers, and professionals. The aim of this study was to compare these perspectives
Model selection via Bayesian information capacity designs for generalised linear models
The first investigation is made of designs for screening experiments where
the response variable is approximated by a generalised linear model. A Bayesian
information capacity criterion is defined for the selection of designs that are
robust to the form of the linear predictor. For binomial data and logistic
regression, the effectiveness of these designs for screening is assessed
through simulation studies using all-subsets regression and model selection via
maximum penalised likelihood and a generalised information criterion. For
Poisson data and log-linear regression, similar assessments are made using
maximum likelihood and the Akaike information criterion for minimally-supported
designs that are constructed analytically. The results show that effective
screening, that is, high power with moderate type I error rate and false
discovery rate, can be achieved through suitable choices for the number of
design support points and experiment size. Logistic regression is shown to
present a more challenging problem than log-linear regression. Some areas for
future work are also indicated
Bayesian Optimal Design for Ordinary Differential Equation Models
Bayesian optimal design is considered for experiments where it is hypothesised that the responses are described by the intractable solution to a system of non-linear ordinary differential equations (ODEs). Bayesian optimal design is based on the minimisation of an expected loss function where the expectation is with respect to all unknown quantities (responses and parameters). This expectation is typically intractable even for simple models before even considering the intractability of the ODE solution. New methodology is developed for this problem that involves minimising a smoothed stochastic approximation to the expected loss and using a state-of-the-art stochastic solution to the ODEs, by treating the ODE solution as an unknown quantity. The methodology is demonstrated on three illustrative examples and a real application involving estimating the properties of human placentas
Balmer-Dominated Shocks Exclude Hot Progenitors for Many Type Ia Supernovae
The evolutionary mechanism underlying Type Ia supernova explosions remains
unknown. Recent efforts to constrain progenitor models based on the influence
that their high energy emission would have on the interstellar medium (ISM) of
galaxies have proven successful. For individual remnants, Balmer-dominated
shocks reveal the ionization state of hydrogen in the immediately surrounding
gas. Here we report deep upper limits on the temperature and luminosity of the
progenitors of four Type Ia remnants with associated Balmer filaments: SN 1006,
0509-67.5, 0519-69.0, and DEM L71. For SN 1006, existing observations of helium
line emission in the diffuse emission ahead of the shock provide an additional
constraint on the helium ionization state in the vicinity of the remnant. Using
the photoionization code Cloudy, we show that these constraints exclude any
hot, luminous progenitor for SN 1006, including stably hydrogen or helium
nuclear-burning white dwarfs, as well as any Chandrasekhar-mass white dwarf
accreting matter at yr via a disk. For
0509-67.5, the Balmer emission alone rules out any such white dwarf accreting
yr. For 0519-69.0 and DEM L71, the inferred
ambient ionization state of hydrogen is only weakly in tension with a recently
hot, luminous progenitor, and cannot be distinguished from e.g., a relatively
higher local Lyman continuum background, without additional line measurements.
Future deep spectroscopic observations will resolve this ambiguity, and can
either detect the influence of any luminous progenitor or rule out the same for
all resolved SN Ia remnants.Comment: 9 pages, 3 figures, 1 table. Accepted for publication in Ap
No hot and luminous progenitor for Tycho's supernova
Type Ia supernovae have proven vital to our understanding of cosmology, both
as standard candles and for their role in galactic chemical evolution; however,
their origin remains uncertain. The canonical accretion model implies a hot and
luminous progenitor which would ionize the surrounding gas out to a radius of
10--100 parsecs for 100,000 years after the explosion. Here we
report stringent upper limits on the temperature and luminosity of the
progenitor of Tycho's supernova (SN 1572), determined using the remnant itself
as a probe of its environment. Hot, luminous progenitors that would have
produced a greater hydrogen ionization fraction than that measured at the
radius of the present remnant (3 parsecs) can thus be excluded. This
conclusively rules out steadily nuclear-burning white dwarfs (supersoft X-ray
sources), as well as disk emission from a Chandrasekhar-mass white dwarf
accreting yr (recurrent novae). The lack of a
surrounding Str\"omgren sphere is consistent with the merger of a double white
dwarf binary, although other more exotic scenarios may be possible.Comment: 17 pages, 2 figures, including supplementary information. Original
accepted manuscript (before copyediting/formatting by Nature Astronomy
Physical Mechanisms for the Variable Spin-down of SGR 1900+14
We consider the physical implications of the rapid spindown of Soft Gamma
Repeater 1900+14, and of the apparent "braking glitch", \Delta P/P = l x 10^-4,
that was concurrent with the Aug. 27th giant flare. A radiation-hydrodynamical
outflow associated with the flare could impart the required torque, but only if
the dipole magnetic field is stronger than ~ 10^14 G and the outflow lasts
longer and/or is more energetic than the observed X-ray flare. A positive
period increment is also a natural consequence of a gradual, plastic
deformation of the neutron star crust by an intense magnetic field, which
forces the neutron superfluid to rotate more slowly than the crust. Sudden
unpinning of the neutron vortex lines during the August 27th event would then
induce a glitch opposite in sign to those observed in young pulsars, but of a
much larger magnitude as a result of the slower rotation.
The change in the persistent X-ray lightcurve following the August 27 event
is ascribed to continued particle heating in the active region of that
outburst. The enhanced X-ray output can be powered by a steady current flowing
through the magnetosphere, induced by the twisting motion of the crust. The
long term rate of spindown appears to be accelerated with respect to a simple
magnetic dipole torque. Accelerated spindown of a seismically-active magnetar
will occur when its persistent output of Alfven waves and particles exceeds its
spindown luminosity. We suggest that SGRs experience some episodes of relative
inactivity, with diminished spindown rates, and that such inactive magnetars
are observed as Anomalous X-ray Pulsars (AXPs). The rapid reappearence of
persistent X-ray emission following August 27 flare gives evidence against
accretion-powered models.Comment: 24 pages, no figure
Instrument for measuring potentials on two dimensional electric field plots Patent
Instrument for measuring potentials on two dimensional electric field plo
- …