25,150 research outputs found
Criteria for Bayesian model choice with application to variable selection
In objective Bayesian model selection, no single criterion has emerged as
dominant in defining objective prior distributions. Indeed, many criteria have
been separately proposed and utilized to propose differing prior choices. We
first formalize the most general and compelling of the various criteria that
have been suggested, together with a new criterion. We then illustrate the
potential of these criteria in determining objective model selection priors by
considering their application to the problem of variable selection in normal
linear models. This results in a new model selection objective prior with a
number of compelling properties.Comment: Published in at http://dx.doi.org/10.1214/12-AOS1013 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Objective Bayes testing of Poisson versus inflated Poisson models
The Poisson distribution is often used as a standard model for count data.
Quite often, however, such data sets are not well fit by a Poisson model
because they have more zeros than are compatible with this model. For these
situations, a zero-inflated Poisson (ZIP) distribution is often proposed. This
article addresses testing a Poisson versus a ZIP model, using Bayesian
methodology based on suitable objective priors. Specific choices of objective
priors are justified and their properties investigated. The methodology is
extended to include covariates in regression models. Several applications are
given.Comment: Published in at http://dx.doi.org/10.1214/074921708000000093 the IMS
Collections (http://www.imstat.org/publications/imscollections.htm) by the
Institute of Mathematical Statistics (http://www.imstat.org
Prospects for near-infrared characterisation of hot Jupiters with VSI
In this paper, we study the feasibility of obtaining near-infrared spectra of
bright extrasolar planets with the 2nd generation VLTI Spectro-Imager
instrument (VSI), which has the required angular resolution to resolve nearby
hot Extrasolar Giant Planets (EGPs) from their host stars. Taking into account
fundamental noises, we simulate closure phase measurements of several
extrasolar systems using four 8-m telescopes at the VLT and a low spectral
resolution (R = 100). Synthetic planetary spectra from T. Barman are used as an
input. Standard chi2-fitting methods are then used to reconstruct planetary
spectra from the simulated data. These simulations show that low-resolution
spectra in the H and K bands can be retrieved with a good fidelity for half a
dozen targets in a reasonable observing time (about 10 hours, spread over a few
nights). Such observations would strongly constrain the planetary temperature
and albedo, the energy redistribution mechanisms, as well as the chemical
composition of their atmospheres. Systematic errors, not included in our
simulations, could be a serious limitation to these performance estimations.
The use of integrated optics is however expected to provide the required
instrumental stability (around 10^-4 on the closure phase) to enable the first
thorough characterisation of extrasolar planetary emission spectra in the
near-infrared.Comment: 10 pages, 8 figures, Proc. SPIE conference 7013 "Optical and Infrared
Interferometry" (Marseille 2008
Incommensurate magnetic ordering in (X=Cl,Br) studied by neutron diffraction
We present the results of the first neutron powder and single crystal
diffraction studies of the coupled spin tetrahedra systems {\CuTeX} (X=Cl,
Br). Incommensurate antiferromagnetic order with the propagation vectors
{\bf{k}_{Cl}}\approx[0.150,0.422,\half],
{\bf{k}_{Br}}\approx[0.158,0.354,\half] sets in below =18 K for X=Cl
and 11 K for X=Br. No simple collinear antiferromagnetic or ferromagnetic
arrangements of moments within Cu tetrahedra fit these observations.
Fitting the diffraction data to more complex but physically reasonable models
with multiple helices leads to a moment of 0.67(1)/Cu at 1.5 K
for the Cl-compound. The reason for such a complex ground state may be
geometrical frustration of the spins due to the intra- and inter-tetrahedral
couplings having similar strengths. The magnetic moment in the Br- compound,
calculated assuming it has the same magnetic structure as the Cl compound, is
only 0.51(5)/Cu at 1.5 K. In neither compound has any evidence
for a structural transition accompanying the magnetic ordering been found
The VLQ Calorimeter of H1 at HERA: A Highly Compact Device for Measurements of Electrons and Photons under Very Small Scattering Angles
In 1998, the detector H1 at HERA has been equipped with a small backward
spectrometer, the Very Low Q^2 (VLQ) spectrometer comprising a silicon tracker,
a tungsten - scintillator sandwich calorimeter, and a Time-of-Flight system.
The spectrometer was designed to measure electrons scattered under very low
angles, equivalent to very low squared four - momentum transfers Q^2, and high
energy photons with good energy and spatial resolution. The VLQ was in
operation during the 1999 and 2000 run periods. This paper describes the design
and construction of the VLQ calorimeter, a compact device with a fourfold
projective energy read-out, and its performance during test runs and in the
experiment.Comment: 32 pages, 25 figures, 2 tables (To be submitted to Nucl. Instrum.
Meth. A
Superfield Realizations of Lorentz and CPT Violation
Superfield realizations of Lorentz-violating extensions of the Wess-Zumino
model are presented. These models retain supersymmetry but include terms that
explicitly break the Lorentz symmetry. The models can be understood as arising
from superspace transformations that are modifications of the familiar one in
the Lorentz-symmetric case.Comment: 10 page
Reference priors for high energy physics
Bayesian inferences in high energy physics often use uniform prior
distributions for parameters about which little or no information is available
before data are collected. The resulting posterior distributions are therefore
sensitive to the choice of parametrization for the problem and may even be
improper if this choice is not carefully considered. Here we describe an
extensively tested methodology, known as reference analysis, which allows one
to construct parametrization-invariant priors that embody the notion of minimal
informativeness in a mathematically well-defined sense. We apply this
methodology to general cross section measurements and show that it yields
sensible results. A recent measurement of the single top quark cross section
illustrates the relevant techniques in a realistic situation
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