83,091 research outputs found
Robust system identification and model predictions in the presence of systematic uncertainty
Model-based data-interpretation techniques are increasingly used to improve the knowledge of complex system behavior. Physics-based models that are identified using measurement data are generally used for extrapolation to predict system behavior under other actions. In order to obtain accurate and reliable extrapolations, model-parameter identification needs to be robust in terms of variations of systematic modeling uncertainty introduced when modeling complex systems. Approaches such as Bayesian inference are widely used for system identification. More recently, error-domain model falsification (EDMF) has been shown to be useful for situations where little information is available to define the probability density function (PDF) of modeling errors. Model falsification is a discrete population methodology that is particularly suited to knowledge intensive tasks in open worlds, where uncertainty cannot be precisely defined. This paper compares conventional uses of approaches such as Bayesian inference and EDMF in terms of parameter-identification robustness and extrapolation accuracy. Using Bayesian inference, three scenarios of conventional assumptions related to inclusion of modeling errors are evaluated for several model classes of a simple beam. These scenarios are compared with results obtained using EDMF. Bayesian model class selection is used to study the benefit of posterior model averaging on the accuracy of extrapolations. Finally, ease of representation and modification of knowledge is illustrated using an example of a full-scale bridge. This study shows that EDMF leads to robust identification and more accurate predictions than conventional applications of Bayesian inference in the presence of systematic uncertainty. These results are illustrated with a full-scale bridge. This example shows that the engineering knowledge necessary to perform parameter identification and remaining-fatigue-life predictions of a complex civil structure is easily represented by the EDMF methodology. Model classes describing complex systems should include two components: (1) unknown physical parameters that are identified using measurements; (2) conservative modeling error estimations that cannot be represented only as uncertainties related to physical parameters. In order to obtain accurate predictions, both components need to be included in the model-class definition. This study indicates that Bayesian model class selection may lead to over-confidence in certain model classes, resulting in biased extrapolation
The LHCb experiment: status and recent results
The LHCb experiment is one of the major research projects at the Large Hadron
Collider. Its acceptance and instrumentation is optimised to perform
high-precision studies of flavour physics and particle production in a unique
kinematic range at unprecedented collision energies. Using large data samples
accumulated in the years 2010-2012, the LHCb collaboration has conducted a
series of measurements providing a sensitive test of the Standard Model and
strengthening our knowledge of flavour physics, QCD and electroweak processes.
The status of the experiment and some of its recent results are presented here.Comment: 8 pages, 12 figure
Embedded Model Control calls for disturbance modeling and rejection
Robust control design is mainly devoted to guaranteeing the closed-loop stability of a model-based control law in the presence of parametric uncertainties. The control law is usually a static feedback law which is derived from a (nonlinear) model using different methodologies. From this standpoint, stability can only be guaranteed by introducing some ignorance coefficients and restricting the feedback control effort with respect to the model-based design. Embedded Model Control shows that, the model-based control law must and can be kept intact in the case of uncertainty, if, under certain conditions, the controllable dynamics is complemented by suitable disturbance dynamics capable of real-time encoding the different uncertainties affecting the âembedded model', i.e. the model which is both the design source and the core of the control unit. To be real-time updated the disturbance state is driven by an unpredictable input vector, the noise, which can only be estimated from the model error. The uncertainty-based (or plant-based) design concerns the noise estimator, so as to prevent the model error from conveying uncertainty components (parametric, cross-coupling, neglected dynamics) which are command-dependent and thus prone to destabilizing the controlled plant, into the embedded model. Separation of the components in the low and high frequency domain by the noise estimator itself allows stability recovery and guarantee, and the rejection of low frequency uncertainty components. Two simple case studies endowed with simulated and experimental runs will help to understand the key assets of the methodolog
Physics of W bosons at LEP
The high-energy and high-luminosity data-taking campaigns of the LEP e^+e^-
collider provided the four collaborations, ALEPH, DELPHI, L3 and OPAL, with
about 50000 W-boson pairs and about a thousand singly-produced W bosons. This
unique data sample has an unprecedented reach in probing some aspects of the
Standard Model of the electroweak interactions, and this article reviews
several achievements in the understanding of W-boson physics at LEP. The
measurements of the cross sections for W-boson production are discussed,
together with their implication on the existence of the coupling between Z and
W bosons. The precision measurements of the magnitude of triple gauge-boson
couplings are presented. The observation of the longitudinal helicity component
of the W-boson spin, related to the mechanism of electroweak symmetry breaking,
is described together with the techniques used to probe the CP and CPT
symmetries in the W-boson system. A discussion on the intricacies of the
measurement of the mass of the W boson, whose knowledge is indispensable to
test the internal consistency of the Standard Model and estimate the mass of
the Higgs boson, concludes this review.Comment: To appear in a special issue of Physics Reports celebrating the 50th
anniversary of CER
Measurement of the W, Z and photon production in lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector
The ATLAS experiment measures yields of isolated photons and of Z and W
bosons via leptonic decay modes in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV. The
data samples used in the analysis were obtained in the year 2010 and year 2011
LHC runs and correspond to 5 ub-1 and 0.15 nb-1 of integrated luminosity
respectively. The measured yields of all bosons are consistent with the scaling
proportional to the number of nucleon-nucleon collisions. The transverse
momentum distributions are measured for isolated photons and for Z bosons for
different centrality bins. The shapes of measured distributions do not change
with centrality. The transverse momentum and rapidity distributions of Z boson
are consistent in shape with Pythia model and the second harmonic coefficient
of the azimuthal distribution of Z bosons with respect to the event plane is
consistent with zero.Comment: 8 pages 5 figur
Top Quark Production
Recent measurements of top quark pair and single top production are
presented. The results include inclusive cross sections as well as studies of
differential distributions. Evidence for single top quark production in
association with a W-boson in the final state is reported for the first time.
Calculations in perturbative QCD up to approximate next-to-next-to-leading
order show very good agreement with the data.Comment: Physics in Collision, Slovakia, 2012 PSNUM 0
Heavy-flavour production in Pb-Pb collisions at the LHC, measured with the ALICE detector
We present the first results from the ALICE experiment on the nuclear
modification factors for heavy-flavour hadron production in Pb-Pb collisions at
sqrt{s_NN}=2.76 TeV. Using proton-proton and lead-lead collision samples at
sqrt{s}=7 TeV and sqrt{s_NN}=2.76 TeV, respectively, nuclear modification
factors R_AA(pt) were measured for D mesons at central rapidity (via displaced
decay vertex reconstruction), and for electrons and muons, at central and
forward rapidity, respectively.Comment: 8 pages, 5 figures, plenary talk at Quark Matter 2011, Annecy, Franc
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