145,162 research outputs found
Combined search for the Standard Model Higgs boson in pp collisions at s√=7 TeV with the ATLAS detector
No full textA combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The data sets used correspond to integrated luminosities from 4.6 fb−1 to 4.9 fb−1 of proton-proton collisions collected at s√=7 TeV in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations (σ). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110–600 GeV is estimated to be ∼15%, corresponding to a significance of approximately 1σ
Combined search for the Standard Model Higgs boson in pp collisions at √s=7TeV with the ATLAS detector
No full textA combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The data sets used correspond to integrated luminosities from 4.6fb -1 to 4.9fb -1 of proton-proton collisions collected at √s=7TeV in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations (σ). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110-600 GeV is estimated to be ∼15%, corresponding to a significance of approximately 1σ
The Charged Particle Multiplicity at Center of Mass Energies from 900 GeV to 7 TeV Measured with the ATLAS Experiment at the Large Hadron Collider
No full textThe first measurements made by the ATLAS experiment at the LHC are presented. The charged particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and pseudorapidity are measured for events with at least one charged particle in the kinematic range |η| 500 MeV. The charged particle multiplicity distributions are measured at the three centre of mass energies at which protons have been collided in the LHC: 900 GeV, 2.36 TeV and 7 TeV. The results are compared to predictions from Monte Carlo models of proton-proton collisions. All models predicted a multiplicity at least 10% lower than was measured. They also failed to predict a sufficient increase in the multiplicity when the centre of mass energy increased from 900 GeV to 7 TeV. Updated models have already been produced using these data, which provide a significantly better description of the properties of proton-proton collisions at LHC energies
Final results of the searches for neutral Higgs bosons in e+ e- collisions at s**(1/2) up to 209-GeV
No full textThe final results of the ALEPH search for the Standard Model Higgs boson at LEP, with data collected in the year 2000 at centre-of-mass energies up to 209 GeV, are presented. The changes with respect to the preceding publication are described and a complete study of systematic effects is reported. The findings of this final analysis confirm the preliminary results published in November 2000 shortly after the closing down of the LEP collider: a significant excess of events is observed, consistent with the production of a 115 \Gcs Standard Model Higgs boson. The final results of the searches for the neutral Higgs bosons of the MSSM are also reported, in terms of limits on \mh, \mA and \tanb. Limits are also set on \mh in the case of invisible decays
Measurement of triple gauge boson couplings from W⁺W⁻ production at LEP energies up to 189 GeV
No full textA measurement of triple gauge boson couplings is presented, based on W-pair data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass energy of 189 GeV with an integrated luminosity of 183 pb⁻¹. After combining with our previous measurements at centre-of-mass energies of 161–183 GeV we obtain κ = 0.97_{-0.16}^{+0.20}, g_{1}^{z} = 0.991_{-0.057}^{+0.060} and λ = -0.110_{-0.055}^{+0.058}, where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to their Standard Model values. These results are consistent with the Standard Model expectations
Measurement of Standard Model Processes in e+e- Collisions at √s >202 GeV
No full textBased on the analysis of about 205 pb of data in e e collisions at centre-of-mass energies between 203 GeV and 209 GeV recorded with the OPAL detector at LEP, several preliminary measurements and comparisons with the Standard Model expectations are described. In particular, results on crosssections for hadronic, lepton pair, and multi-photon nal states and on forward-backward asymmetries for the leptonic nal states are presented. Preliminary determinations of the cross-sections for WW and ZZ production are described. All results are consistent with Standard Model expectations
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Conceptual Design of A 1-2 GeV Synchroton Radiation Source
No full textA description is presented of the conceptual design of the Lawrence Berkeley Laboratory 1-2 GeV Synchrotron Radiation Source, which is designed to produce ultraviolet and soft x-ray radiation. The facility consists of an injection system (linac plus booster synchrotron), a low emittance storage ring optimized at 1.5 GeV, several insertion devices (wigglers and undulators) located in the storage ring straight sections, and beam lines from the insertion devices and bending magnets. Storage ring performance is analyzed in terms of lattice, collective instabilities and beam lifetime. The injection system and its performance are discussed. Spectral characteristics of the radiation are presented
OPAL Physics Note PN462 6th October 2000 Measurement of Standard Model Processes in
No full textBased on the analysis of about 175 pb of data in e e collisions at centre-of-mass energies between 203 GeV and 209 GeV recorded with the OPAL detector at LEP, several preliminary measurements and comparisons with the Standard Model expectations are described. In particular, results on crosssections for hadronic, lepton pair, and multi-photon nal states and on forward-backward asymmetries for the leptonic nal states are presented. Preliminary determinations of the cross-sections for WW and ZZ production are described. QCD studies of hadronic events are also presented at these highest e centre-of-mass energies. All results are consistent with Standard Model expectations
Higgs Boson Studies at the Tevatron
No full textWe combine searches by the CDF and D0 Collaborations for the standard model Higgs boson with mass in the range 90--200 GeV produced in the gluon-gluon fusion, , , , and vector boson fusion processes, and decaying in the , , , , and modes. The data correspond to integrated luminosities of up to 10 fb and were collected at the Fermilab Tevatron in collisions at TeV. The searches are also interpreted in the context of fermiophobic and fourth generation models. We observe a significant excess of events in the mass range between 115 and 140 GeV/. The local significance corresponds to 3.0 standard deviations at GeV/, consistent with the mass of the Higgs boson observed at the LHC, and we expect a local significance of 1.9 standard deviations. We separately combine searches for , , , and . The observed signal strengths in all channels are consistent with the presence of a standard model Higgs boson with a mass of 125 GeV/
Random Covariance Heterogeneity in Discrete Choice Models
No full textThe area of discrete choice modelling has developed rapidly in recent years. In particular, continuing refinements of the Generalised Extreme Value (GEV) model family have permitted the representation of increasingly complex patterns of substitution and parallel advances in estimation capability have led to the increased use of model forms requiring simulation in estimation and application. One model form especially, namely the Mixed Multinomial Logit (MMNL) model, is being used ever more widely. Aside from allowing for random variations in tastes across decision-makers in a Random Coefficients Logit (RCL) framework, this model additionally allows for the representation of inter-alternative correlation as well as heteroscedasticity in an Error Components Logit (ECL) framework, enabling the model to approximate any Random Utility model arbitrarily closely. While the various developments discussed above have led to gradual gains in modelling flexibility, little effort has gone into the development of model forms allowing for a representation of heterogeneity across respondents in the correlation structure in place between alternatives. Such correlation heterogeneity is however possibly a crucial factor in the variation of choice-making behaviour across decision-makers, given the potential presence of individual-specific terms in the unobserved part of utility of multiple alternatives. To the authors' knowledge, there has so far only been one application of a model allowing for such heterogeneity, by Bhat (1997). In this Covariance NL model, the logsum parameters themselves are a function of socio-demographic attributes of the decision-makers, such that the correlation heterogeneity is explained with the help of these attributes. While the results by Bhat show the presence of statistically significant levels of covariance heterogeneity, the improvements in terms of model performance are almost negligible. While it is possible to interpret this as a lack of covariance heterogeneity in the data, another explanation is possible. It is clearly imaginable that a major part of the covariance heterogeneity cannot be explained in a deterministic fashion, either due to data limitations, or because of the presence of actual random variation, in a situation analogous to the case of random taste heterogeneity that cannot be explained in a deterministic fashion. In this paper, we propose two different ways of modelling such random variations in the correlation structure across individuals. The first approach is based on the use of an underlying GEV structure, while the second approach consists of an extension of the ECL model. In the former approach, the choice probabilities are given by integration of underlying GEV choice probabilities, such as Nested Logit, over the assumed distribution of the structural parameters. In the most basic specification, the structural parameters are specified as simple random variables, where appropriate choices of statistical distributions and/or mathematical transforms guarantee that the resulting structural parameters fall into the permissible range of values. Several extensions are then discussed in the paper that allow for a mixture of random and deterministic variations in the correlation structure. In an ECL model, correlation across alternatives is introduced with the help of normally distributed error-terms with a mean of zero that are shared by alternatives that are closer substitutes for each other, with the extent of correlation being determined by the estimates of the standard deviations of the error-components. The extension of this model to a structure allowing for random covariance heterogeneity is again divided into two parts. In the first approach, correlation is assumed to vary purely randomly; this is obtained through simple integration over the distribution of the standard deviations of the error-terms, superseding the integration over the distribution of the error-components with a specific draw for the standard deviations. The second extension is similar to the one used in the GEV case, with the standard deviations being composed of a deterministic term and a random term, either as a pure deviation, or in the form of random coefficients in the parameterisation of the distribution of the standard deviations. We next show that our Covariance GEV (CGEV) model generalises all existing GEV model structures, while the Covariance ECL (CECL) model can theoretically approximate all RUM models arbitrarily closely. Although this also means that the CECL model can closely replicate the behaviour of the CGEV model, there are some differences between the two models, which can be related to the differences in the underlying error-structure of the base models (GEV vs ECL). The CECL model has the advantage of implicitly allowing for heteroscedasticity, although this is also possible with the CGEV model, by adding appropriate error-components, leading to an EC-CGEV model. In terms of estimation, the CECL model has a run-time advantage for basic nesting structures, when the number of error-components, and hence dimensions of integration, is low enough not to counter-act the gains made by being based on a more straightforward integrand (MNL vs advanced GEV). However, in more complicated structures, this advantage disappears, in a situation that is analogous to the case of Mixed GEV models compared to ECL models. A final disadvantage of the CECL model structure comes in the form of an additional set of identification conditions. The paper presents applications of these model structures to both cross-sectional and panel datasets from the field of travel behaviour analysis. The applications illustrate the gains in model performance that can be obtained with our proposed structures when compared to models governed by a homogeneous covariance structure assumption. As expected, the gains in performance are more important in the case of data with repeated observations for the same individual, where the notion of individual-specific substitution patterns applies more directly. The applications also confirm the slight differences between the CGEV and CECL models discussed above. The paper concludes with a discussion of how the two structures can be extended to allow for random taste heterogeneity. The resulting models thus allow for random variations in choice behaviour both in the evaluation of measured attributes C as well as the correlation across alternatives in the unobserved utility terms. This further increases the flexibility of the two model structures, and their potential for analysing complex behaviour in transport and other areas of research.
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