8,760 research outputs found

    Top pair production at CMS (includes charge asymmetry and ttbar resonance)

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    Measurements of top quark pair production are presented based on data recorded with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) in 2011 and 2012. Inclusive and differential top quark pair production cross section measurements in various channels are described. Inclusive as well as differential measurements of the charge asymmetry in top quark pair events are summarised and a method for the extraction of the strong coupling alpha(s) from the measured top pair production cross section is discussed. All measurements are in agreement with the Standard Model predictions

    Fisher Vectors Derived from Hybrid Gaussian-Laplacian Mixture Models for Image Annotation

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    In the traditional object recognition pipeline, descriptors are densely sampled over an image, pooled into a high dimensional non-linear representation and then passed to a classifier. In recent years, Fisher Vectors have proven empirically to be the leading representation for a large variety of applications. The Fisher Vector is typically taken as the gradients of the log-likelihood of descriptors, with respect to the parameters of a Gaussian Mixture Model (GMM). Motivated by the assumption that different distributions should be applied for different datasets, we present two other Mixture Models and derive their Expectation-Maximization and Fisher Vector expressions. The first is a Laplacian Mixture Model (LMM), which is based on the Laplacian distribution. The second Mixture Model presented is a Hybrid Gaussian-Laplacian Mixture Model (HGLMM) which is based on a weighted geometric mean of the Gaussian and Laplacian distribution. An interesting property of the Expectation-Maximization algorithm for the latter is that in the maximization step, each dimension in each component is chosen to be either a Gaussian or a Laplacian. Finally, by using the new Fisher Vectors derived from HGLMMs, we achieve state-of-the-art results for both the image annotation and the image search by a sentence tasks.Comment: new version includes text synthesis by an RNN and experiments with the COCO benchmar

    Integration of noise modelling into RF receiver design

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    A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2017The scientific requirements for Radio Frequency (RF) receivers especially for Radio Astronomy have become more demanding, requiring: compact, low-profile, multi and wideband antennas and more sensitive receivers. Integration of the antenna into the receiver system is often critical to meet these demands. Noise theory to model these more complex systems is well developed but is not implemented in commercial solvers, given the niche market of the receivers it is only available using specialised software. If the system is closely coupled, it becomes necessary for design to incorporate Electromagnetic (EM) and Microwave (MW) modelling into the multi port noise modelling. CAESAR, a combined noise and EM/MW modelling code is available, but to use it requires the exclusive use of the CAESAR software, which is impractical given the utility and wide use of commercial solvers. Mathematical methods are developed to incorporate commercial solvers into the more specialised CAESAR, validated using a folded dipole and applied to a wideband Eleven antenna system, a compact form of a log periodic dipole array. The Eleven antenna consist of eight single ended or four differential ports, with a closely coupled feeding arrangement. Cryogenic measurements are done to verify the modelling, the measured sensitivity matches with the model closely in amplitude and shape, giving confidence to the approach, and allowing modelling but not system optimisation. Optimising the antenna based on receiver design and still being able to use commercial code requires the external scripting of a commercial solver. The EDITFEKO (card based) module of FEKO (a powerful and versatile solver) is used along with the meshing software GMSH and GNU Octave. Optimisation of system sensitivity is demonstrated on a choke horn fed reflector system at 1420MHz. This optimisation method is applied to a practical application, an octave band system (4:5 GHz to 9 GHz) for the Hartebeesthoek Radio Observatory. The design is split into smaller simulations using waveguide modes and the associated S–parameters, the techniques are presented and checked on a truncated system. Initial design and optimisation are given. The combining of specialised multiport noise modelling design and optimisation within commercial EM/MW solvers allows more sensitive and specialised receivers to be built. Index terms— noise modelling, wideband, multiport, corrugated horn, octave band receiver, EM/MW optimisationMT201

    Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

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    Compressive fluctuations are a minor yet significant component of astrophysical plasma turbulence. In the solar wind, long-wavelength compressive slow-mode fluctuations lead to changes in βp8πnpkBTp/B2\beta_{\parallel \mathrm p}\equiv 8\pi n_{\mathrm p}k_{\mathrm B}T_{\parallel \mathrm p}/B^2 and in RpTp/TpR_{\mathrm p}\equiv T_{\perp \mathrm p}/T_{\parallel \mathrm p}, where TpT_{\perp \mathrm p} and TpT_{\parallel \mathrm p} are the perpendicular and parallel temperatures of the protons, BB is the magnetic field strength, and npn_{\mathrm p} is the proton density. If the amplitude of the compressive fluctuations is large enough, RpR_{\mathrm p} crosses one or more instability thresholds for anisotropy-driven microinstabilities. The enhanced field fluctuations from these microinstabilities scatter the protons so as to reduce the anisotropy of the pressure tensor. We propose that this scattering drives the average value of RpR_{\mathrm p} away from the marginal stability boundary until the fluctuating value of RpR_{\mathrm p} stops crossing the boundary. We model this "fluctuating-anisotropy effect" using linear Vlasov--Maxwell theory to describe the large-scale compressive fluctuations. We argue that this effect can explain why, in the nearly collisionless solar wind, the average value of RpR_{\mathrm p} is close to unity.Comment: 11 pages, published in Ap

    Preventive Detention in American Theory and Practice

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    It is something of an article of faith in public and academic discourse that preventive detention runs counter to American values and law. This meme has become standard fare among human rights groups and in a great deal of legal scholarship. It treats the past nine years of extra-criminal detention of terrorism suspects as an extraordinary aberration from a strong American constitutional norm, under which government locks up citizens pursuant only to criminal punishment, not because of mere fear of their future acts. This argument further asserts that any statutory counterterrorism administrative detention regime would be a radical departure from this norm, an institutionalization of the aberration that the detention practices of the Bush and Obama years have represented. The more careful commentators acknowledge that the rule has exceptions – sometimes even many of them. But they describe these exceptions as narrow and limited, deviations from a generally strong rule that the American system tolerates to accommodate exceptional circumstances. The trouble with such civic mythology is that it is, ultimately, an inaccurate description of past practice, and thus a misleading indicator of the theory on which such practice rested. Our purpose in this paper is to describe just how mythological this particular civic myth is – indeed, to show that nearly every aspect of it is false: Preventive detention is not prohibited by U.S. law or especially frowned upon in tradition or practice. The circumstances in which it arises are not isolated exceptions to a strong rule against it; rather, they are relatively frequent. The federal government and all 50 states together possess a wide range of statutory preventive detention regimes that are frequently used, many of which provoke little social or legal controversy