Understanding the origin or Electroweak symmetry breaking within the Standard Model was a key motivation for the construction of the Large Hadron Collider (LHC) experiment at CERN. This thesis presents a search for evidence of Higgs boson production in the 4.7 fb−1 of collision data recorded at a centre-of-mass energy of 7 TeV at the ATLAS detector during 2011. This search is focused on signal events in which a Higgs boson is produced in the mass range 100 < mH < 180 GeV/c2 and subsequently decays to a pair of W bosons or a pair of tau leptons to final states with one hadronically decaying tau lepton and one light lepton. After an event selection criteria has been applied, the number of events in this data sample is consistent with the total background estimate and an upper limit is placed on the SM Higgs boson production rate at 95% confidence level. In addition, the prospects for measuring the SM Higgs coupling strength to tau leptons with the associated Higgs production channels and the full LHC dataset are also presented.

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One of the main motivating factors for the construction of the Large Hadron Collider (LHC) was the search for the Higgs boson, postulated to explain the origin of fundamental gauge boson masses. This thesis presents the results of the first search for the Higgs boson at the LHC, using 35 pb^−1 of proton-proton collision data with a centre of mass energy of 7 TeV collected by the ATLAS experiment throughout 2010. The search is performed in the H -> WW channel, since the branching ratio for Higgs boson decays to W bosons is large for a wide range of Higgs boson masses. Two different search methods are presented: a straightforward cut-based method and a method using a matrix-element-based discriminant to provide additional separation between signal and background. The matrix element method is shown to give better expected sensitivity at all Higgs boson masses. Using these methods, a SM-like Higgs boson with a mass of 160 GeV with a production rate of 1.2 times the SM rate is excluded at 95% Confidence Level and limits are placed on the production rate of the SM Higgs boson in the range of masses from 120 < mH < 200 GeV. In addition, a measurement of the SM WW cross section is performed. It is essential to understand this channel since it is the major background to the H -> WW search. SM WW production is also sensitive to new physics processes, which would enhance its cross section. The SM WW cross section is measured to be σ(WW) = 40+20−16(stat.)±7(syst.) pb, which is consistent with the NLO SM expectation of 46 ± 3 pb.

The ATLAS experiment is used to observe the √s=7 TeV proton-proton collisions produced by the LHC at CERN. This gives an unprecedented opportunity to search for physics beyond the Standard Model at hitherto unexplored kinematic regimes. Supersymmetry (SUSY) provides interesting solutions to a variety of theoretical problems that may be encountered in the Standard Model at high energy scales, while providing signatures that may be observed at the LHC. However, in order to produce a search that is sensitive to SUSY it is vital to understand how the physics that has been discovered to date may produce signatures that mimic those expected from SUSY. Statistical models are constructed using both Monte Carlo and data-driven predictions of various background processes. The expectations are compared to the observed data for selections containing one electron or muon, each in association with jets and missing transverse momentum. Kinematic variable shapes, in the form of histograms, are used to enhance the sensitivity of the search. Squark and gluino masses in a MSUGRA SUSY model are excluded up to 1200 GeV, while gluino masses up to 900 GeV are excluded in a simplified SUSY model. Model-independent limits are also set, excluding theoretical models with efficiency times cross section above 1 fb.

The ensemble Kalman filter (EnKF) is a reliable data assimilation tool for high-dimensional meteorological problems. On the other hand, the EnKF can be interpreted as a particle filter, and particle filters (PF) collapse in high-dimensional problems. We explain that these seemingly contradictory statements offer insights about how PF function in certain high-dimensional problems, and in particular support recent efforts in meteorology to 'localize' particle filters, i.e. to restrict the influence of an observation to its neighbourhood.

Apresentamos um estudo do setor da quebra espontânea de simetria da teoria eletrofraca através do formalismo de lagrangianas quirais, analisando os operadores que geram vértices anômalos genuinamente quárticos entre os bósons de gauge, os quais podem ser sondados pela próxima geração de aceleradores: LHC e NLC. Com o objetivo de obter vínculos aos acoplamentos anômalos relativos a esses operadores, estudamos as reações e POT. +e POT. - W POT. + W POT. Z e e POT. +e POT. - ZZZ no NLC operando com energias no centro de massa de s = 0.5 TeV e 1 TeV, estendendo as nossas análises para a colisão com o feixe de elétrons polarizado. Avaliamos também o impacto desses mesmos acoplamentos ao processo pp VV + 2 jatos (V = W±, Z) no LHC. No intuito de desenvolver cálculos realísticos, todas as amplitudes de espalhamento das reações estudadas foram determinadas sem qualquer tipo de aproximação. / We present a study of electroweak symmetry breaking sector in the framework or chiral Lagrangians, analyzing the operators that lead to genuine quartic gauge boson couplings, which could be probed by the next generation accelerators; LHC and NLC. In order to get bounds on the anomalous couplings related to these operators, we studied the reactions e+e- W+W-Z and e+e- ZZZ at the NLC operating with center-or-mass energy of s = 0.5 TeV and 1 TeV, extending our analysis to polarized electron beam collisions. We also estimate the impact of these couplings to the process pp VV + 2 jets (V = W±, Z) at the LHC. In order to develop realistic calculations, all the scattering amplitudes considered have been determined without any kind of approximation.

In Particle Image Velocimetry (PIV), the number of particle images per interrogation region, or particle image density, impacts the strength of the correlation and, as a result, the number of valid vectors and the measurement uncertainty. Therefore, any a-priori estimate of the accuracy and uncertainty of PIV requires knowledge of the particle image density. An autocorrelation-based method for estimating the local, instantaneous, particle image density is presented. Synthetic images were used to develop an empirical relationship based on how the autocorrelation peak magnitude varies with particle image density, particle image diameter, illumination intensity, interrogation region size, and background noise. This relationship was then tested using images from two experimental setups with different seeding densities and flow media. The experimental results were compared to image densities obtained through using a local maximum method as well as manual particle counts and are found to be robust. The effect of varying particle image intensities was also investigated and is found to affect the particle image density.

<p>The Standard Model of particle physics has been very successful in predicting a wide range of phenomena and has so far been confirmed by all existing data to a very high precision.</p><p>The work described in this thesis tests the limits of validity of the Standard Model (SM) in two areas believed to be sensitive to deviations from the theory: the observation of unpredicted particles and CP violation. The studies were performed within the framework of experiments at two hadron colliders, the future ATLAS detector scheduled for operation in 2007 at the Large Hadron Collider in Geneva and the currently running DØ experiment at the Tevatron in Chicago.</p><p>The tau lepton’s distinctive signature is a useful tool in many new physics searches where it is present in the final state. As a first study in ATLAS a Monte Carlo analysis of two-tau final states, which are sensitive to the underlying structure of supersymmetric models, was performed. </p><p>Several extensions of the SM predict the existence of a charged Higgs boson.The major part of this thesis has consisted in using tau leptons to search for the charged Higgs in the context of the Minimal Supersymmetric extension of the SM (MSSM). Results from this thesis show that searches for the <i>H</i>^± → τν_τ decay channel extend the charged Higgs discovery reach for the ATLAS experiment compared to previous studies of other channels: the charged Higgs can be observed for masses up to ~ 600 GeV for tan<i>β</i> > 10. Its mass can be determined with an uncertainty of 1 to 2%, dominated by statistical errors. The tan<i>β</i> parameter can be derived from the absolute rate of this decay to a precision around 6% for 20 < tan <i>β</i> < 50.</p><p>By measuring precisely the unitarity triangle parameter sin2<i>β</i> the SM description of CP violation can be put to a test. As a separate study a Monte Carlo analysis was performed in ATLAS, which shows that the systematic uncertainty is half the attainable statistical uncertainty. </p><p>As part of the effort to search for the charged Higgs in the DØ experiment a trigger algorithm for tau leptons was written, extensively tested and implemented in the experiment. Trigger strategies for events containing taus were designed. These trigger studies will be useful also for many other new physics searches at DØ.</p>

<p>A search for cosmic point sources of neutrinos has been performed using data taken in 1999 with the AMANDA-B10 neutrino telescope. </p><p>This work describes methods for signal and background separation and the statistical analysis of the final data sample. In particular, the multivariate method Support Vector Machines has been applied to achieve good background rejection while at the same time retaining high signal efficiency. </p><p>A grid search covering the complete northern hemisphere revealed no statistical significant excess of events over the expected background from mis-reconstructed cosmic ray induced muons and muons induced by atmospheric neutrinos. Thus, no cosmic point sources of neutrinos have been detected. Upper limits on the neutrino flux for each cell of the grid are presented.</p><p>Twenty potential sources of neutrinos chosen among three classes of astronomical objects (Blazars, Super Nova Remnants and Microquasars) have been preselected. Upper Limits on the flux of cosmic neutrinos from those are presented.</p><p>The presence of systematic uncertainties makes the calculation of confidence limits an intricate problem. A method is presented which makes it possible to include these uncertainties into the frequentist construction of confidence intervals. Statistical properties of the presented method have been studied.</p>

Particle size distribution (PSD) of gypseous soils is important in the soil science community. When gypsum constitutes a major portion of the soil, its removal prior to PSD analysis distorts the results and may lead to textures that do not relate to conditions in the field. In order to understand the true characterization of the soil and the gypsum particles, the entire soil sample should be analyzed. Four different approaches to the BaCl2 method presented in the literature (Hesse, 1976, Matar and Douleimy, 1978, Viellefon, 1979) were used to evaluate the use of BaCl2 solution to reduce the solubility of gypsum by forming a protective coating of BaSO4 around gypsum particles. Results showed that the BaCl2 method was unsatisfactory, as dispersion of clays was not sufficient to allow particle size analysis using the pipette method. A procedure using a laser diffraction particle size analyzer (LPSA) was also evaluated. As gypsum is insoluble in methanol, methanol was selected as a possible solution, but it caused flocculation of clays and could not be used to analyze samples containing silicate clays. Gypsum saturated water containing Na hexametaphosphate was evaluated as a solution. First, 20 non-gypseous samples were analyzed on a sand-free basis using saturated gypsum water with Na hexametaphosphate. Results were used to establish a relationship comparing LPSA results and pipette results. An equation y = 1.37x + 2.03 was established relating LPSA clay percent by volume (x) to the pipette clay percent by weight (y). The equation had a R2 value of 0.84 and was significant at the 1% level. From this equation a comparison of 21 gypseous samples was made, between clay percentages of the pipette method and the LPSA method. Results indicate that LPSA can be used to give a satisfactory particle size distribution of gypseous soils when coupled with sand analysis by sieving.