Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-02-11T11:12:28Z No. of bitstreams: 1 viniciusbarbosaschettino.pdf: 29158699 bytes, checksum: 73147768684934f5b11f6c844b4bbc4d (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-02-26T11:57:35Z (GMT) No. of bitstreams: 1 viniciusbarbosaschettino.pdf: 29158699 bytes, checksum: 73147768684934f5b11f6c844b4bbc4d (MD5) / Made available in DSpace on 2016-02-26T11:57:35Z (GMT). No. of bitstreams: 1 viniciusbarbosaschettino.pdf: 29158699 bytes, checksum: 73147768684934f5b11f6c844b4bbc4d (MD5) Previous issue date: 2014-02-14 / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / O LHC é o mais energético acelerador de partículas já construído. Operado pelo CERN, essa fantástica máquina encontra-se no subterrâneo, na região da fronteira franco-suíça. Um de seus principais experimentos, o ATLAS, está atualmente passando por um período de parada técnica planejada, visando concomitantemente a consolidação da estrutura eletrônica do detector e a substituição de tecnologias, preparando o experimento para um novo estágio de operação do LHC, em que haverá um grande aumento na taxa de colisões entre prótons. Nesse contexto, essa dissertação abordará o estudo e o desenvolvimento de sistemas eletrônicos para o TileCal, um dos calorímetros do ATLAS. Principalmente, serão discutidos dois novos sistemas: O Sistema Móvel de Checagem da Integridade das Gavetas, conhecido como Mobi- DICK, e o projeto Tile-Muon. Relativamente ao primeiro, serão expostos detalhes da quarta versão de um sistema utilizado para realizar checagens quantitativas e qualitativas da eletrônica do TileCal. Esta nova versão, implementa ainda a estrutura que possibilita a realização de testes nas novas tecnologias que serão utilizadas no calorímetro. O ambicioso projeto Tile-Muon visa a utilização do TileCal para identificação de múons, com o objetivo de tornar mais eficiente a seleção de eventos do ATLAS. Essa proposta requer o desenvolvimento de uma nova estrutura eletrônica de suporte, que será apresentada nessa dissertação. Apresentaremos análises feitas com o MobiDICK sobre a eletrônica do TileCal, que mostrou boa performance, e também uma caracterização do sinal de múons, estudo fundamental para a validação e o desenvolvimento do projeto Tile-Muon. / The LHC is the most energetic particle accelerator ever built. Run by CERN, this fantastic machine lays underground, near the Franco-Swiss border. One of its main experiments, ATLAS, is currently going through a planned technical stop, targeting both a consolidation of the detector’s electronics and a technology replacement, preparing the experiment to a new stage in the LHC operation, where the protons collision rate will increase largely. In such context, this dissertation addresses the study and the development of electronic systems for TileCal, one of ATLAS calorimeters. Primarily, two new systems will be discussed: the Mobile Drawer Integrity Checking System, known as MobiDICK, and the Tile-Muon project. Regarding the former, we’ll bring details of the fourth version of a system used to check quantitatively and qualitatively the electronics of TileCal. This new version also provides the system with the ability to test the new technologies that will be used in the calorimeter. The ambitious Tile-Muon project aims at using TileCal for muon identification, with the goal of raising the efficiency of event selection in ATLAS. This proposal requires the development of a new electronic structure for support, which will also be presented in this dissertation. Analysis of TileCal’s electronics using MobiDICK will be shown, revealing good performance, along with a characterization of the muon signal, a fundamental study for validation and development of the Tile-Muon project.

The ATLAS Experiment at the CERN Large Hadron Collider is a particle physics experiment to study fundamental particles and their interactions at very high energies. Supersymmetry is a theory of new physics beyond the Standard Model of particle physics. A search for directly produced pairs of the supersymmetric partner of the top quark was performed using 20 fb^-1 of proton--proton collision data at a centre of mass energy of 8 TeV taken in 2012. The search targeted a model where the supersymmetric partner of the top quark (``scalar top'') decays via the supersymmetric partner of the tau lepton (``scalar tau'') into the supersymmetric partner of the graviton (``gravitino''). Scalar top candidates were searched for in pp collision events with either two hadronically decaying taus, two light leptons (electrons or muons), or one hadronically decaying tau and one light lepton. The numbers of events passing the analysis selection criteria agree with the Standard Model expectations. Exclusion limits at the 95% confidence level were set as a function of the scalar top and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV limit set by the LEP experiments to a few GeV below the scalar top mass, lower limits between 490 GeV and 640 GeV were placed on the scalar top mass within the model considered. / Graduate

The first measurement of the spin correlation strength in pairs of top quarks at √s=8 TeV is presented, using data collected using the ATLAS detector, corresponding to an integrated luminosity of 20.3 fb⁻¹. Dileptonically decaying tt^- events are selected with two high-pT leptons and at least two jets, one of which is required to be b-tagged. The azimuthal angle between the two charged leptons, Δφ, is used to measure the degree of spin correlation of top quark pairs. The extracted value of fSM, which is a measure of the spin-correlation strength, is fSM=1.20 ± 0.14. This is the most precise spin-correlation measurement to date. Using the Δφ distribution, a search for Supersymmetric top squarks is performed, with the assumption that the top squarks decay to a top quark and a neutralino. No excess of events is observed and top squarks with masses between the top quark mass and 191 GeV are excluded at 95% confidence level, extending previous limits.

No description available.

In its first three years of operation, the Large Hadron Collider at CERN has already proved its worth as a discovery machine and reinvigorated the prospects for discovering new dynamics beyond the Standard Model. This thesis presents a range of experimental methods devised for discovery of supersymmetry, one of the most important extensions of the Standard Model being tested at the LHC. The first chapter discusses the concepts behind a set of transverse mass-bound variables, and explores connections between these variables and others in the literature. Not only are these variables important tools for mass measurement, should sparticles be seen at the LHC, they are also critical components of experimental searches. Accordingly, their performance has been compared in both mass measurement and search contexts, and the results are reported here. This is followed by the details of a search for hadronically-decaying squarks and gluinos, carried out on 4.7 fb−1 of data collected at 7 TeV in 2011 by the ATLAS detector. The search strategy is described, focusing on the implementation of a background estimation method using photon events to determine the background contamination due to invisible Z boson decays. The results of the search are then presented. Stringent limits are set on the production of squarks and gluinos in a variety of model scenarios.

This thesis presents measurements of the the spin correlation strength in top anti-top quark pair production at the LHC using the ATLAS detector. The data used corresponds to 4.6 fb−1 of integrated luminosity taken during 2011 at the LHC at a center of mass energy of 7 TeV. The spin correlation is studied utilising different observables with different sensitivities to the production mechanism, in particular to gluon-gluon fusion in the like or unlike helicity state, quark anti- quark annihilation in the unlike helicity state, or a combination of the three. In addition cuts are made on the invariant mass of the ttbar system to enhance or suppress contributions from different initial state production mechanisms. The analysis presented is a precision test of both ttbar production and decay in the SM. These measurements are compared to the most current theoretical predictions. No deviation from the SM expectation was observed. In a subset of the data, corresponding to an integrated luminosity of 2.1 fb−1, the hypothesis of zero spin correlation is excluded at 5.1 standard deviations.

A search for heavy exotic diboson resonances decaying to llqq final states is presented using pp collision data collected with the ATLAS detector at the Large Hadron Collider. The analysis uses a data sample corresponding to an integrated luminosity of 20.3 fb^-1 at sqrt(s) = 8 TeV collected between April and December 2012. No significant excess of data events over the predicted Standard Model background is observed and 95% confidence level upper limits are set on the product of the production cross-section and the branching ratio for spin-2 Kaluza-Klein gravitons predicted by the bulk Randall-Sundrum model and for Extended Gauge Model W’ bosons. These results are subsequently combined with limits obtained from searches using the lvll, lvqq, and qqqq final states, and new mass limits are set on both signal models.

No description available.

Data collected at the LHC are analyzed by the ATLAS collaboration for evidence of dark matter. In this thesis, a fast simulation of the ATLAS detector response using the Delphes software is assessed for dark matter models with a leptonically decaying Z boson and a pair of dark matter particles in the final state. Limits for the Two Higgs Doublet plus pseudoscalar dark matter model are obtained using simplified systematics, and found to be nearly indistinguishable to limits obtained using the more complex standard ATLAS analysis. / Graduate

<p>The high luminosity phase of the LHC, poised to start taking data in 2027, aims to increase the instantaneous luminosity of the machine to 7.5 x 10³⁴ cm^-2 s^-1. This will make it possible for experiments at CERN to make higher precision measurements on known physics phenomenon as well as to search for “new physics”. However, this motivates the need for hardware upgrades at the various experiments in order to ensure compatibility with the HL-LHC. This thesis describes some of the efforts to upgrade the inner-most layers of the Compact Muon Solenoid, namely the CMS silicon pixel tracking detector. </p> <p>Silicon sensors used to track particles are installed in the detector as part of a pixel sensor module. Modules consist of a silicon sensor-readout chip assembly that is wire-bonded to an HDI, or High Density Interconnects to provide power and signals. </p> <p>As part of the upgrade, 2,541 modules need to be assembled delicately and identically with alignment error margins as low as 10 microns. Assembly will be across three production sites in clean rooms to avoid dust and humidity contamination.</p> <p>In addition, the modules need to survive high magnetic fields and extended close-range radiation as part of the HL-LHC.</p> <p>In line with this effort, new materials and assembly procedures able to sustain such damage are investigated. Techniques to assemble modules are explored, specifically precision placing of parts with a robotic gantry and techniques to protect wirebonds. This is followed by a discussion of the accuracy and repeatability.</p>