<p>This thesis presents two measurements of the top quark pair production cross section at sqrt{s} = 1.96 TeV using data from the DØ experiment. Both measurements are performed in the dilepton final state and make use of secondary vertex b-tagging. With 158 pb^-1 of data in the electron-muon final state, the measured cross section is:</p><p>σ(top-antitop) = 11.1 +5.8 -4.3 (stat) +- 1.4 (syst) +- 0.7 (lumi) pb.</p><p>With 425 pb^-1 of data in the electron+track and muon+track final states, the measured cross section is:</p><p>sigma(top-antitop) = 6.3 +2.1 -1.8 (stat) +- 1.1 (syst) +- 0.4 (lumi) pb.</p><p>Both measurements are in agreement with the prediction from perturbative QCD calculations. In addition, an estimate of the DØ silicon detector lifetime is presented. The radiation damage is determined by studying the depletion voltage of the silicon sensors as a function of time. Based on this data the silicon detector is estimated to remain operational up to delivered luminosities of 6-8 fb^-1.</p>

This thesis presents two measurements of the top quark pair production cross section at sqrt{s} = 1.96 TeV using data from the DØ experiment. Both measurements are performed in the dilepton final state and make use of secondary vertex b-tagging. With 158 pb-1 of data in the electron-muon final state, the measured cross section is: σ(top-antitop) = 11.1 +5.8 -4.3 (stat) +- 1.4 (syst) +- 0.7 (lumi) pb. With 425 pb-1 of data in the electron+track and muon+track final states, the measured cross section is: sigma(top-antitop) = 6.3 +2.1 -1.8 (stat) +- 1.1 (syst) +- 0.4 (lumi) pb. Both measurements are in agreement with the prediction from perturbative QCD calculations. In addition, an estimate of the DØ silicon detector lifetime is presented. The radiation damage is determined by studying the depletion voltage of the silicon sensors as a function of time. Based on this data the silicon detector is estimated to remain operational up to delivered luminosities of 6-8 fb-1.

While the dark matter has so far only revealed itself through the gravitational influence it exerts on its surroundings, there are good reasons to believe it is made up by WIMPs – a hypothetical class of heavy elementary particles not encompassed by the Standard Model of particle physics. The Inert Doublet Model constitutes a simple extension of the Standard Model Higgs sector. The model provides a new set of scalar particles, denoted inert scalars because of their lack of direct coupling to matter, of which the lightest is a WIMP dark matter candidate. Another popular Standard Model extension is that of supersymmetry. In the most minimal scenario the particle content is roughly doubled, and the lightest of the new supersymmetric particles, which typically is a neutralino, is a WIMP dark matter candidate. In this thesis the phenomenology of inert scalar and supersymmetric dark matter is studied. Relic density calculations are performed, and experimental signatures in indirect detection experiments and accelerator searches are derived. The Inert Doublet Model shows promising prospects for indirect detection of dark matter annihilations into monochromatic photons. It is also constrained by the old LEP II accelerator data. Some phenomenological differences between the Minimal Supersymmetric Standard Model and a slight extension, the Beyond the Minimal Supersymmetric Standard Model, can be found. Also, supersymmetric dark matter models can be detected already within the early LHC accelerator data.

For more than 30 years the Standard Model has been the theoretical foundation for particle physics. The theory has been verified successfully by experimental tests. Its biggest shortcoming is the non-discovery of the Higgs boson,responsible for giving the other particles masses. Despite its success there are hints that the Standard Model is not the complete theory and many extensions of it, such as supersymmetry, have been proposed. Extended theories often predict the existence of a charged Higgs boson and its detection will be a clear sign of physics beyond the Standard Model. The main focus in this thesis is on various phenomenological aspects of the charged Higgs boson. For favorable mass and couplings direct detection is shown to be possible at the Large Hadron Collider in production with an associated W boson. It is also shown how a light charged Higgs can have measurable effects on spin correlations in decays of pair-produced top quarks. The charged Higgs boson can also be seen indirectly, in for example B-meson decays, which can be used to put constraints on its mass and fermion couplings. Exclusion limits in two supersymmetric models are given together with a comparison with the discovery potentials for the LHC experiments. A tool for calculating properties, such as masses and decays, of both charged and neutral Higgs bosons in the Two-Higgs-Doublet Model is also presented. B-meson decays can also be used to test aspects of the strong interaction. Part of this thesis deals with improving and applying phenomenological models to B-meson decays. Although these models are not derived from first principles, their success shows that they capture important features of non-perturbative strong interactions.

One of the goals of the LHC is to test the existence of the Higgs Boson. This thesis presents a study of the potential to discover the Standard Model Higgs boson in the vector boson fusion (VBF) channel for the Higgs mass range 120-200 GeV/c2. The decay of Higgs bosons into WW* final state with both W-bosons decaying leptonically is considered. The main backgrounds are tt_+j and W+W-jj. This study, based on a full simulation of the CMS detector at the LHC, shows that a 5(Sigma) discovery can be done with an integrated luminosity of 12-72 fb-1 for 130-200 GeV/c2 Higgs bosons. Due to the uncertainties in the backgrounds, it is important to measure the backgrounds from data. This study shows that the major background can be measured directly to 7% with 30 fb-1. After discovering the Higgs boson mass using transverse mass template distributions is investigated in the VBF channel. The performance of the combined CMS electromagnetic and hadronic calorimeters (EB+HB) was measured at the H2 test beam at the CERN SPS during 2006 with various partivles in a large momentum range, 1-350 GeV/c. Another major contribution of this thesis is developing the method to optimize the energy reconstruction for the combined EB+HB system with which the corrected responses become 100% with 6% fluctuation and the stochastic resolution is improved from 111% to 94%.

The discovery reach for the ADD-type Large Extra Dimension (LED) scenario in the CMS Experiment at the LHC is presented by looking at the Monojet + Missing Energy signature, which arises as a result of a single graviton emission accompanied by a quark or gluon. Using Monte Carlo generated events, two LHC run scenarios were considered and compared namely a center-of-mass energy of 14 TeV and integrated luminosity of 100 pb&amp / #8722 / 1, and a center-of-mass energy of 10 TeV and integrated luminosity of 200 pb&amp / #8722 / 1. Details from extensive trigger studies are presented and offline selection techniques that optimize the signal excess over backgrounds are highlighted. As a result of this study, it is shown that the existing Tevatron limits on the ADD model can be improved through the implementation of this analysis in CMS Experiment with a factor of 3 using the the early LHC data.

Prompt photons are produced primarily by quark-gluon compton scattering and quark-anti-quark annihilation mechanisms that their measurements are driven by several motivations at hadron colliders. Measurement of prompt photons can be used for probing perturbative Quantum Chromodynamics (pQCD). Prompt photons are produced in the final states of important decays providing evidence for low mass Higgs boson and new physics searches. Compact Muon Solenoid (CMS) is one of the multi-purpose experiments conducted at the Large Hadron Collider (LHC) to study the Standard Model (SM) physics including prompt photons and new theories in proton-proton collisions. In this thesis, photon reconstruction is discussed along with the photon identification methods in the CMS experiment. Photon efficiency measurements are presented as an important ingredient for photon+jet cross section measurement. Photon High Level Trigger (HLT) efficiencies, reconstruction (RECO) efficiencies, and identification (ID) efficiencies are presented that are measured from collision data recorded by the CMS detector and Monte Carlo simulation data. Efficiency corrected differential photon+jet cross section measurement results are presented in this study. The collision data used in this thesis corresponds to 2.2 /fb integrated luminosity collected by the CMS detector at a center-of-mass energy of 7 TeV from 2011 LHC proton-proton collision running.

In this work an analysis of the CHORUS (anti-)neutrino-nucleon scattering data taken on lead--scintillator calorimeter during the 1998 run is presented. The differential cross-sections are measured in the range of $0.01le xle 0.7$, $0.05le y le 0.95$, $10le E_nu le 200 GeV$ for both anti-neutrino and neutrino beam modes. The anti-neutrino and neutrino--nucleon structure functions Fone, Ftwo and Fthree is extracted by making 2, 3 and 6-parameter fits to the measured differential cross-sections. The comparisons of these results with the earlier experiments, CDHSW and CCFR is given.

The Standard Model of particle physics is the best existing theory for describing the interactions between elementary particles. Even though the Standard Model has been confirmed in many experiments, there remain unanswered questions. One of the main questions is how fermions and most gauge bosons get masses; the Standard Model begins with them as massless. The Higgs effect is a mechanism to explain how fermions and several gauge bosons do get masses in the Standard Model. The corresponding Higgs boson is the only particle that has not yet been detected. This Thesis gives a complete review of the Higgs effect and Higgs related topics. It starts with theoretical basics and develops the theory of the Higgs effect within the electroweak section of the Standard Model. The discussion then considers the topics of radiative corrections and the effect of the Higgs boson as a virtual particle, concentrating on the example of the rho-parameter. In addition, experimental and theoretical constraints for the Higgs mass M_H will be given with special emphasis on the Hierarchy Problem which leads to a physically unacceptable Higgs mass when using high energies (of the Grand Unification scale) as a cutoff for the radiative corrections. Furthermore experimental attempts to detect the Higgs boson at LEP2, TEVATRON and LHC will be described and the different decay channels discussed. Finally, alternative theoretical models beyond the Standard Model are motivated and presented, such as supersymmetry, a vectorlike Standard Model and a possible relation between the Higgs and the Inflaton of Cosmology.

O ensino da Física Moderna e Contemporânea (FMC) vem sendo palco para uma ampla discussão nas últimas décadas. No entanto, a realidade escolar ainda se encontra profundamente afastada de uma prática sistematizada do ensino de tópicos relativos a este período de construção do conhecimento científico. Ao contrário dos argumentos apresentados nos documentos educacionais e nas pesquisas acadêmico-científicas, o que ainda se enxerga é uma prática dominada por tópicos associados à física clássica e enviesada por uma visão de ciência profundamente comprometida com o caráter empírico-indutivista de apreensão da realidade. Neste contexto, esta pesquisa buscou promover uma reflexão sobre a dimensão epistemológica do problema de renovação curricular. Para isto, baseou-se em referenciais das denominadas teorias de currículos, compreendendo a constituição dos saberes escolares segundo as reflexões do sociólogo da educação francês Jean-Claude Forquin, e no trabalho epistemológico do filósofo da ciência francês Gaston Bachelard, acerca da novidade essencial do pensamento científico contemporâneo. Coloca-se como hipótese central que a concepção positivista de ciência, dominante na educação científica, vem exercendo enorme influência sobre a forma como discussões e propostas sobre o ensino da FMC tem sido conduzidas - ocasionando um afastamento cada vez maior entre os estudantes e as formas próprias de pensar deste período de extrema importância para a ciência. Para materialização destas reflexões, foram analisadas diversas propostas para o ensino e divulgação da Física de Partículas Elementares no nível médio, apontando que grande parcela delas vem se concentrando em disseminar uma visão reduzida à enunciação da composição da matéria por quarks e da classificação e organização das espécies de partículas. Apontamos para a necessidade de encontrar alternativas para o ensino de física de partículas, tomando seu desenvolvimento histórico como parâmetro de análise. / The teaching of Contemporary and Modern Physics (CMP) have been the stage for a wide discussion in the past few decades. However, the actual school is far away from a systematized practices on teaching topics related to this period of scientific development of knowledge. Contraty to the arguments presented in the educational documents and academic researches, what we\'ve been seen is a practice dominated by topics of classical physics e skewed by a deeply committed view of Science with an empirical-inductive character of apprehension of reality. In this context, this research sought to promote a reflection on the epistemological dimension of the curriculum renewal problem. For this, it was based on references of the curriculum theories, understading the constitution of scholarly knowledge, according to the work of the French sociologist Jean-Claude Forquin, and on the epistemological work of the French philosopher of Science Gaston Bachelard, on the essential novelty of the contemporary scientific thought. The central hypothesis is that the positivist conception of Science, dominant in the scientific education, has been exerting a strong influence in discussions and proposals about on teaching of the CMP - leading to a great distance between the students and the own ways of thinking of this period of for science. In order to materialize these reflections, several proposals for the teaching and dissemination of Elementary Particle Physics in the middle level were analyzed, pointing out that a large number of them have been concentrating on disseminate a view of particle physics reduced as a composition of matter by quarks and the classification and organization of particles species. We\'re pointing to a necessite of finding alternative looks for particle physics teaching, taking its historical development as a parameter of analysis.