Search for Third Generation Squarks in the Missing Transverse Energy Plus Jet Sample at CDF Run II. PDF Download

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Languages : en
Pages : 185

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The twentieth century leaves behind one of the most impressive legacies, in terms of human knowledge, ever achieved. In particular the StandardModel (SM) of particle physics has proven to be one of the most accurate descriptions of Nature. The level of accuracy of some theoretical predictions has never been attained before. It includes the electromagnetic interaction, and the weak and strong force, developing the Lagrangian from symmetry principles. There are two different types of fundamental constituents of Nature, in the framework of the Standard Model: bosons and fermions. Bosons are those particles responsible for carrying the interactions among the fermions, which constitute matter. Fermions are divide into six quarks and six leptons, forming a three-folded structure. All these fermions and bosons have an antimatter partner. However, several difficulties point along with the idea that the Standard Model is only an effective low energy theory. These limitations include the difficulty to incorporate gravity and the lack of justification to fine tuning of some perturbative corrections. Moreover, some regions of the theory are not understood, like the mass spectrum of the Standard Model or the mechanism for electroweak symmetry breaking. Supersymmetry is a newer theoretical framework, thought to adress the problems found in the Standard Model, while preserving all its predictive power. It introduces a new symmetry that relates a new boson to each SM fermion and a new fermion to each SM boson. In this way, for every existing boson in the SM it must exist a fermionic super-partner (named with a sufix ino), and likewise, for every fermion a bosonic super-partner (named with a prefix s) must also exist. Moreover, another symmetry called R-parity is introduced to prevent baryon and lepton number violating interactions. If R-parity is conserved, super-particles can only be pair-produced and they cannot decay completely in SM particles. This implies the existence of a lightest SUSY particle (LSP) which would provide a candidate for cold dark matter, that account for 23% of the universe content, as strongly suggested by recent astrophysical data [1]. The Tevatron is a hadron collider operating at Fermilab, USA. This accelerator provides proton-antiproton (p{bar p}) collisions with a center of mass energy of (square root)s = 1.96 TeV. CDF and D0 are the detectors built to analyse the products of the collisions provided by the Tevatron. Both experiments have produced a very significant scientific output in the last few years, like the discovery of the top quark or the measurement of the B{sub s} mixing. The Tevatron experiments are also reaching sensitivity to the SM Higgs boson. The scientific program of CDF includes a broad spectrum on searches for physics signatures beyond the Standard Model. Tevatron is still the energy frontier, what means an unique opportunity to produce a discovery in physic beyond the Standard Model. The analyses presented in this thesis focus on the search for third generation squarks in the missing transverse energy plus jets final state. The production of sbottom ({tilde b}) and stop ({tilde t}) quarks could be highly enhanced at the Tevatron, giving the possibility of discovering new physics or limiting the parameter space available in the theory. No signal is found over the predicted Standard Model background in both searches. Instead, 95% confidence level limits are set on the production cross section, and then translated into the mass plane of the hypothetical particles. This thesis sketches the basic theory concepts of the Standard Model and the Minimal Supersymmetric Extension in Chapter 2. Chapter 3, describes the Tevatron and CDF. Based on the CDF subsystems information, Chapter 4 and 5 describe the analysis objet reconstruction and the heavy flavor tagging tools. The development of the analyses is shown in Chapter 6 and Chapter 7. Finally, Chapter 8 is devoted to discuss the results and conclusions of this work, and future prospects.