| The Standard Model(SM) of particle physics, which is a theory concerning the electromagnetic, weak, and strong nuclear interactions, has made a great success. It precisely predicts the W/Z boson mass and also has been tested by a number of exper-iments for decades. However, Standard Model is still not an ultimate theory. There are still some serious issues for Standard Model, such as fine tuning problem. Supersym-metry (SUSY) is one of the theoretically favored candidates for new physics beyond the Standard Model, which provides a solution to protect the Higgs boson mass from quadratically diverging radiative corrections, and the LSP(such as neutralino?10) is almost an ideal candidate for the dark matter particle of the Universe. SUSY predicts the existence of partners with identical quantum numbers to all Standard Model par-ticles except for a spin that differs by half a unit. However, SUSY must be broken, since no super-partner has been observed so far. The Minimal Super-symmetric Stan-dard Model (MSSM) proposes a common approach to the phenomenological study of SUSY, which is to assume the minimal possible particle content, and parameterizes the SUSY-breaking Lagrangian as the sum of all the terms which do not reintroduce quadratic divergences into the theory. MSSM is characterized by a large number of parameters (>100). The Light Stop Model is one scenario of MSSM. The Light Stop Model is characterized by a light stop, with a predominantly right-handed component and a mass close to that of the top quark, which allows for a sufficiently strong first order electroweak phase transition. All other supersymmetric scalers are much heav-ier to comply with the present bounds on the Higgs mass and neutron electric dipole moments. The Light Stop Model also fells into a narrow enough strip of cosmology requiement. A Monte Carlo study is performed in the feasibility of searching for light stop at the ATLAS experiment.Large Hadron Collider(LHC) at CERN is a proton-proton collider with a designed center of mass at 14TeV and beam luminosity 1034cm-2s-1. ATLAS is a general purpose particle detector at LHC. They were under design and construction in 1995-2009, and started taking data in 2009. The data recorded by the ATLAS experiment during 2009-2011 correspond a total integrated luminosity of 1fb-1,with the center of mass energy 7TeV.For the Light Stop analysis, We have produced Light Stop Monte Carlo full simu-lation sample, which has been qualified as ATLAS official sample. Using this sample, this thesis studies the feasibility of measuring ttt1t1/ttt1t1?l±l±2bjets+2cjets+ (?)+X channel within the gluino(g) mass range from 300 to 550 GeV. This analysis studies trigger, pileup effects, charge misidentification rate as well as all kinds of pos-sible systematic uncertainties, at gluino mass of 300GeV. With the detailed systematics uncertainties studied, the 5 deviation signal excess can be observed with gluino mass between 300GeV and 400GeV, in two same sign di-leptons final states research. This is the first time for Light Stop complete studies.The most difficult part of Light Stop Model analysis is to extract signal from fake events in abundant backgrounds, since the tt cross section is very large. Fake same sign signal events will be produced in b quark semi-decay, where b quark decayed from top quark. Therefor the final states will be much similar to Light Stop Model final states, and will be difficult to be eliminated. Lepton isolation and lepton jet overlap removal procedure are taken to suppress the tt background, by a factor of 10. With the same sign lepton,4 jets and large missing transverse energy requirement, the background is reduced by a factor of 1000.Electron charge misidentification rate uncertainty is one of the important sys-tematic uncertainty sources for the Light Stop analysis. The electron reconstruction efficiency will also be important in this research. The fake electron from jet, elec-tron bremsstrahlung and photon converted electron will introduce the bias of trajec-tory measurement, and affect the electron charge identification. With around 35pb-1 data, the electron reconstruction efficiency and charge mis-identification rate of Ro-bustMedium and RobusterTight are estimated by tag-and probe method. The result from both MC and real data coincide with each other, which is a good supplement for Light Stop Study. There are seven parts in my thesis. The first part is the "History and Evolu-tion of High Energy Physics", which tells the development of high energy physics and give a simple introduction to ATLAS experiment research and the motivation of Light Stop analysis. The second one is the "LHC and ATLAS Introduction", which gives an introduction to LHC parameters and ATLAS design report. The third one is the "Electron, Muon, Jet and Missing Transverse Energy Reconstruction", in which lepton, jet and missing transverse energy reconstruction algorithms are described. The fourth one is "Electron Selection Efficiency", in which the tag-and-probe method is presented into detail, and which also tells application of this method in three levels electron selection efficiency study. The fifth one is "Electron Charge Misidentifica-tion Measurement", which is also an application of tag-and-probe method to electron charge misidentification measurement. The sixth one is "Light Stop Monte Carlo Sample Generation and Standard Model Sample Discription ", which will give a detail introduction to Light Stop signal generation with Pythia and signal simulation, recon-struction processes. The seventh part is "Light Stop Analysis Strategy", in which a detail strategy for Light Stop research is presented, where the systematics study is implemented. The last part is the " Summary and Expectation "... |
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