The hierarchy problem of the electroweak scale is an intriguing puzzle which can conceivably be solved during upcoming runs of the Large Hadron Collider (LHC). Supersymmetry (SUSY) is an attractive potential solution to this problem, though the fact that no supersymmetric particles have been discovered thus far raises fine tuning of most models to &order;(1&percnt;). Extradimensional SUSY is especially interesting in light of this fact since certain models are easier to reconcile with LHC data. This thesis discusses two different extradimensional SUSY scenarios: auto-concealment and Maximally Natural SUSY (MNSUSY). The auto-concealment mechanism applies when the lightest ordinary superymmetric particle (LOSP), a brane localized state, promptly decays to the Kaluza-Klein (KK) tower of a bulk lightest supersymmetric particle (LSP). This dynamically realizes the compression mechanism for hiding SUSY as decays into the more numerous heavier KK LSP states are favored. LHC limits on LOSP squarks weaken to ≲ 450 GeV while limits on LOSP right-handed sleptons evaporate. Slepton searches perform poorly in this case because LHC analyses are blind to compressed slepton spectra. To help fill this gap, this thesis presents a monojet-like search sensitive to the very compressed range 3 GeV < m _&itilde; − m^&Tilde;<sub style='position: relative; left: -.7em;'>x</sub>⁰<sub style='position: relative; left: -.6em;'>1</sub> < 24 GeV. The analysis should allow LHC14 with 100 fb^−1 to search for degenerate left-handed selectrons and smuons in the compressed region up to m _{&itilde;_L} ≲ 150 GeV. In addition, it should be sensitive to m _{&itilde;_L} ≲ 110 GeV for auto-concealed SUSY. To expand the class of extradimensional SUSY models the LHC is able to test this thesis also describes the Lagrangian and mass matrices of MNSUSY, which are needed to implement this model in the Feynman rules generator program FeynRules for use in Monte Carlo programs for collider simulations. It also describes benchmark scenarios useful for the first collider phenomenology studies.

Attempting to find evidence for supersymmetry (SUSY) is one of the key aims of the ATLAS experiment at the Large Hadron Collider. This thesis is concerned with searching for supersymmetry in final states with 2-4 hadronic jets, missing transverse energy and no electrons or muons. In the first part, a search strategy is developed using 1.04 fb−1 of data from the first half of 2011. No excess over the Standard Model expectation is observed, so the data are used to set limits on two SUSY simplified models, in which pair-produced squarks or gluinos decay directly to neutralinos and jets. Good limits are achieved for scenarios where the neutralino is nearly as massive as the squark/gluino, compared to an earlier ATLAS analysis using the same dataset. For example, for pair-production of squarks decaying directly to neutralinos, all neutralino masses below 200 GeV are excluded at 95% confidence level when the squark mass is 300 GeV. Similarly, for pair-produced gluinos, neutralino masses below 300 GeV are excluded when the gluino mass is 400 GeV. The equivalent neutralino mass limits in the earlier analysis are 130 GeV and 240 GeV respectively. In the second part, the performance of the ATLAS missing transverse energy trigger is studied, and its suitability for use in the SUSY search is evaluated. The behaviour is found to be consistent with expectations, and the trigger strategy for 2010 data-taking is described.

Supersymmetry is a promising candidate theory that could solve the hierarchy problem and explain the dark matter density in the Universe. The ATLAS experiment at the Large Hadron Collider is sensitive to a variety of such supersymmetric models. This thesis reports on a search for pair production of the supersymmetric scalar partners of bottom and top quarks in 20.1 fb⁻¹ of pp collisions at a centre-of-mass energy of 8 TeV using the ATLAS experiment. The study focuses on final states with large missing transverse momentum, no electrons or muons and two jets identified as originating from a b-quark. This final state can be produced in a R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom decays exclusively to a bottom quark and a neutralino and the scalar top decays to a bottom quark and a chargino, with a small mass difference with the neutralino. As no signal is observed above the Standard Model expectation, competitive exclusion limits are set on scalar bottom and top production, surpassing previously existing limits. Sbottom masses up to 640 GeV are excluded at 95% CLs for neutralino masses of up to 150 GeV. Differences in mass between ^~<sub style='position: relative; left: -.7em;'>b</sub>₁ and ^~<sub style='position: relative; left: -.7em;'>X</sub>⁰<sub style='position: relative; left: -.5em;'>1</sub> larger than 50 GeV are excluded up to sbottom masses of 300 GeV. In the case of stop pair production and decay ^~<sub style='position: relative; left: -.7em;'>t</sub>₁ → b + ^~<sub style='position: relative; left: -.7em;'>X</sub>^±<sub style='position: relative; left: -.5em;'>1</sub> and ^~<sub style='position: relative; left: -.7em;'>X</sub>^±<sub style='position: relative; left: -.5em;'>1</sub> → ^~<sub style='position: relative; left: -.7em;'>X</sub>⁰<sub style='position: relative; left: -.5em;'>1</sub> + W* with mass differences &triangle;m = m_{^~<sub style='position: relative; left: -.7em;'>X}^±<sub style='position: relative; left: -.5em;'>1</sub></sub> − m_{^~<sub style='position: relative; left: -.7em;'>X}⁰<sub style='position: relative; left: -.5em;'>1</sub></sub> = 5 GeV (20 GeV), stop masses up to 580 GeV (440 GeV) are excluded for m_{^~<sub style='position: relative; left: -.7em;'>X}⁰<sub style='position: relative; left: -.5em;'>1</sub></sub> = 100 GeV. Neutralino masses up to 280 GeV (230 GeV) are excluded for m_{^~<sub style='position: relative; left: -.7em;'>t}₁</sub> = 420 GeV for &triangle;m = 5 GeV (20 GeV). In an extension of this analysis, sbottom quarks cascade-decaying to at least a Higgs boson are searched for in final states with large missing transverse momentum, at least 3 b-tagged jets and no electrons or muons, using neural network discriminants.

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A feasibility study is presented which measures the masses of the chargino X̃₁^± and neutralinos X̃₁⁰ and X̃₂⁰ using the processes of e⁺e⁻→ X̃₁⁺X̃₁^- → X̃₁⁰X̃₁⁰W⁺W⁻ and e⁺e⁻→ X̃₂⁰X̃₂⁰ → X̃₁⁰X₁⁰ZZ at the International Linear Collider. The detector simulation is based on the Silicon Detector (SiD) concept and an integrated luminosity of 500 fb⁻¹ is considered at the centre-of-mass energy of 500 GeV. A template-fitting method is employed to measure the chargino and neutralino masses, which results in uncertainties of 0.16 GeV, 0.5 GeV and 1.0 GeV for the mass of X̃₁⁰, X̃₁^± and X̃₂⁰ respectively. A study on the ISIS2 sensor, a technology for the ILC vertex detector, is also presented. The characteristic of the sensors are studied for both its test structure and main array pixels. The operation conditions are optimized and the sensor successfully demonstrated its capabilities of in-situ charge storage and charge transfer. The charge transfer efficiency is measured to be better than 98%.

La supersymétrie représente le cadre théorique phare en physique des particules pour prendre la relève du modèle standard. Cependant, en tant que cadre général, il dispose de beaucoup de liberté dans sa mise en oeuvre, notamment au niveau des modèles de brisure. Le travail principal de cette thèse consiste à tenter de réduire l'espace des paramètres par des considérations théoriques, en trouvant un sous-espace réduit dans les modèles de brisure par médiation gravitationnelle, en utilisant les méthodes de minimisation du potentiel effectif développé dans les scénarios dits no-scale. / Supersymmetry is the most preferred theoretical framework that could replace and complete the standard model of particle physics. However it is hard to distinguish clearly between all the possible models allowed.During this thesis, we tried to reduce the arbitrariness in the choice of some parameters in supergravity breaking models, by looking at minimisation methods introduced in no-scale supergravity scenarios.

Gauge theories is one of the most pervasive and important subject of modern theoretical physics, and there are still many things about them we do not understand. In particular dealing with strongly coupled theories where normal perturbative techniques do not apply is a fundamental open problem. In this thesis, we study a particular class of toy-models that have supersymmetry, which makes them much easier to deal with. We employ the mathematical technique of localization, which for supersymmetric theories lets us evaluate certain path integrals exactly and for any value of the coupling. This is used to study the 5d N=1 theories placed on toric Sasaki-Einstein manifolds and compute their partition functions, finding that they factorize into a product of contributions from each closed Reeb orbit of the manifold. This computation leads us to define two new hierarchies of special functions associated to these manifolds, and we study their properties. Finally, we use the 5d N=1 theories to construct new 4d N=2 theories on a large class of curved backgrounds. These theories have some interesting features, such as supporting both instantons and anti-instantons, and having a position-dependent complexified coupling constant.

The search for new physics beyond the Standard Model can follow one of two tracks: direct searches for new particles at the collider or indirect probes for new physics from precision measurements. In the direct searches for third generation squarks in SUSY at the LHC, the common practice has been to assume a 100% decay branching fraction for a given search channel. In realistic MSSM scenarios, there is often more than one signicant decay mode present, which signicantly weakens the current search limits on third generation squarks at the LHC. On the other hand, the combination of multiple decay modes as well as the new open decay modes offer alternative discovery channels for third generation squarks searches. In this work, we present the third generation squarks decay and the collider signatures in a few representative mass parameter scenarios. We then analyze the reach of the stop/sbottom signal for the pair production in QCD at the 14 TeV LHC with 300 fb⁻¹ integrated luminosity and of the 100 TeV future collider with 3000 fb⁻¹ integrated luminosity in a few representative scenarios. In the scenario of Bino LSP with Wino NLSP, we investigate stop/sbottom pair production at the LHC with one stop/sbottom decaying via t̃ --> t𝑥[0 1], t𝑥[0 2]/b̃ --> b𝑥[0 1], b𝑥[0 2], and the other one decaying via t̃ --> b𝑥[± 1]/b̃ -->t𝑥[± 1]. With the gaugino subsequent decaying to gauge bosons or a Higgs boson 𝑥[0 2] --> 𝑍𝑥[0 1], h𝑥[0 1] and 𝑥[± 1]--> 𝑊±𝑥[0 1], leading to 𝑏𝑏𝑏𝑏𝑗𝑗𝓁 Ɇᴛ final states for the Higgs channel and 𝑏𝑏𝑗𝑗𝑗𝑗𝓁𝓁Ɇᴛ final states for the 𝑍 channel, we study the reach of those signals at the 14 TeV LHC with 300 fb⁻¹ integrated luminosity. Because the sbottom and stop signals in the same SUSY parameter scenario have indistinguishable final states, they are combined to obtain optimal sensitivity, which is about 150 GeV better than the individual reaches of the sbottom or stop. In the scenario of Bino LSP with Higgsino NLSP. The light stop pair production at the 14 TeV LHC, with stops decaying via t̃₁ --> t𝑥[0 2]/𝑥[0 3] and the neutralino subsequently decaying to a gauge boson or a Higgs boson 𝑥[0 2]/𝑥[0 3] --> 𝑥[0 1]h/𝑍, leads to tt̄hh Ɇᴛ, tt̄h𝑍 Ɇᴛ or tt̄𝑍𝑍 Ɇᴛ final states. The above decay channels give rise to final states containing one or more leptons, therefore our search strategy is to divide the signal regions based on the multiplicity of leptons. We find that the one lepton signal region of channel tt̄h𝑍 Ɇᴛ has the best reach sensitivity of light stop searches at the 14 TeV LHC with 300 fb⁻¹ integrated luminosity. We then combine all the signal regions for a given decay channel or combine all the decay channels for a given signal region to maximize the reach sensitivity of the stop search. For the light stop pair production at the √s = 100 TeV future machine with 3000 fb⁻¹ integrated luminosity, we find that a stop with a mass up to 6 TeV can be discovered at 5𝜎 signicance, while a mass up to 6.8 TeV can be excluded at 95% C.L. for the combined results of all three channels. In the indirect probes for new physics, we utilize the 𝑍-pole Oblique Parameters 𝑆,𝑇, 𝑈 and Higgs precision measurements complementarily in the framework of the Two Higgs Doublet Model at current and future colliders. The 𝑆, 𝑇 , 𝑈 is not that sensitive to the rotation angle 𝛽-𝛼, while the Higgs precision measurements set strong constrains on 𝛽-𝛼. Also the 𝑇 is very sensitive to the mass difference of Higgs bosons, leading to the mass of charged Higgs (H±) aligning either along with the mass of neutral Higgs 𝐻 or 𝐴. As for the Higgs precision measurements, we consider the tree level corrections to Higgs coupling constants as well as the radiative corrections to Higgs coupling constants at one loop level for the future collider. The combination of 𝑍-pole precision measurements and Higgs precision measurements complementarily set strong constraints on the parameter space of the 2HDM, especially in the future 𝑒⁺𝑒⁻ circular collider compared to the current collider due to much cleaner backgrounds and higher luminosity.

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.