Aspects of the Exceptional Supersymmetric Standard Model
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The Standard Model of particle physics (SM) is a gauge field theory that provides a very successful description of the electromagnetic, weak and strong interactions among the elementary particles. It is in very good agreement with the precision measurements and the list of all the fundamental particles predicted by the model was completed with the discovery of the last missing piece, the Higgs boson, at the LHC in 2012. However, it is believed to be valid up to a certain energy scale and widely considered as a low-scale approximation of a more fundamental theory due to some theoretical and phenomenological issues appearing in the model. Among many alternatives, supersymmetry is considered as the most prominent candidate for new physics beyond the SM. Supersymmetry relates two different classes of the particles known as fermions and bosons. The simplest straightforward supersymmetrization of the SM is named as minimal supersymmetric Standard Model (MSSM) where minimal set of new supersymmetric particles is introduced as superpartners of the Standard Model particles. It is the most studied low-scale supersymmetric model since it has very appealing features such as containing a dark matter candidate and providing a solution to the naturalness problem of the SM. After the Higgs discovery, the parameter space of the model has been investigated in great detail and it has been observed that the measured Higgs mass can be achieved only for the parameter regions which generate a severe fine-tuning. Such large fine-tuning can be alleviated by extending the minimal field content of the model via a singlet and/or a triplet. In this thesis, we discuss the triplet extension of the supersymmetric Standard Model where the MSSM field content is enlarged by introducing a triplet chiral superfield with zero hypercharge. The first part of the thesis contains an overview of the SM and the second part is dedicated to the general features of supersymmetry. After discussing aspects of the MSSM in the third part, we discuss the triplet extended supersymmetric Standard Model where we investigate the implications of the triplet on the Higgs phenomenol- ogy. We show that the measured mass of the Higgs boson can be achieved in this model without requiring heavy third generation squarks and/or large squark mixing parameters which reduce the amount of the required fine-tuning. Afterwards, we study the charged Higgs sector where a triplet scalar field with non-zero vacuum expectation value leads to $h_i^{\pm}\,Z W^{\mp}$ coupling at tree level. We discuss how this coupling alters the charged Higgs decay and production channels at the LHC.
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We review the most recent data from e{sup +}e{sup {minus}} and p{bar p} colliders and discuss the resulting constraints on the parameters of the Minimal Supersymmetric Standard Model, and their implications for future supersymmetry searches. We review the patterns of cascade decays of squarks and gluinos and discuss the present status of supersymmetry event generators for hadron colliders. We present the results of detailed simulations of E{sub T} and same sign dilepton events from supersymmetry at the Tevatron. Although the E{sub T} signal continues to be viable, it is concluded that the same sign dilepton signal may be too small unless squarks and gluinos are approximately degenerate. The E{sub T} and the same-sign dilepton signals from supersymmetry and the Standard Model backgrounds at the SSC are also discussed in detail. We also discuss other promising ways of searching for supersymmetry at the SSC including events containing Z{degree} bosons, and events containing n isolated leptons (n {ge} 3). Finally, we discuss how supersymmetry searches might be modified if the Higgs sectors is more complicated or if R-parity is not conserved due to baryon number violating interactions. 49 refs., 12 figs.
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Current experimental constraints on a large parameter space in supersymmetric models rely on the large missing energy signature. This is usually provided by the lightest neutralino which stability is ensured by the R-parity. However, if the R-parity is violated, the lightest neutralino decays into the standard model particles and the missing energy cut is not efficient anymore. In particular, the UDD type R-parity violation induces the neutralino decay to three quarks which potentially leads to the most difficult signal to be searched at hadron colliders. In this paper, we study the constraints on the R-parity violating supersymmetric model using a same-sign dilepton and a multijet signatures. We show that the gluino and squarks lighter than a TeV are already excluded in constrained minimal supersymmetric standard model with R-parity violation if their masses are approximately equal. We also analyze constraints in a simplified model with R-parity violation. We compare how R-parity violation changes some of the observables typically used to distinguish a supersymmetric signal from standard model backgrounds.
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Measuring the Higgs self-coupling is one of the crucial physics goals at the LHC Run-2 and other future colliders. In this work, we attempt to figure out the size of SUSY effects on the trilinear self-coupling of the 125 GeV Higgs boson in the MSSM and NMSSM after the LHC Run-1. Taking account of current experimental constraints, such as the Higgs data, flavor constraints, electroweak precision observables and dark matter detections, we obtain the observations: (1) In the MSSM, the ratio of $\lambda^{MSSM}_{3h}/\lambda^{SM}_{3h}$ has been tightly constrained by the LHC data, which can be only slightly smaller than 1 and minimally reach 97\%; (2) In the NMSSM with $\lambda<0.7$, a sizable reduction of $\lambda^{NMSSM}_{3h_2}/\lambda^{SM}_{3h_2}$ can occur and minimally reach 10\% when the lightest CP-even Higgs boson mass $m_{h_1}$ is close to the SM-like Higgs boson $m_{h_2}$ due to the large mixing angle between the singlet and doublet Higgs bosons; (3) In the NMSSM with $\lambda>0.7$, a large enhancement or reduction $-1.1<\lambda^{NMSSM}_{3h_1}/\lambda^{SM}_{3h_1}<2$ can occur, which is accompanied by a sizable change of $h_1\tau^+\tau^-$ coupling. The future colliders, such as the HL-LHC and ILC, will have the capacity to test these large deviations in the NMSSM.
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We consider a scenario where supersymmetry is broken at a high energy scale, out of reach of the LHC, but leaves a few fermionic states at the TeV scale. The particle content of the low-energy effective theory is similar to that of Split Supersymmetry. However, the gauginos and higgsinos are replaced by fermions carrying the same quantum numbers but having different couplings, which we call fake gauginos and fake higgsinos. We study the prediction for the light-Higgs mass in this Fake Split SUSY Model (FSSM). We find that, in contrast to Split or high-scale supersymmetry, a 126 GeV Higgs boson is easily obtained even for arbitrarily high values of the supersymmetry scale. For a supersymmetry scale greater than roughly 100 PeV, the Higgs mass is almost independent of the supersymmetry scale and the stop mixing parameter, while the observed value is achieved for tan beta between 1.3 and 1.8 depending on the gluino mass.
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The Standard Model of particle physics (SM) is a gauge field theory that provides a very successful description of the electromagnetic, weak and strong interactions among the elementary particles. It is in very good agreement with the precision measurements and the list of all the fundamental particles predicted by the model was completed with the discovery of the last missing piece, the Higgs boson, at the LHC in 2012. However, it is believed to be valid up to a certain energy scale and widely considered as a low-scale approximation of a more fundamental theory due to some theoretical and phenomenological issues appearing in the model. Among many alternatives, supersymmetry is considered as the most prominent candidate for new physics beyond the SM. Supersymmetry relates two different classes of the particles known as fermions and bosons. The simplest straightforward supersymmetrization of the SM is named as minimal supersymmetric Standard Model (MSSM) where minimal set of new supersymmetric particles is introduced as superpartners of the Standard Model particles. It is the most studied low-scale supersymmetric model since it has very appealing features such as containing a dark matter candidate and providing a solution to the naturalness problem of the SM. After the Higgs discovery, the parameter space of the model has been investigated in great detail and it has been observed that the measured Higgs mass can be achieved only for the parameter regions which generate a severe fine-tuning. Such large fine-tuning can be alleviated by extending the minimal field content of the model via a singlet and/or a triplet. In this thesis, we discuss the triplet extension of the supersymmetric Standard Model where the MSSM field content is enlarged by introducing a triplet chiral superfield with zero hypercharge. The first part of the thesis contains an overview of the SM and the second part is dedicated to the general features of supersymmetry. After discussing aspects of the MSSM in the third part, we discuss the triplet extended supersymmetric Standard Model where we investigate the implications of the triplet on the Higgs phenomenology. We show that the measured mass of the Higgs boson can be achieved in this model without requiring heavy third generation squarks and/or large squark mixing parameters which reduce the amount of the required fine-tuning. Afterwards, we study the charged Higgs sector where a triplet scalar field with non-zero vacuum expectation value leads to hi ZW ∓ coupling at tree level. We discuss how this coupling alters the charged Higgs decay and production channels at the LHC.
Effective field theory
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We analyze particle decays including radiative corrections at the next-to-leading order (NLO) within the Minimal Supersymmetric Standard Model (MSSM). If the MSSM is realized at the TeV scale, squark and gluino production and decays yield relevant rates at the LHC. Hence, in the first part of this thesis, we compute decay widths including QCD and electroweak NLO corrections to squark and gluino decays. Furthermore, the Higgs sector of the MSSM is enhanced compared to the one of the Standard Model. Thus, the additional Higgs bosons decay also into supersymmetric particles. These decays and the according NLO corrections are analyzed in the second part of this thesis. The calculations are performed within a common renormalization framework and numerically evaluated in specific benchmark scenarios.
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