This thesis presents a search for singly produced vector-like T quarks with charge +2=3 or Y quarks with charge -4=3. These exotic particles are predicted by several beyond-the-standard-model theories which try to shed light on the mechanism of electro-weak symmetry breaking, thus addressing the Naturalness Problem. The analysis is performed on a proton–proton collision data set with an integrated luminosity of 36:1 fb-1 at a
centre-of-mass energy of √s = 13 TeV recorded in 2015 and 2016 by the ATLAS detector at the LHC. The focus is on leptonically decaying W bosons from Y/T → Wb decays of the heavy quarks and the event selection requires a jet leading in transverse momentum and being b-tagged, exactly one lepton, a forward jet as well as missing transverse energy beside several isolation requirements.
No significant deviation from the expected Standard Model background is observed. Therefore, upper limits on the coupling strength and mixing angle between third generation Standard Model quarks and a T from a singlet model as well as a Y from a (B, Y) doublet or a (T, B, Y) triplet model are computed using the CLs method. This search represents the first analysis in which the interference effects with the Standard Model background are taken into account. The smallest upper limits are found to be |sin θL| = 0.18 for a T singlet for a mass of 800 GeV, |sin θR| = 0.17 in the (B, Y) doublet case at 800 GeV and |sin θL| = 0.16 for the (T, B, Y) triplet model. The observed limits on the mixing parameter |sin θR| are competitive with indirect bounds from electroweak precision observables in the mass range between about 800 GeV and 1250 GeV. For the Y in the (B, Y) doublet model, also an observed mass limit of 1.64 TeV is determined for a right-handed coupling value of cWbR = 1/p2.
|Advisor:||Lacker , Heiko , Lohse , Thomas , Husemann , Ulrich|
|School:||Humboldt Universitaet zu Berlin (Germany)|
|Source:||DAI-C 81/8(E), Dissertation Abstracts International|
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