Doctor of Philosophy / Department of Physics / Andrew G. Ivanov / A search is performed for heavy vector-like top quark partner (T) with electric charge +2/3 by using proton-proton collision events from Large Hadron Collider (LHC) at a centre- of-mass energy of 13 TeV. The data corresponds to an integrated luminosity of 35.9fb−1 collected by the Compact Muon Solenoid (CMS) experiment during 2016. The production of this new hypothetical particle is assumed to be in pairs and strong interaction is responsible for such production mechanism. T quarks can decay to various combinations of third generation quarks and standard model bosons: T → bW, tZ, or tH, and hence the final states consist of pair of opposite-sign leptons consistent with coming from a Z boson and jets. No significant excess has been observed and hence 95% CL upper limits are obtained on TT production cross section by assuming different branching ratios. T quark mass values below 1280 GeV are excluded in case of 100% branching fraction for T → tZ.

The top quark is the most massive fundamental particle in the Standard Model of particle physics. Only experimentally observed in 1995, it can be used as a precise test of Standard Model predictions, and it could lend insight to the problem of what lies beyond the Standard Model. This thesis presents a measurement of top-quark pair production using data collected at a center-of-mass energy √s=7 TeV in 2011, and a search for production of vector-like quarks using data collected at √s=8 TeV in 2012. Both datasets were recorded by the ATLAS detector, a multipurpose proton-proton collider located at the CERN LHC outside of Geneva Switzerland. The top-quark pair production cross-section is measured as a function of four different variables and the results are presented as normalized, differential spectra. The variables considered are the transverse momentum of the top quark, and the mass, rapidity, and transverse momentum of the top-quark pair system. Events are selected in the lepton+jets channel, and the measured spectra are corrected for detector resolution and efficiency. The final results are compared with predictions from various Monte Carlo generators, theoretical calculations and proton parton distribution functions and found to be in reasonable agreement. Data is found to be softer than all predictions, particularly for high values of top-quark transverse momentum and the top-quark pair invariant mass. The search for vector-like quarks focuses on new heavy quarks that decay with a large branching ratio to a Z boson and a third generation Standard Model quark. Events are selected with at least two leptons (electrons or muons), and two of the leptons are required to reconstruct a Z boson with high transverse momentum. No significant excess of events is observed above the Standard Model prediction. Upper limits on the masses of vector-like T and B quarks are derived for various branching ratio hypotheses.

Since the discovery of the Higgs boson in 2012, the search for new physics beyond the Standard Model has been greatly intensified. At the CERN Large Hadron Collider (LHC), ATLAS searches for new physics entail looking for new particles by colliding protons together. Presented here is a search for a new form of quark matter called Vector-like Quarks (VLQ), which are hypothetical particles that are expected to have mass around a few TeV. VLQ can come in a variety of forms and can couple to their Standard Model (SM) quark counterparts, particularly to the third generation. They are necessary in several beyond the SM theories in order to solve the hierarchy problem. This search uses 36.1 fb−1of proton-proton collision data collected with the ATLAS detector at the LHC from August 2015 to October 2016. Only events with two leptons of the same charge, or three leptons, plus b-jets and high missing transverse energy are considered in the main analysis. This signature is rarely produced in the SM, which means the backgrounds in this analysis are relatively low. This analysis is sensitive to specific predicted decay modes from pair production of an up-type VLQ with a charge of +2/3, T, an up-type VLQ with a charge of +5/3, T5/3, and a down-type quark with a charge of −1/3, B, as well as single production of T5/3. There is another theorized VLQ that this analysis is not sensitive to: B−4/3, due to its primary decay mode, which is unable to produce the final-state signature of interest. The results from this analysis suggest only a slight deviation of data from SM backgrounds reaching as high as 1.89σ, which does not indicate evidence for VLQ. A mostly frequentist statistical technique, called the CLS Method, is used to interpret the data and set limits on the T, B, and T5/3 signal models. Using this method, exclusion limits are set at the 95% confidence level, effectively excluding T mass below 0.98 TeV, T5/3 mass below 1.2 TeV, and B mass below 1.0 TeV, assuming singlet branching ratios. Also, branching ratio independent limits are set on the T and B VLQ.

In dieser Arbeit wird die Suche nach vektorartigen und angeregten Quarks mit einer elektrischen Ladung von Q = 1/3 mit dem ATLAS-Detektor vorgestellt. Die Existenz solcher Quarks wird von verschiedenen Modellen jenseits des Standardmodells vorhergesagt, die bestimmte Schwächen der Theorie beseitigen können. Es wird nach einzeln produzierten Quarks aus zwei verschiedenen Modellen gesucht, da für diesen Produktionsmodus höhere Wirkungsquerschnitte im untersuchten Massenbereich vorhergesagt sind als für die Paarproduktion. Die Suche beschränkt sich auf Zerfälle der Quarks in W-Bosonen und Top-Quarks, wobei letztere fast ausschließlich in W-Bosonen und Bottom-Quarks zerfallen. Verlangt man, dass eines der W-Bosonen leptonisch zerfällt, erhält man Endzustände mit einem Lepton (Elektron oder Myon in diesem Fall), mehreren Jets, von denen einer als von einem Bottom-Quark stammend identifiziert werden kann, und fehlender Energie in der Transversalebene, die durch das beim leptonischen Zerfall entstandene und nicht detektierbare Neutrino hervorgerufen wird. Die daraus berechnete invariante Masse des neuen Quarks wird zur Unterscheidung zwischen Signal und Untergrund verwendet. Die Suche wurde auf dem am LHC im Jahr 2012 aufgenommenen Datensatz von Proton-Proton-Kollisionen bei einer Schwerpunktsenergie von 8 TeV ausgeführt. Es wurde kein signifikanter Unterschied zum vorhergesagten Untergrund gemessen und deshalb Ausschlussgrenzen für Parameter der untersuchten Modelle berechnet. Da die vorgestellte Suche Gebrauch von der geboosteten Topologie der Signalereignisse im untersuchten Massenbereich macht, übertreffen die Ergebnisse bei Weitem die Ausschlussgrenzen für das angeregte Quarkmodell, die in der Vorgängersuche auf dem Datensatz von 2011 erhalten wurden und sind vergleichbar mit kürzlich auf dem Datensatz von 2012 mit dem CMS-Detektor gewonnenen Ergebnissen. Darüber hinaus ist es die erste Suche nach einzeln produzierten vektorartigen Quarks im untersuchten Modell. / A search for vector-like and excited down-type (Q = 1/3) quarks with the ATLAS detector is presented in this thesis. The existence of these quarks is predicted by various models beyond the Standard Model, motivated by some limitations of that theory. Quarks from two specific models are searched for in single production mode, as this is predicted to yield higher cross-sections than pair production in the investigated mass range. The search focuses on decays of the heavy quarks to a W boson and a top quark, the latter of which subsequently decays almost exclusively into another W boson and a bottom quark. Requiring one of the two W bosons to decay leptonically leads to final states containing exactly one lepton (electron or muon in this case), several jets, one of which can be identified as originating from a bottom quark, and some amount of missing energy in the transverse plane stemming from the neutrino of the leptonic W decay. The reconstructed mass of the heavy quark is used to discriminate between signal and background. This search was performed on 20.3 inverse femtobarns of proton-proton collision data collected at the Large Hadron Collider in the year 2012 with a centre-of-mass energy of 8 TeV. No significant excess of the data over the background could be observed and therefore exclusion limits were set on model parameters. The search presented here makes use of the boosted event topology that is prominent in the explored range of signal masses, allowing it to by far exceed the exclusion bounds obtained in the preceding search for the excited quark model on the 2011 data set, and producing results that are comparable to those from a recent search performed with the CMS detector on the 2012 data set. Additionally, it is the first search in the single production mode for that specific vector-like quark model.

In dieser Dissertation wird die Suche nach einzeln erzeugten vektorartigen T-Quarks mit einer elektrischen Ladung +2/3 und Y-Quarks mit einer elektrischen Ladung von -4/3 vorgestellt. Diese exotischen Teilchen werden von verschiedenen Modellen jenseits des Standardmodells vorhergesagt, welche versuchen das Verständnis der elektorschwachen Symmetriebrechung zu erweitern und somit das Naturalness Problem zu lösen. Hierfür werden die in 2015 und 2016 bei einer Schwerpunktsenergie von 13 TeV und einer integrierten Luminosität von 36.1 inversen fb vom ATLAS Detektor am LHC aufgezeichneten Proton-Proton-Kollisionen ausgewertet. Die Ereignisselektion ist auf leptonisch zerfallende W-Bosonen der Y/T nach Wb Zerfälle ausgerichtet, weshalb ein hochenergetischer, von einem Bottom-Quark stammender Jet, ein einzelnes Lepton, ein in Vorwärtsrichtung emittierter Jet sowie fehlende transversale Energie, neben verschiedenen Isolationskriterien, verlangt wird. Es konnte keine signifikante Abweichung zur Vorhersage des Standardmodells gemessen werden. Aus diesem Grund wurden obere Grenzen for die Kopplungsstärke und den Mischungswinkel von T-Quarks in einem Singulett-Modell sowie Y-Quarks eines (B,Y)-Dublett bzw. (T,B,Y)-Triplett-Modells mit Quarks der dritten Generation des Standard Modells bestimmt. Hierbei wurden erstmalig Interferenzeffekte mit Standardmodellprozessen berücksichtigt. Die obere Grenze für den Mischungswinkel wurden für ein T-Singulett-Modell bei einer Masse von 800 GeV zu |sin(theta_L)|= 0.18, für ein Y eines (B,Y)-Dublett-Modells zu |sin(theta_R)|= 0.17 sowie für ein Y eines (T,B,Y)-Triplett-Modells zu |sin(theta_L)|= 0.16 bestimmt. Die ermittelten Grenzen innerhalb des (B,Y)-Dublett-Modells im Massenbereich 800 GeV bis 1250 GeV sind zu indirekten, aus elektroschwachen Präzisionsvariablen abgeleiteten Schranken kompetitiv. / 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 inverse fb at a centre-of-mass energy of 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 to 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(theta_L)|= 0.18 for a T singlet for a mass of 800 GeV, |sin(theta_R)|= 0.17 in the (B,Y) doublet case at 800 GeV and |sin(theta_L)|= 0.16 for the (T,B,Y) triplet model. The observed limits on the mixing parameter |sin(theta_R)| are competitive with indirect bounds from electroweak precision observables in the mass range between about 800 GeV and 1250 GeV.

No description available.

La recherche de la production de gluons lourds gKK, de type Kaluza-Klein dans la théorie avec dimensions supplémentaires, qui se désintègre en une paire de SM quark top (t¯t), ou en un VLQ top et un SM quark top (tT ), est présentée dans cette thèse. Cette recherche est basée sur les collisions de proton proton à 8 TeV, enregistrées durant l’année 2012 par le détecteur ATLAS au LHC, au CERN. La luminosité, correspondante à ces énergies de collisions, a été de 20.3 fb^−1. Dans le cas de gKK -> tT , trois types d’analyses ont été effectuées de façon indépendante: VLQ top (T) se désintégrant en un boson W et un quark b (T -> W b), avec un large rapport de branchement, VLQ top se désintégrant en un boson de Higgs composite et un SM quark top (T -> H t), avec un large rapport de branchement, et VLQ top se désintégrant en un boson Z et un SM quark top (T -> Z t). Dans chaque cas, les données, issues des collisions, sont étudiées dans des états finaux avec un lepton et des jets. Ces états sont caractérisés par un electron ou un muon isolé ayant une impulsion transverse large, une énergie transverse manquante et jets. Les spectres de la masse invariante de t¯t et de t¯t+jets, sont ensuite examinés, afin de vérifier un éventuel excès (ou un déficit) inconsistant avec les prédictions du Modèle Standard. Aucune évidence de résonance de paire de quarks top a été observée, et une limite d’échelle de confiance de 95% a été déterminée pour des résonances lourdes prévues par les modèles benchmark. D’une part, une résonance correspondant à un gluon de type Kaluza–Klein (de largeur de 15.3%) prévu par le modèle Randall–Sundrum, qui se désintègre avec un taux d’embranchement de 100% en une paire de SM quarks top a été exclue avec 95% de degré de confiance, jusque pour les masses inférieures à 2.0 TeV. D’autre part, les excitations de gluon de type Kaluza–Klein, dans le modèle composite avec des paramètres de benchmark pour des angles de mélange et une largeur de 10%, de masse 1.0 TeV, qui se désintègrent en un SM quark top, et, un VLQ top de masse 0.6 TeV pour tous les embranchements propres à VLQ top, ont été exclues avec 95% de degré de confiance. Finalement, les excitations de gluon de type Kaluza–Klein, de largeur de 10%, ayant une masse de 2.0 TeV, qui se désintègrent en un SM quark top et un VLQ top de masse 1.4 TeV, avec des scenarios qui respectent BR (T -> Wb) < 0.80, ont été exclues avec 95% de degré de confiance. Les échantillons utilisés lors de cette analyse ont été validés par la collaboration ATLAS, cependant les résultats montrés dans cette thèse n’ont pas été vérifiés par la collaboration ATLAS en vue d’une publication. / A search for production of the lightest Kaluza-Klein excitation of gluon (gKK), in theories with a warped extra dimension, that decays into Standard Model (SM) top quark pairs (t¯t) or into one vector-like top quark (VLQ top) in association with a SM top quark (Tt) is presented in this thesis. The search is based on proton proton collisions at 8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb^−1. For the analysis of gKK -> tT , we performed targeting three cases: VLQ top (T) with significant branching ratio to a W boson and a b-quark (T -> W b), VLQ top with significant branching ratio to a composite Higgs boson and a standard model top quark (T -> H t) and VLQ top with significant branching ratio to a Z boson and a standard model top quark (T -> Z t). Data are analyzed in the lepton-plus-jets final state, characterized by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. The invariant mass spectrum of t¯t and t¯t + jets is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pairs resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. On the one hand, a Kaluza–Klein excitation of the gluon of width 15.3% with a branching ratio of 100% into a SM top quark pairs in a Randall–Sundrum model, is excluded for masses below 2.0 TeV. On the other hand, in a composite model where the heavy gluon decays to tT , using benchmark parameters for mixing angles, a 1.0 TeV Kaluza–Klein excitation of the gluon of width 10% that decay to one SM and a 0.6 TeV VLQ top, considering any VLQ top branching ratio scenario, is excluded at 95% Confidence Level (CL). Finally, a 2.0 TeV Kaluza–Klein excitation of the gluon, of width 10%, that decays into one SM in association with a 1.4 TeV VLQ top, with any scenario with BR (T -> Wb) < 0.80, is excluded at 95% CL. The samples used in these analyses were validated by ATLAS collaboration, however the results shown in this thesis were not reviewed by the ATLAS collaboration for publication.

An evaluation of how sensitive some ATLAS searches for new physics are to a new beyond standard model (BSM) vector-like quark (VLQ) and a pseudo Nambu-Goldstone boson (pNGB) scalar. This was done by simulating a signal containing these new particles and making a recast of it onto existing verified ATLAS searches for new physics at center-of-mass (CM) energy of 13 TeV (Run 2) at the Large Hadron Collider (LHC). Signals for recasting were tailored such that their final states would be appropriate in relation to each respective ATLAS search in order to use the same selection criteria as applied in the existing searches. The results are summarized in the form of significances (Z) for each masspoint of the new top-partner and S particle. Significances did not show any expectiation of excluding any masspoint in the examined mass range for the recasts at 95% CL. This suggests that a dedicated search for these particles in the considered masspoints would be required.