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.

No description available.

A search is presented for vector-like quarks using 20.3 fb⁻¹ of proton-proton collisions at a center-of-mass energy of √S = 8 TeV collected with the ATLAS detector in 2012 at the CERN Large Hadron Collider. Vector-like quarks are predicted to exist in many theories beyond the Standard Model of particle physics that attempt to resolve the hierarchy problem. Events are selected containing jets including at least one b-jet, sizable missing transverse momentum, large scalar sum of the jet and lepton transverse momenta, and either three leptons or two leptons with the same electric charge. Standard Model processes rarely produce this final state and production of vector-like quarks would lead to an enhanced rate of such events. The data are interpreted in the context of a variety of models including pair production of vector-like quarks T and B, which have the same electric charge as the corresponding Standard Model quarks and can appear in either SU(2) weak isospin singlets, doublets or triplets. In addition single and pair production of the vector-like quark T₅/₃ which appears in an SU(2) weak isospin doublet or triplet and has electric charge 5/3, is considered. A moderate excess of data above the SM background expectation is observed with a significance of less than two standard deviations. The data are used to set limits at 95% Confidence Level (CL) on the new heavy quark mass for the various VLQ models considered using the CL_S method. The vector-like quarks T and B in the singlet model are excluded at 95% CL below a mass of 0.59 TeV and 0.62 TeV respectively. The T₅/₃ is excluded at 95% CL below a mass of 0.74 TeV when only pair production is considered and below 0.75 TeV when both pair and single production are considered.

A search for vector-like quarks (VLQs) decaying to a Z boson using multi-stage machine learning was compared to a search using a standard square cuts search strategy. VLQs are predicted by several new theories beyond the Standard Model. The searches used 20.3 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV collected with the ATLAS detector in 2012 at the CERN Large Hadron Collider. CLs upper limits on production cross sections of vector-like top and bottom quarks were computed for VLQs produced singly or in pairs, Tsingle, Bsingle, Tpair, and Bpair. The two stage machine learning classification search strategy did not provide any improvement over the standard square cuts strategy, but for Tpair, Bpair, and Tsingle, a third stage of machine learning regression was able to lower the upper limits of high signal masses by as much as 50%. Additionally, new test statistics were developed for use in the Neyman construction of confidence regions in order to address deficiencies in current frequentist methods, such as the generation of empty set confidence intervals. A new method for treating nuisance parameters was also developed that may provide better coverage properties than current methods used in particle searches. Finally, significance ratio functions were derived that allow a more nuanced interpretation of the evidence provided by measurements than is given by confidence intervals alone.

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 Dissertation wird eine Suche nach vektorartigen Quarks (VLQs) beim Zerfall in ein W-Boson und ein b-Quark vorgestellt. Die Suche wird mit Proton-Proton-Kollisionsdaten durchgeführt, die vom ATLAS-Detektor am CERN bei einer Schwerpunktsenergie von √s = 13 TeV zwischen 2015 und 2018 während dem LHC Run II gesammelt wurden. Die Existenz von VLQs wird von mehreren Theorien jenseits des Standardmodells postuliert, zudem sind VLQs ein einfaches Beispiel für Fermionen mit Farbladung, die immer noch mit experimentellen Beobachtungen konsistent sind. Diese Arbeit konzentriert sich auf den Fall, dass ein einzelnes VLQ erzeugt wird und in ein b-Quark und ein W-Boson zerfällt, das dann wiederum in ein geladenes Lepton und ein Neutrino zerfällt. Diese Suche schließt sich an mehrere Suchen nach einzeln und paarweise erzeugten VLQs am LHC an, die sowohl bei ATLAS als auch bei CMS durchgeführt wurden und deren Existenz bis zu einer Masse von über 1 TeV ausgeschlossen haben. Bei einer Masse von etwa 1.8 TeV wird ein Überschuss von 3.7 σ gegenüber der Vorhersage des Standardmodells beobachtet. Einige mögliche Masse/Kopplungskombinationen werden durch diese Analyse ausgeschlossen. Dies ist jedoch weniger Parameterraum als aufgrund eines unerwarteten Überschusses an beobachteten Ereignissen erwartet wurde. / In this thesis, a search for vector-like quarks (VLQs) decaying into a W-boson and a b-quark is presented. The search is performed on proton-proton collision data collected by the ATLAS detector at CERN at a center-of-mass energy of √s = 13 TeV between 2015 and 2018, during Run II of the LHC. The existence of VLQs is consistent with several Beyond the Standard Model theories and VLQs are a simple example of coloured fermions that are still consistent with experimental observations. This work focuses on the case where a single VLQ is produced and decays to a b- quark and a W-boson, which then decays to a charged lepton and a neutrino. This search follows on several searches for singly- and pair-produced VLQs at the LHC from both ATLAS and CMS, which have excluded their existence up to over 1 TeV, and this search considers VLQs in a mass range from 1 TeV to 2.7 TeV. An excess over the Standard Model (SM) prediction of 3.7 σ is observed at a mass of around 1.8 TeV. Some possible mass/coupling combinations are excluded by this analysis. However, this is less parameter space than was expected due to an unexpected excess of observed events.

Utilisant les plus récentes données recueillies par le détecteur ATLAS lors de collisions pp à 7 et 8 TeV au LHC, cette thèse établira des contraintes sévères sur une multitude de modèles allant au-delà du modèle standard (MS) de la physique des particules. Plus particulièrement, deux types de particules hypothétiques, existant dans divers modèles théoriques et qui ne sont pas présentes dans le MS, seront étudiés et sondés. Le premier type étudié sera les quarks-vectoriels (QV) produits lors de collisions pp par l’entremise de couplages électrofaibles avec les quarks légers u et d. On recherchera ces QV lorsqu’ils se désintègrent en un boson W ou Z, et un quark léger. Des arguments théoriques établissent que sous certaines conditions raisonnables la production simple dominerait la production en paires des QV. La topologie particulière des évènements en production simple des QV permettra alors la mise en oeuvre de techniques d’optimisation efficaces pour leur extraction des bruits de fond électrofaibles. Le deuxième type de particules recherché sera celles qui se désintègrent en WZ lorsque ces bosons de jauges W, et Z se désintègrent leptoniquement. Les états finaux détectés par ATLAS seront par conséquent des évènements ayant trois leptons et de l’énergie transverse manquante. La distribution de la masse invariante de ces objets sera alors examinée pour déterminer la présence ou non de nouvelles résonances qui se manifesterait par un excès localisé. Malgré le fait qu’à première vue ces deux nouveaux types de particules n’ont que très peu en commun, ils ont en réalité tous deux un lien étroit avec la brisure de symétrie électrofaible. Dans plusieurs modèles théoriques, l’existence hypothétique des QV est proposé pour annuler les contributions du quark top aux corrections radiatives de la masse du Higgs du MS. Parallèlement, d’autres modèles prédisent quant à eux des résonances en WZ tout en suggérant que le Higgs est une particule composite, chambardant ainsi tout le sector Higgs du MS. Ainsi, les deux analyses présentées dans cette thèse ont un lien fondamental avec la nature même du Higgs, élargissant par le fait même nos connaissances sur l’origine de la masse intrinsèque des particules. En fin de compte, les deux analyses n’ont pas observé d’excès significatif dans leurs régions de signal respectives, ce qui permet d’établir des limites sur la section efficace de production en fonction de la masse des résonances. / Using the most recent data collected by the ATLAS detector in pp collisions delivered by the LHC at 7 and 8 TeV, this thesis shall establish severe constraints on a variety of models going beyond the Standard Model (SM) of particle physics. More particularly, two types of hypothetical particles, existing in various theoretical models shall be studied and probed. The first type will be the search for vector-like quarks (VLQ) produced in pp collisions through electroweak couplings with the u and d quarks. The quest for these particles will be made as they decay into either W(lν)+jet or Z(ll)+jet. There exist theoretical arguments that establish that, under certain reasonable conditions, single production of VLQ dominates over production in pairs. The particular topology of such events enables the implementation of effective techniques to extract signal over electroweak background. The second type is the search for resonant particles decaying to WZ when the gauge bosons W and Z decay leptonically. The final states detected by ATLAS therefore contain three leptons (e, or μ) and missing transverse energy. The distribution of the invariant mass of these objects will then be examined to determine the presence or absence of new resonances that manifest themselves as localized excesses in m(WZ). Despite the fact that, at first glance, these two new types of particles have very little in common, they are in fact both closely linked to electroweak symmetry breaking. In many theoretical models, the hypothetical existence of VLQ is put forward to counteract the top quark’s contribution to radiative loop corrections of the Higgs mass, a calculation which assumes that the Higgs is an elementary particle. Concurrently, other models foretelling the existence WZ resonances alternatively suggest that the Higgs is a composite particle, completely rewriting the whole Higgs sector of the SM. In this perspective, the two analyses presented in this thesis have a fundamental link with the very nature of the Higgs, thereby extending our knowledge of the origin of particle masses. Ultimately, the two analyses did not observe any significant excess in their respective signal regions, paving the way for the computations of limits on the production cross section as a function of the mass of the resonances.

Utilisant les plus récentes données recueillies par le détecteur ATLAS lors de collisions pp à 7 et 8 TeV au LHC, cette thèse établira des contraintes sévères sur une multitude de modèles allant au-delà du modèle standard (MS) de la physique des particules. Plus particulièrement, deux types de particules hypothétiques, existant dans divers modèles théoriques et qui ne sont pas présentes dans le MS, seront étudiés et sondés. Le premier type étudié sera les quarks-vectoriels (QV) produits lors de collisions pp par l’entremise de couplages électrofaibles avec les quarks légers u et d. On recherchera ces QV lorsqu’ils se désintègrent en un boson W ou Z, et un quark léger. Des arguments théoriques établissent que sous certaines conditions raisonnables la production simple dominerait la production en paires des QV. La topologie particulière des évènements en production simple des QV permettra alors la mise en oeuvre de techniques d’optimisation efficaces pour leur extraction des bruits de fond électrofaibles. Le deuxième type de particules recherché sera celles qui se désintègrent en WZ lorsque ces bosons de jauges W, et Z se désintègrent leptoniquement. Les états finaux détectés par ATLAS seront par conséquent des évènements ayant trois leptons et de l’énergie transverse manquante. La distribution de la masse invariante de ces objets sera alors examinée pour déterminer la présence ou non de nouvelles résonances qui se manifesterait par un excès localisé. Malgré le fait qu’à première vue ces deux nouveaux types de particules n’ont que très peu en commun, ils ont en réalité tous deux un lien étroit avec la brisure de symétrie électrofaible. Dans plusieurs modèles théoriques, l’existence hypothétique des QV est proposé pour annuler les contributions du quark top aux corrections radiatives de la masse du Higgs du MS. Parallèlement, d’autres modèles prédisent quant à eux des résonances en WZ tout en suggérant que le Higgs est une particule composite, chambardant ainsi tout le sector Higgs du MS. Ainsi, les deux analyses présentées dans cette thèse ont un lien fondamental avec la nature même du Higgs, élargissant par le fait même nos connaissances sur l’origine de la masse intrinsèque des particules. En fin de compte, les deux analyses n’ont pas observé d’excès significatif dans leurs régions de signal respectives, ce qui permet d’établir des limites sur la section efficace de production en fonction de la masse des résonances. / Using the most recent data collected by the ATLAS detector in pp collisions delivered by the LHC at 7 and 8 TeV, this thesis shall establish severe constraints on a variety of models going beyond the Standard Model (SM) of particle physics. More particularly, two types of hypothetical particles, existing in various theoretical models shall be studied and probed. The first type will be the search for vector-like quarks (VLQ) produced in pp collisions through electroweak couplings with the u and d quarks. The quest for these particles will be made as they decay into either W(lν)+jet or Z(ll)+jet. There exist theoretical arguments that establish that, under certain reasonable conditions, single production of VLQ dominates over production in pairs. The particular topology of such events enables the implementation of effective techniques to extract signal over electroweak background. The second type is the search for resonant particles decaying to WZ when the gauge bosons W and Z decay leptonically. The final states detected by ATLAS therefore contain three leptons (e, or μ) and missing transverse energy. The distribution of the invariant mass of these objects will then be examined to determine the presence or absence of new resonances that manifest themselves as localized excesses in m(WZ). Despite the fact that, at first glance, these two new types of particles have very little in common, they are in fact both closely linked to electroweak symmetry breaking. In many theoretical models, the hypothetical existence of VLQ is put forward to counteract the top quark’s contribution to radiative loop corrections of the Higgs mass, a calculation which assumes that the Higgs is an elementary particle. Concurrently, other models foretelling the existence WZ resonances alternatively suggest that the Higgs is a composite particle, completely rewriting the whole Higgs sector of the SM. In this perspective, the two analyses presented in this thesis have a fundamental link with the very nature of the Higgs, thereby extending our knowledge of the origin of particle masses. Ultimately, the two analyses did not observe any significant excess in their respective signal regions, paving the way for the computations of limits on the production cross section as a function of the mass of the resonances.

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.