Techn. Hochsch., Diss., 2001--Aachen.

Techn. Hochsch., Diss., 2002--Aachen.

Techn. Hochsch., Diss., 2004--Aachen.

Techn. Hochsch., Diss., 2005--Aachen.

This dissertation presents searches for direct pair production of a scalar partner of the top quark in events with only jets and \acrlong{met} in proton--proton collisions recorded during LHC Run 1 and Run 2 with the ATLAS detector. In the supersymmetry scenario, the partner is called top squark or stop. The stop ($\stop$) is assumed to decay via $\stop \to t \ninoone$, $\stop\to b\chinoonepm \to b W^{\left(\ast\right)} \ninoone$, or $\stop\ra bW\ninoone$, where $\ninoone$ ($\chinoonepm$) denotes the lightest neutralino (chargino). Exclusion limits are reported in terms of the stop and neutralino masses. The LHC Run 1 analysis uses an integrated luminosity of 20.1~{\ifb} at $\acrshort{sqrts}=8~\tev$ to exclude top squark masses in the range $270$--$645~\GeV$ for $\ninoone$ masses below $30~\GeV$, assuming a 100\% $\stop \to t \ninoone$ \gls{br}. For a \gls{br} of $50\%$ to either $\stop \to t \ninoone$ or $\stop\to b\chinoonepm$, and assuming $m_{\chinoonepm} = 2 m_{\ninoone}$, stop masses in the range $250$--$550~\GeV$ are excluded for $\ninoone$ masses below $60~\GeV$. The LHC Run 2 analysis uses an integrated luminosity of 13.3~{\ifb} at $\acrshort{sqrts}=13~\tev$. Assuming a 100\% $\stop \to t \ninoone$ \gls{br}, stop masses in the range $310$--$820~\GeV$ are excluded for $\ninoone$ masses below $160~\GeV$. For $\mstop\sim m_t+\mLSP$ scenario, the search excludes stop masses between $23$--$380~\GeV$. Additionally, scenarios where stops are produced indirectly through gluino decay but have very low $p_T$ signature due to a very small $\Delta (\mstop, \mLSP)$, have been considered. The result is interpreted as an upper limit for the cross section in terms of the gluino and stop masses. This excludes all models considered which include $m_{\gluino}$ up to 1600~\GeV\ with $\mLSP<560~\GeV$ at 95\% CL. Finally, the analysis strategy from the LHC Run 1 search is applied in the broader scope of supersymmetry called \gls{pmssm}. This dissertation presents a summary of the results that related to the stop search.

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Cette thèse présente une étude du secteur scalaire dans le cadre du modèle standard (MS), ainsi que la recherche d'un secteur scalaire étendu dans des théories au-delà du MS. Ces analyses ont en commun la désintégration d'au moins un des bosons scalaires en une paire de leptons tau. Les résultats sont basés sur les données collectées par le détecteur CMS pendant le Run-1 du LHC, lors de collisions proton-proton à une énergie dans le centre de masse de 7 ou 8 TeV. Le boson découvert en 2012, H, semble compatible avec un boson de Brout-Englert-Higgs (BEH) du MS, mais ce constat se base essentiellement sur l'étude des modes de désintégration bosoniques. Le mode de désintégration H->tau tau est le canal fermionique le plus sensible, et permet de tester les couplages de Yukawa du nouveau boson. Cette thèse décrit dans un premier temps la recherche d'un boson scalaire se désintégrant en leptons tau et produit en association avec un boson vecteur. Les données du Run-1 ne sont pas sensibles aux petites sections efficaces de production prédites dans le SM, mais la combinaison avec les autres modes de production montre avec évidence l'existence de désintégrations du boson H en taus. De nombreux modèles au-delà du MS, tels que l'extension supersymétrique minimale du MS (MSSM) ou les modèles avec deux doublets scalaires (2HDM), prédisent l'existence de plusieurs boson scalaires. Quatre recherches d'un secteur scalaire étendu au-delà du MS sont présentées dans cette thèse. La première analyse recherche un pseudoscalaire avec une masse entre 220 et 350 GeV, se désintégrant en un boson scalaire similaire à celui du MS et en un boson Z, dans l'état final avec deux taus et deux leptons légers. La deuxième analyse explore la possibilité d'une désintégration exotique de la nouvelle particule en deux bosons scalaires plus légers dans l'état final avec deux muons et deux leptons tau. Dans le cadre de la troisième analyse, une région en masse quasiment inexplorée auparavant au LHC est testée par la recherche d'un pseudoscalaire avec une masse entre 25 et 80 GeV, se désintégrant en leptons tau et produit en association avec deux quarks b, dans le contexte des 2HDM. La dernière analyse recherche une résonance lourde se désintégrant en une paire de leptons tau dans le contexte du MSSM. Aucun indice de nouvelle physique n'a été trouvé dans les analyses décrites ci-dessus, mais des limites strictes sur les sections efficaces des différents signaux ont été déterminées. / This thesis presents a study of the scalar sector in the standard model (SM), as well as different searches for an extended scalar sector in theories beyond the standard model (BSM). All analyses have in common the fact that at least one scalar boson decays to a pair of tau leptons. The results exploit the data collected by the CMS detector during LHC Run-1, in proton-proton collisions with a center-of-mass energy of 7 or 8 TeV.The particle discovered in 2012, H, looks compatible with a SM Brout-Englert-Higgs boson, but this statement is driven by the bosonic decay modes. The H-> tau tau decay mode is the most sensitive fermionic decay channel, and allows to test the Yukawa couplings of the new particle. The search for the SM scalar boson decaying to tau leptons, and produced in association with a massive vector boson W or Z, is described in this thesis. Even though a good background rejection can be achieved by selecting the leptons originating from the vector boson, Run-1 data are not sensitive to the small production cross sections predicted in the SM for the scalar boson. The combination with the gluon-gluon fusion and vector boson fusion production searches leads to an evidence for the decay of the H boson to tau leptons.Many BSM models, such as the minimal supersymmetric SM (MSSM) or models with two scalar doublets (2HDM), predict the existence of several scalar bosons. The decays of these bosons to tau leptons can be enhanced in some scenarios depending on the model parameters, which makes the di-tau decay mode powerful to discover BSM physics. Four searches for an extended scalar sector are detailed in this thesis. The first analysis searches for a pseudoscalar boson with a mass between 220 and 350 GeV, decaying to an SM-like scalar boson and a Z boson, in the final state with two light leptons and two tau leptons. Second, a search for the exotic decay of the new particle H to a pair of light pseudoscalar bosons, which is still allowed by all measurements made up to now, in the final state with two muons and two tau leptons is performed. Third, a mass region almost never explored at the LHC is probed by the search of a light pseudoscalar, with a mass between 25 and 80 GeV, decaying to tau leptons and produced in association with b quarks. The last analysis describes the search for a heavy resonance in the MSSM, decaying to a pair of tau leptons. None of these analyses has found any hint of new physics beyond the SM, but stringent limits on the cross section of such signals could be set. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

A search for a light charged Higgs boson decaying into cs is presented using data recorded in pp collisions at √s = 7 TeV. The analysis uses a data sample corresponding to an integrated luminosity of 35.3 pb⁻¹ collected by the ATLAS detector at the LHC between June and November 2010. The search is based on the semi-leptonic tt channel searching for the process t → H⁺b where H⁺ → cs⁻. The invariant mass distribution of the dijet system consistent with the hadronic decay of a W⁺ is used to search for a secondary bump from hadronic H⁺ decays. With no observation of the charged Higgs signal, 95% confidence level upper limits on the decay branching ratio of top quarks to charged Higgs bosons are set as a function of the charged Higgs boson mass.

This thesis investigates the prospects of measuring anomalous triple gauge boson couplings in the ATLAS detector at the Large Hadron Collider (LHC). The most general(V ZZ, V = Z, g) vertex is parametrised by four couplings, fV = Z,gi=4,5 , all of which are zero in the Standard Model. Non-zero couplings would manifest themselves as an excess of events in ZZ diboson channels, and, if observed, would be a direct probe of new physics beyond the Standard Model. A set of criteria are outlined to select events recorded by ATLAS in two such channels, ZZ to llll(l = e, mu) and ZZ to llnunu. With 1 fb-1 of integrated luminosity at a centre of mass energy of sqrt(s) = 7 TeV, ATLAS can expect to observe 10+/-1 events in the ZZ to llllchannel, with 0.5+0.9-0.2 background events. In the ZZ to llnunu channel, 6.2+/-0.7 signal events are expected, with a background of 1.9+2.0-0.2 events. The expected sensitivity of ATLAS to non-zero anomalous couplings is calculated by performing extended, unbinned maximum-likelihood fits to the Z boson transverse momentum spectrum. For 1 fb-1 of integrated luminosity at sqrt(s) = 7 TeV, ATLAS has the potential to place constraints on the coupling parameters of |fZi | < 0.06 and |fig | < 0.07 at the 95% confidence level. These limits assume a form factor with a cutoff of Lambda_FF = 1.2 TeV. As a prelude to ZZ observation, criteria are defined to select Z to ll(l = e, mu) events in the first 315 nb-1 of ATLAS pp collision data at sqrt(s) = 7 TeV. In total 57 events are observed in the electron channel, with 109 in the muon channel, leading to cross-section measurements of sigma (Z to ee) = 0.70+/-0.09 (stat)+/-0.10 (syst)+/-0.08 (lumi) pband sigma(Z to mumu) = 0.90+/-0.09 (stat) +/-0.07 (syst) +/-0.10 (lumi) pb, both of which are consistent with the Standard Model predictions. In addition, this thesis presents a summary of developments made to the Data Acquisition (DAQ) system of the ATLAS Semiconductor Tracker (SCT). These include the construction of a test system, involving a scaled-down version of the entire SCT readout chain. The test system was subsequently used to develop a number of new DAQ features, including a hardware-based event simulator and monitoring framework.

For several theoretically and experimentally motivated reasons, super- symmetry (SUSY) has for some time been identified as an interesting candidate for a theory of fundamental particle physics beyond the Stan- dard Model. The ATLAS collaboration, of which I am a member, possess a detector emplaced in the Large Hadron Collider experiment at CERN. If SUSY does in fact describe our universe, then it is hoped that evidence of it will be visible in data collected in the ATLAS detector. I present an analysis looking for a particular signature that could indicate the presence of SUSY; events containing two like-charge leptons (e or μ). This signature benefits from having both low Standard Model backgrounds as well as potential to observe several SUSY scenarios, par- ticularly those involving strong production processes. These include pair production of squarks and gluinos. The latter of these are particularly relevant for the analysis presented herein since gluinos are Majorana fermions; hence they can decay to produce like-charge leptons. The analysis considers several SUSY production topologies determined from a variety of simplified and phenomenological models. One of the core pieces of any ATLAS analysis is estimating the expected backgrounds in the signal regions. These backgrounds arise both from known Standard Model production processes that can produce the same final state as the SUSY models being targeted, as well as detector mismeasurement effects. One important background in the like-charge analysis is that of “fake” leptons; these are jets that have been misclassified as either electrons or muons by the reconstruction algorithms. A large portion of this thesis is dedicated to introducing novel techniques for robustly estimating these backgrounds, and evaluating their relative performance.