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In dieser Dissertation wird eine Suche nach der Produktion von ''down''-artigen b'' Anti-b'' Quark-Antiquark-Paaren einer sequentiellen vierten Generation von Fermionen präsentiert. Für die Suche wurden Endzustände mit zwei gleich geladenen Leptonen (e oder mu), mindestens zwei Jets (davon ein b-Jet), fehlender transversaler Energie und einer großen Summe der Transversalimpulse aller Elektronen, Muonen und Jets betrachtet. Für den Zerfall dieser b'' Anti-b'' Quark-Antiquark-Paare wurden zwei verschiedene Szenarien untersucht. Im ersten Szenario wird angenommen, dass beide b'' Quarks ausschliesslich in ein t Quark und ein W Boson zerfallen. Für das zweite Szenario werden variable Verzweigungsverhältnisse für die Zerfälle der b'' Quarks nach tW, cW und uW erlaubt. Die Suche wurde mit Daten durchgeführt, die von pp Kollisionen mit einer Schwerpunktsenergie von sqrt(s) = 8 TeV vom ATLAS Detektor am Large Hadron Collider aufgezeichnet wurden. Der benutzte Datensatz entspricht einer integrierten Luminosität von 14.3 fb^-1. Es wurde kein signifikanter Überschuss an Ereignissen über den erwarteten Ereignissen vom Untergrund beobachtet. Daher wurde im ersten Szenario eine untere Grenze auf die b''-Masse (m_b'') von 0.724 TeV mit einem Konfidenzniveau von 95% bestimmt. Im Falle des zweiten Szenarios mit variablen Verzweigungsverhältnissen wurden Ausschlussregionen mit einem Konfidenzniveau von 95% als Funktion der Masse des b''-Quarks und seiner Verzweigungsverhältnisse bestimmt. Die erhaltenen Grenzen auf m_b'' sind schwächer verglichen mit dem Ergebnis für ein Verzweigungsverhältniss von 100% für b''->tW. / This thesis presents a search for the production of down-type b'' anti-b'' quark-antiquark pairs of a sequential fourth generation of fermions. In this search, final states with two same-sign charged leptons (e or mu), at least two jets (including at least one b-jet), missing transverse energy and a large total sum of the transverse momenta of all electrons, muons and jets were considered. Two different scenarios were assumed for the decays of these $b'' anti-b'' quark pairs. In the first scenario the two b'' quarks were considered to decay exclusively into a t quark and a W boson. For the second scenario, variable branching fractions for the decays of the b'' quarks into tW, cW and uW were allowed. The search was performed using data from pp collisions at a center-of-mass energy of sqrt(s) = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The used dataset corresponds to an integrated luminosity of 14.3 fb^-1. No significant excess of events over the background expectation was observed. Hence a lower limit on the b'' mass of 0.724 TeV was set with 95% confidence level in the first scenario. In case of the second scenario with variable branching fractions, exclusion areas with 95% confidence level were derived as a function of the b'' mass and its decay branching fractions. These limits are less stringent compared to the result assuming a branching fraction of 100% for b''->tW.

Weiche inelastische QCD Prozesse dominieren am LHC. Über 20 solcher Kollisionen werden innerhalb einer Strahlkreuzung bei ATLAS stattfinden, sobald der LHC die nominelle Luminosität von L = 1034 cm−2 s−1 und die Schwerpunktsenergie von p s = 14 TeV erreicht. Diese inelastischen Wechselwirkungen sind durch einen geringen Impulsübertrag gekennzeichnet, welche theoretisch lediglich durch phänomenologische Modelle angenähernd beschrieben werden können. Zu Beginn des Strahlbetriebs des LHC’s 2009 war die Luminosität relativ niedrig mit L = 1027 bis 1031 cm−2 s−1, was ein sehr gutes Szenario bot, um einzelne Proton-Proton Kollisionen zu selektieren und deren allgemeine Eigenschaften experimentell zu untersuchen. Zunächst wurde ein Minimum-Bias Trigger entwickelt, um Daten mit ATLAS aufzunehmen. Dieser Trigger, mbSpTrk, verarbeitet Signale der Silizium-Spurdetektoren und verwirft effizient Ereignisse ohne eine Proton-Wechselwirkung, wobei zugleich eine mögliche Verschiebung zu bestimmten Ereignistypen hin minimier wird. Um einen flexiblen Einsatz des Triggers zu gewährleisten, wurde er mit einer Sequenz ausgestattet, welche effizient Machinenuntergrund unterdrückt. Im zweiten Teil der Arbeit wurden geladenen Teilchenmultiplizitäten im zentralen Bereich in zwei kinematisch definierten Phasenräumen gemessen. Mindestens ein geladenes Teilchen mit einer Pseudorapidität kleiner als 0.8 und einem Transversalimpuls von pT > 0.5 bzw. 1 GeV musste vorhanden sein. Vier typische Minimum-Bias Verteilungen wurden bei zwei Schwerpunktsenergien von p s = 0.9 und 7 TeV gemessen. Die Ergebnisse sind derart präsentiert, dass sie nur minimal von Monte Carlo Modellen abhängen. Die vorgestellten Messungen stellen zudem den Beitrag der ATLAS Kollaboration dar für die erste, LHC-weit durchgeführte Analyse, der auch die CMS und ALICE Kollaborationen zustimmten. Ein Vergleich konnte mit den Pseudorapiditätsverteilungen angestellt werden. / Soft inelastic QCD processes are the dominant proton-proton interaction type at the LHC. More than 20 of such collisions pile up within a single bunch-crossing at ATLAS, when the LHC is operated at design luminosity of L = 1034 cm−2 s−1 colliding proton bunches with an energy of p s = 14 TeV. Inelastic interactions are characterised by a small transverse momemtum transfer and can only be approximated by phenomenological models that need experimental data as input. The initial phase of LHC beam operation in 2009, with luminosites ranging from L = 1027 to 1031 cm−2 s−1, offered an ideal period to select single proton-proton interactions and study general aspects of their properties. As first part of this thesis, a Minimum Bias trigger was developed and used for data-taking in ATLAS. This trigger, mbSpTrk, processes signals of the silicon tracking detectors of ATLAS and was designed to fulfill efficiently reject empty events, while possible biases in the selection of proton-proton collisions is reduced to a minimum. The trigger is flexible enough to cope also with changing background conditions allowing to retain low-pT events while machine background is highly suppressed. As second part, measurements of inelastic charged particles were performed in two phase-space regions. Centrally produced charged particles were considered with a pseudorapidity smaller than 0.8 and a transverse momentum of pT > 0.5 or 1 GeV. Four characteristic distributions were measured at two centre-of-mass energies of p s = 0.9 and 7 TeV. The results are presented with minimal model dependency to compare them to predictions of different Monte Carlo models for soft particle production. This analysis represents also the ATLAS contribution for the first common LHC analysis to which the ATLAS, CMS and ALICE collaborations agreed. The pseudorapidity distributions for both energies and phase-space regions are compared to the respective results of ALICE and CMS.

No description available.

<p>In recent years, there has been a large interest in the subject of extra dimensions in particle physics. In particular, a number of models have been suggested which provide solutions to some of the problems with the current Standard Model of particle physics, and which could be tested in the next generation of high-energy experiments. Among the most important of these models are the large extra dimensions model by Arkani-Hamed, Dimopoulos, and Dvali, the universal extra dimensions model, and models allowing right-handed neutrinos to propagate in the extra dimensions. In this thesis, we study phenomenological aspects of these three models, or simple modifications of them.</p><p> </p><p>The Arkani-Hamed-Dimopoulos-Dvali model attempts to solve the gauge hierarchy problem through a volume suppression of Newton's gravitational constant, lowering the fundamental Planck scale down to the electroweak scale. However, this solution is unsatisfactory in the sense that it introduces a new scale through the radius of the extra dimensions, which is unnaturally large compared to the electroweak scale. It has been suggested that a similar model, with a hyperbolic internal space, could provide a more satisfactory solution to the problem, and we consider the hadron collider phenomenology of such a model.</p><p> </p><p>One of the main features of the universal extra dimensions model is the existence of a potential dark matter candidate, the lightest Kaluza-Klein particle. In the so-called minimal universal extra dimensions model, the identity of this particle is well defined, but in more general models, it could change. We consider the indirect neutrino detection signals for a number of different such dark matter candidates, in a five- as well as a six-dimensional model.</p><p> </p><p>Finally, right-handed neutrinos propagating in extra dimensions could provide an alternative scenario to the seesaw mechanism for generating small masses for the left-handed neutrinos. Since extra-dimensional models are non-renormalizable, the Kaluza-Klein tower is expected to be cut off at some high-energy scale. We study a model where a Majorana neutrino at this cutoff scale is responsible for the generation of the light neutrino masses, while the lower modes of the tower could possibly be observed in the Large Hadron Collider. We investigate the bounds on the model from non-unitarity effects, as well as collider signatures of the model.</p>

A ce jour la détermination de l’angle γ du Triangle d’Unitarité de la matrice de Cabibbo-Kabayashi-Maskawa est statistiquement limitée par la rareté des transitions b→ u. La précision obtenue en combinant les résultats des expériences BABAR et BELLE est proche de 10°. L’expérience LHCb auprès du LHC présente un fort potentiel d’amélioration pour ce paramètre de violation de CP, notamment via la désintégration Bd0 -> D0K*0, un des canaux clés de cette mesure. Les désintégrations D0 -> Kπ, D0 -> Kππ0 et D0 -> Kπππ sont étudiées dans cette thèse. L’utilisation de désintégrations faisant intervenir des π0 nécessite un bon étalonnage en énergie des calorimètres électromagnétiques. Une méthode d’étalonnage intercellules du ECAL basée sur le flux d’énergie, permet d’obtenir une inter-calibration de l’ordre de 1,5 %, l’échelle d’énergie absolue étant obtenue par des méthodes indépendantes non étudiées ici. La détermination du rapport d’embranchement de la désintégration Bs0 -> D0K*0, première étape du programme aboutissant à la mesure de la violation de CP dans le canal Bd0 -> D0K*0, est réalisée relativement au rapport d’embranchement de Bd0 -> D0ρ0. Le résultat final obtenu avec 36 pb−1 de données collectées par LHCb en 2010 reste dominé par l’erreur statistique :BR(Bs0 -> D0K*0)/BR(Bd0 -> D0ρ0) = 1, 48 ± 0, 34 (stat) ± 0, 15 (syst) ± 0, 12 (fd/fs). / The present status on the determination of the angle γ of the Cabibbo-Kabayashi-Maskawa Unitary Triangle is statistically limited by the rarity of b→ u transitions. The obtained precision combining results from the BABAR and BELLE experiments is close to 10°. The LHCb experiment at the LHC has a strong potential to reduce the uncertainty on this CP violation parameter, especially through the Bd0 -> D0K*0 decay, one of the key channels for this measurement. The D0 -> Kπ, D0 -> Kππ0 and D0 -> Kπππ decays are studied in this thesis. Decays involving π0 require an accurate energy calibration of electromagnetic calorimeters. An inter-calibration technique based on the energy flow allows to obtain the 1.5 % level, the absolute scale being obtained from independent methods studied elsewhere. The determination of the Bs0 -> D0K*0 decay branching ratio, first step towards a measurement of CP violation in the Bd0 -> D0K*0 channel, is performed relatively to the Bd0 -> D0ρ0 decay. The final result, obtained with 36 pb−1 of data collected by LHCb in 2010 is dominated by the statistical uncertainty:BR(Bs0 -> D0K*0)/BR(Bd0 -> D0ρ0) = 1, 48 ± 0, 34 (stat) ± 0, 15 (syst) ± 0, 12 (fd/fs).

Grâce à la grande section efficace de production de paires bb, LHC offre une excellente occasion de faire des études de courants neutres changeant la saveur. Ces transitions sont sensibles aux effets de nouvelle physique. Cette thèse porte sur l'analyse des événements B->K*ee qui permettent de mesurer la fraction de photon avec une polarisation droite et donc de rechercher des signaux de nouvelle physique émis dans la transition b ->s. En effet, dans le Modèle Standard, la polarisation des photons est gauche. La paire e+e, lors que la masse invariante de la paire de leptons est basse, provient d'un photon virtuel et permet donc de sonder la polarisation de celui-ci. Cette mesure se fait grâce à l'étude des distribution angulaires de cette désintégration à quatre corps. Une première étape est la mesure du rapport d'embranchement dans le domaine de masse 30-1000MeV=c2. En effet, cette désintégration n'a jamais été observée dans cette région, y compris auprès des usines a B a cause du très faible rapport d'embranchement. Cette analyse comportant des électrons de basse impulsion transverse est expérimentalement complexe dans un environnement tel que celui du LHC. La mesure est faite relativement au rapport d'embranchement de la désintégration B->J/Psi(ee)K*. En effet, cela permet de s'affranchir de nombreux effets expérimentaux ainsi que de la détermination absolue des efficacités. Le résultat, repose sur les données collectées par LHCb en 2011 et correspondant a une luminosité intégrée de 1 fb-1: B(B->K*ee)30-1000MeV = (3:19+0:75-0:68(stat) +/- 0:21(syst) =/-0.15(PDG)) x10-7 en utilisant la valeur PDG pour le rapport d'embranchement de la désintégration B->J/Psi(ee)K*. La dernière partie de la thèse porte sur des études Monte Carlo qui montrent que la précision sur la fraction de photon avec une polarisation droite que l'on peut espérer obtenir avec l'inclusion des données de 2012 est d'environ 0.1, comparable à la moyenne mondiale obtenue avec des méthodes différentes. / The high bb cross section produced by the LHC offers an excellent opportunity for thestudy of flavour changing neutral current B decays, where the effects of new physics can be probed. This thesis presents an analysis of the rare decay B->K*ee which can be used to measure the polarisation of the photon in the b -> s transition. When the dilepton mass is low, the ee pair comes predominantly from a virtual photon, and the polarisation can be accessed via an angular analysis. It is predicted to be predominantly left handed in the Standard Model, and therefore an enhanced right handed amplitude would be a sign of new physics. A first step is to measure the branching fraction in the dilepton mass range, 30 MeV to 1 GeV. This decay has not yet been observed in this region, due to its small branching ratio. The analysis involves electrons with low transverse momentum, and is thus experimentally complex in the hadronic environment at the LHC. The branching ratio is measured relative to that of B->J/Psi(ee)K*, which eliminates both certain experimental effects, and the need to determine absolute effciencies. The result is obtained with an integrated luminosity of 1 fb-1 of pp collisions, collected by LHCb during 2011 and is found to be:B(B->K*ee)30-1000MeV = (3:19+0:75-0:68(stat) +/- 0:21(syst) =/-0.15(PDG) x10-7 when using the PDG value for the B->J/Psi(ee)K* branching ratio. The last part of the thesis presents Monte Carlo studies, showing that with the inclusion of the 2012 data sample, the expected sensitivity on the fraction of right handed polarisation is approximately 0.1, which is comparable with the world average obtained with different methods.

This thesis is an investigation of the subject of extra dimensions in particle physics. In recent years, there has been a large interest in this subject. In particular, a number of models have been suggested that provide solutions to some of the problem with the current Standard Model of particle physics. These models typically give rise to experimental signatures around the TeV energy scale, which means that they could be tested in the next generation of high-energy experiments, such as the LHC. Among the most important of these models are the universal extra dimensions model, the large extra dimensions model by Arkani-Hamed, Dimopolous, and Dvali, and models where right-handed neutrinos propagate in the extra dimensions. In the thesis, we study phenomenological aspects of these models, or simple modifications of them. In particular, we focus on Kaluza–Klein dark matter in universal extra dimensions models, different aspects of neutrino physics in higher dimensions, and collider phenomenology of extra dimensions. In addition, we consider consequences of the enhanced renormalization group running of physical parameters in higher-dimensional models. / QC 20120427

ATLAS is a multi-purpose detector which has recently started to take data at the LHC at CERN. This thesis describes the tests and calibrations of the central calorimeters of ATLAS and outlines a search for heavy top quark pair resonances.The calorimeter tests were performed before the ATLAS detector was assembled at the LHC, in such a way that particle beams of known energy were targeted at the calorimeter modules. In one of the studies presented here, modules of the hadronic barrel calorimeter, TileCal, were exposed to beams of pions of energies between 3 and 9 GeV. It is shown that muons from pion decays in the beam can be separated from the pions, and that the simulation of the detector correctly describes the muon behaviour. In the second calorimeter study, a scheme for local hadronic calibration is developed and applied to single pion test beam data in a wide range of energies, measured by the combination of the electromagnetic barrel calorimeter and the TileCal hadronic calorimeter. The calibration method is shown to provide a calorimeter linearity within 3%, and also to give a reasonable agreement between simulations and data. The physics analysis of this thesis is the proposed search for heavy top quark resonances, and it is shown that a narrow uncoloured top pair resonance, a Z', could be excluded (or discovered) at 95% CL for cross sections of 4.0±1.6 pb (in the case of M=1.0 TeV/c2) or 2.0±0.3 pb (M=2.0 TeV/c2), including systematical uncertainties in the model, assuming √s = 10 TeV and an integrated luminosity of 200 pb-1. It is also shown that an important systematical uncertainty is the jet energy scale, which further underlines the importance of hadronic calibration.

This thesis is concerned with experimental investigations of properties of the top quark and processes involving this particle. In the first part of the thesis, the first determination of the electric charge of the top quark is presented. The measurement was made using top quark pair events produced in proton-antiproton collisions recorded by the D0 detector at the Fermilab Tevatron. It is based on the reconstruction of the charge of the top quarks decay products from the dominant decay to a W boson and a b-quark. The method uses a jet charge algorithm, calibrated with data, to discriminate between b- and antib-quark jets. A constrained kinematic fit is also performed to resolve the ambiguities of the pairing of the top quark decay products and to extract the top quark electric charge. The result is in good agreement with the Standard Model top quark electric charge of 2e/3 and an upper limit of 0.8 at 90\% confidence level on the fraction of exotic quarks with charge 4e/3 in the data sample is obtained. The second part of the thesis concerns the estimation of the top quark pair background to searches for new physics, such as supersymmetry, with the ATLAS  experiment at the CERN Large Hadron Collider. These searches will require a robust estimation of standard model backgrounds in order to make any claims of discovery or to exclude models of new physics. For searches with a final state signature characterized by two isolated charged leptons, multiple jets and large missing transverse energy the largest source of background is expected to be top quark pairs with leptonic decay of the two W bosons from the top quarks in the event. A data-driven method to estimate this contribution based on full kinematic reconstruction of the top quark pair events is studied using simulated proton-proton collisions. It is shown that the method is capable of estimating the top quark pair background to within 12% using data corresponding to approximately 1fb-1. The systematic uncertainty is of the order of 20% and, depending on the model, the contamination of signal events can potentially be large. / QC 20100730