Dissertations / Theses on the topic 'Large Hadron Collider (France and Switzerland)'

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.
Includes bibliographical references (p. 49-50).
In this thesis I explore the identification of [tau] leptons from simulated reconstructed data that will be collected by the Compact Muon Solenoid (CMS) detector on the Large Hadron Collider (LHC) at CERN. The two components of particle identification, efficiencies of [tau] identification from generator level information, along with fake rates of the current default algorithm have been determined and analyzed for a photon plus jets background sample and QCD background sample. I propose a new [tau] lepton identification algorithm that employs a signal cone parametrized with respect to the 7 transverse energy, and an isolation cone parametrized with respect to charged particle density surrounding the [tau] jet. Using the default algorithm an efficiency of 27.7% is achieved along with a photon plus jets fake rate of 1.96%. Using the proposed algorithm and matching the efficiency of the default algorithm, an efficiency of 26.9% and a fake rate of 0.44% is achieved. Approximately matching fake rates, an efficiency of 37.4% is achieved with a fake rate of 2.36%.
by by Philip James Ilten.
S.B.

The ATLAS Experiment is one of the four large detectors located at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. In summer 2008, ATLAS is expected to start collecting data from proton-proton collisions at 14 TeV centre-of-mass energy. In the centre of the detector, the reconstruction of charged particle tracks is performed by silicon and drift tube based sub-detectors. In order to achieve the ATLAS physics goals the resolutions of the measured track parameters should not be degraded by more than 20% due to misalignment. Thus, the relative positions of the silicon detector elements have to be known to an accuracy of about 10 micrometers in the coordinate with the best measurement precision. This requirement can be achieved by track based alignment algorithms combined with measurements from hardware based alignment techniques. A robust track based alignment method based on track residual and overlap residual optimisation has been developed and implemented into the ATLAS offline software framework. The alignment algorithm has been used to align a test beam setup and also part of the final ATLAS detector using cosmic ray muons. Several simulation studies showed that the algorithm will be able to align the full detector with collision data. In addition to detector misalignments, limitations in the knowledge of the proton structure are going to affect physics discoveries at the LHC. Therefore, parton distribution uncertainties in high-mass Drell-Yan processes have been determined. This study includes the analysis of the forward-backward asymmetry. It has been performed on the level of next-to-leading order in both, Monte Carlo simulation using k-factors and parton distribution functions. This analysis is crucial for new physics searches with the ATLAS detector.

Orientador: José Augusto Chinellato
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-29T05:01:52Z (GMT). No. of bitstreams: 1 MedinaJaime_Miguel_D.pdf: 6686309 bytes, checksum: 48654e10cf1138cdc89eb810557a427d (MD5) Previous issue date: 2015
Resumo: A produção exclusiva de bósons Z é um processo difrativo, teoricamente representado pela troca de um objeto com os números quânticos do vácuo (pomeron- IP), o qual pode ser tratado no modelo padrão no enfoque do mecanismo de dipolo, via fotoprodução. A caraterística principal deste tipo de processo é que os dois prótons emergem intactos após a colisão com momentum transverso pequeno. Assim, os prótons são espalhados muito frontais e escapam à detecção., restandonos como criterio de e. Espera-se , onde um próton irradia um fóton virtual, o qual flutua para um par quark anti-quark os quais são espalhados elasticamente pela troca de dois glúons emergentes do outro próton, e se materializa num bóson Z. Além disso, os dois prótons emergem intactos após a colisão com momentum transverso pequeno, assim prótons são espalhados muito frontais e escapam à detecção. Portanto, o critério usado para a identificação de eventos exclusivos de bósons Z decaindo em par múons é o número de traços adicionais no vértice, mas ainda resta a possibilidade de haver uma dissociação de pelo menos um dos prótons. Neste caso, a medida da seção de choque respectiva será contaminada. São expostos três diferentes métodos para a pré-seleção des eventos contendo eventos exclusivos. A estratégia de análise proposta para selecionar eventos exclusivos é baseada no decaimento de bósons Z em pares ???? caraterizados por grande momento transverso, para o qual exploramos múons em uma região cinemática de momento transverso Pt(?)>20 GeV, massa invariante M(????)>40 GeV/c² e pseudo-rapidez |?|<2,5. Adicionalmente a estes cortes, exigimos "zero traços" extras no vértice e, por último que o par de múons sejam back-to-back com balanço no Pt. Nesta Tese se apresenta um estudo da produção exclusiva de bósons Z em colisões próton-próton para energias de centro de massa igual a ?s=7 TeV no experimento CMS/LHC. Esta análise foi feita usando simulações para processos de Drell-Yan, fotoprodução de múons e dados coletados pelo detector CMS de colisões próton-próton com ?s=7 TeV durante o ano de 2011, correspondendo a uma luminosidade integrada de 5,09 fb?¹. Nenhum excesso significante de sinal foi extraído nas produções exclusivas de fazemos uma estimativa de eventos esperados nas produções exclusiva e bósons. Nenhum candidato a bóson Z exclusivo foi possível de observar, isto devido ao fato da baixa estatística e o enorme background proveniente de processos de Drell-Yan., que mesmo sendo processos não exclusivos contaminarem nossa amostra de dados devido ao fato de que os prótons escapam de nossa detecção e o único critério de exclusividade é o de o número de traços extras no vertice do evento seja igual a zero, mas devido a que os prótons podem dissociar sim ser detetados e nós foi impossível dete devido a que assim prótons são espalhados muito frontais e escapam à detecção. Entretanto, a seção de choque para processos de produção exclusiva de bósons Z é encontrada a ser ?excl(Z?????)<13,3 fb para um nível de confiança de 95%. Este é o primeiro resultado preliminar para este tipo de processo na escala de energia de 7 TeV, onde cabe destacar que este limite foi melhorado com respeito a resultados publicados pela colaboração CDF para ?s = 1,9 TeV
Abstract: he exclusive Z boson production is a diffractive process theoretically represented by the exchange of an object with the quantum numbers of vacuum (pomeron-IP), which is predicted by the Standard Model in the dipole mechanism approach, via photoproduction. The main feature of this type of process is two forward outgoing protons intact after the collision with small transverse momentum, the protons are scattering very front and escape detection. Then, the approach taken to detect exclusive Z bosons events decaying in to muon pair is the number of additional tracks on the vertex, but there is still the possibility of dissociation of at least one proton, then the measurement of cross section will be contaminated. It's exposed three different methods for the pre-selection events containing exclusive Z bosons. The strategy proposed to exclusive selection events, is based on the decay of Z bosons on pairs ???? featured by large transverse momentum, for which we explored muons in a kinematic region with transverse momentum Pt(?)>20 GeV, Invariant mass M(????)>40 GeV/c² and pseudorapidity |?|<2,5. The additional cuts, require zero extra tracks on vertex and finally the pair of muons are back-to-back with momentum transverse conservation . This thesis presents a search for exclusive Z boson production in proton proton collisions at ?s=7 TeV center of mass energy in the experiment CMS/LHC. This analysis was made using simulations to Drell-Yan processes, photoproduction of muon and data collected by the CMS detector on proton proton collisions with ?s=7 TeV in 2011, for 5.09 fb?¹ integrated luminosity record. No exclusive Z????? candidates are observed, because low statistical and huge background contribution from Drell-Yan processes. But even so, the cross section for the exclusive Z bosons production is found to be ?excl(Z?????)<13.3 fb at 95% C.L. This is the first very preliminary results for 7 TeV, where we highlight that it limit has been improved respect to the results published by the CDF collaboration to ?s = 1.9 TeV
Doutorado
Física
Doutor em Ciências
147301/2011-04
CNPQ

The analysis presented in this thesis focuses on two methods developed to estimate, from data, the multi-jet and the V+jets background processes for top quark pair production occuring during proton-proton at LHC. Top quark paires are reconstructed using the CMS detector, exploiting the semi-leptonic decay channel. Both methods have been developed and studied using Monte-Carlo simulated data.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

This PhD thesis presents the measurement of the differential cross section for the production of a Z boson in association with jets in proton-proton collisions taking place at the Large Hadron Collider (LHC) at CERN, at a centre-of-mass energy of 8 TeV. A development of a data acquisition (DAQ) system for the Triple-Gas Electron Multiplier (GEM) detector in view of the Compact Muon Solenoid (CMS) detector upgrade is also presented.

The events used for the data analysis were collected by the CMS detector during the year 2012 and constitute a sample of 19:6 fb-&
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Le sujet de cette thèse porte sur la recherche de nouvelles résonances massives se désintégrant en une paire d’électrons ou une paire électron-muon avec le détecteur CMS, installé auprès du Grand Collisionneur du Hadrons (LHC) au CERN. Les données analysées correspondent à l’ensemble des collisions proton-proton enregistrées par le détecteur en 2012 à une énergie dans le centre de masse de 8 TeV. Après une brève introduction au modèle standard des particules élémentaires et à quelques unes des théories allant au-delà, le LHC et le détecteur CMS sont présentés. La reconstruction des différentes particules créées lors des collisions, en particulier des électrons et muons de haute énergie, est ensuite discutée. Deux analyses séparées sont menées.

La première consiste en la recherche d’une nouvelle résonance étroite, plus massive que le boson Z, dans le spectre de masse invariante des paires d’électrons, dont la principale contribution, dans le modèle standard, provient du processus de Drell–Yan. De telles résonances sont notamment prédites par des modèles dits de grande unification ou à dimensions spatiales supplémentaires. Le bruit de fond provenant des processus du modèle standard étant réduit dans la région étudiée, quelques événements localisés peuvent suffire pour mener à une découverte, et la sélection des électrons est optimisée afin de ne perdre aussi peu d’événements que possible. Les différentes contributions des bruits de fond sont partiellement estimées à partir de simulations. Une méthode basée sur le spectre de masse invariante des paires électron-muon mesuré dans les données est développée pour valider la contribution du second bruit de fond en terme d’importance. Aucun excès n’est observé par rapport aux prédictions du modèle standard et des limites supérieures à 95% de niveau de confiance sont placées sur le rapport entre la section efficace de production multipliée par le rapport de branchement d’une nouvelle résonance et celle au pic du boson Z. Ces limites sont ensuite converties en limites inférieures sur la masse de différentes particules hypothétiques de spin 1 ou de spin 2.

La seconde analyse consiste en une recherche de résonances massives et étroites dans le spectre de masse invariante des paires électron-muon. De telles résonances briseraient la conservation du nombre leptonique tel que prédit par le modèle standard. Cette possibilité existe cependant dans certains modèles de nouvelle physique. C’est notamment le cas pour un modèle à dimensions supplémentaires où apparaissent des nouveaux bosons neutres lourds. La sélection des événements demande un électron de haute énergie comme dans l’analyse précédente, et un muon de grande impulsion transverse. La stratégie de recherche est similaire au cas des paires d’électrons :le fait de rechercher un signal étroit rend l’analyse statistique très peu sensible aux erreurs systématiques affectant la normalisation absolue du spectre de masse électron-muon. Comme aucune déviation significative n’est observée par rapport aux prévisions du modèle standard, des limites supérieures sur la section efficace multipliée par le rapport de branchement sont établies pour le modèle à dimensions spatiales supplémentaires. Étant données les faibles valeurs théoriques de la section efficace de production des résonances violant la conservation de la saveur dans ce modèle, la quantité de données analysées ne permet pas d’en déduire une limite inférieure sur leur masse. Cette analyse représente néanmoins la première recherche directe avec l’expérience CMS, de bosons massifs, se désintégrant avec violation du nombre leptonique, en une paire électron-muon.

Doctorat en Sciences
info:eu-repo/semantics/nonPublished

For more than half a century, colliders have been in the forefront of studying the Standard Model (SM) predictions. The pinnacle of both SM and colliders occurred when on July the 4th 2012, at CERN Large Hadron Collider, the Higgs discovery was announced, almost 50 years after being postulated. Until 2012 the Higgs was the only missing piece of the SM, and its discov- ery was a milestone in the LHC. The complete analysis of Run 1 data and the preliminary ones from Run 2 data, indicate that this new particle is a scalar bo- son, with CP–even properties, as in the SM Higgs scenario. Furthermore, it seems that the observed state is directly connected to the electroweak symmetry breaking (EWSB), after analyzing interactions with gauge bosons. Until the moment there is no evidence of any physics BSM at the LHC in the form of new states. Clearly this disfavours any extension of the SM that predicts new particles at the TeV scale. This was the scenario when this Thesis was initiated. The Run 1 of LHC had reached its final luminosity, and the Run 2 was starting its operation. The lack of NP states at LHC served us as motivation to look for an alternative approach, instead of constraining ourselves to an specific SM completion. Here enters the model-independent philosophy, where through the use of an effective Lagrangian we start to confront all the existing available data and search for any possible de- viation of the SM predictions, by making use of the framework of Effective Field Theories (EFT). In the context of an EFT, we follow an atheist path: NP is expected to manifest directly at a scale Λ, which is higher than the scale at which the experiments are performed. Any effect of new physics (NP) at the low scale can be parametrized by a set of higher dimensional operators, that are suppressed by powers of the high energy scale. With this aim, in Chapter 2 we introduce the SM as an EFT at low energy. Our ignorance about the ultraviolet (UV) theory, and with no guidance of where the scale of NP could be laying, lead us to a bottom-up approach, where the higher dimensional operators used are driven by the existing data on the EW sector. This Chapter is supposed to set the roots of the analysis done in Chapters 3, 4 and 5. The results presented in these Chapters represent a step in the determination of the precision at which the different interactions in the EW sectors of the SM are being tested by the available data. The ultimate purpose of these analyses is to look for deviations that would be translated in the future on information regarding the UV completion of the SM. Up to this point our focus was on the scrutiny of the EWSB at LHC, and so the origin of the particles’ mass from a model-independent point of view. But still there are other alternatives to study physics beyond the SM, like direct searches at the LHC for some particular signature in a model. Clearly this approach only applies to neutrino models which are testable at LHC. In Chapter 6 we will study one such model of Type-III see-saw where the heavy states can live in the TeV region and, not less important, the couplings of these states are determined by the light neutrino masses and mixings.
Esta tesis se centra en la búsqueda de física mas allá del modelo estándar en el sector electrodébil, y en particular en el mecanismo de la rotura expontánea de la simetría electrodébil y de la generación de masas, usando datos recogidos por los experimentos del Large Hadron Collider. El zenit, tanto del modelo estándar como de la física experimental de colisionadores se alcanzó en 2012 cuando CERN anunció el descubrimiento de una nueva partícula que podría ser el boson de Higgs. Hasta 2012 el boson de Higgs era la única pieza que faltaba en la construcción del modelo estándard. Sin embargo, sabemos de forma feaciente que el modelo estándar no puede ser la teoría de la Naturaleza basandonos en hechos puramente observacionales tales como la existencia de materia oscura, de neutrinos masivos, y de la simetría materia-antimateria en el Universo. En particular, la falta de nuevos estados observados en el LHC nos sirvió como motivación para el uso de Lagrangianos efectivos como herramienta para testear en una forma independiente de modelo una amplia variedad de datos con el fin de buscar posibles desviaciones sobre las predicciones del modelo estándar. En este contexto, los efectos de nueva física, que se manifestarían de forma directa a una escala Lambda superior a aquella alcanzable en los experimentos estudiados, pueden parametrizarse en una serie de operadores de dimensión mayor que cuatro y que aparecen en el Lagrangiano suprimidos por potencias de Lambda. Para ello el capítulo 2 tras una breve introdución sobre como se construye una teoría efectiva. En los siguientes tres capítulos 3–5 presentamos un análysis en que el objetivo es buscar desviaciones de las predicciones del modelo estándar que pudieran darnos información sobre el modelo completo en el sector responsable para la generación de masas. Finalmente en el capítulo 6 presentamos los resultados de un estudio realizado en el contexto de un modelo de masa de neutrinos. Dado que el LHC es nuestra principal herramienta para testear las extensiones del modelo estándar, una cuestión obvia es si LHC puede darnos información acerca de las extensiones que permiten generar masas para los neutrinos.

Détection et identification de la réaction quark + anti-quark -> e+ + e- à l'aide du détecteur CMS (Compact Muon Solenoid) auprès du Grand Collisionneur de Hadrons du CERN, le LHC. Cette réaction permet de tester avec précision le Modèle Standard et de rechercher d'éventuelles nouvelles particules (Z') prédites par les théories de grande unification (GUT) et par les modèles à dimensions spatiales supplémentaires.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Cette these a pour sujet la recherche de nouvelle physique et l'etude de la production Drell-Yan dans le canal di-electron a l'aide du detecteur CMS au LHC. Certaines theories au dela du Modele Standard (extra dimensions, theories de grande unification) predisent l'existence de particules massives pouvant se desintegrer en une paire d'electrons. La selection des evenements recherches est presentee et etudiee. La strategie d'analyse est introduite et testee. Enfin, l'analyse des premieres donnees a 7 TeV est decrite et les resultats sont commentes.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Orientador: Jun Takahashi
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-19T21:22:39Z (GMT). No. of bitstreams: 1 Chinellato_DavidDobrigkeit_D.pdf: 12088648 bytes, checksum: c79560f4593e813870fc0e788aadf075 (MD5) Previous issue date: 2012
Resumo: Neste trabalho, estudamos a produção de hádrons estranhos em colisões próton-próton (pp) a energias de (s)^(1/2)= 7 TeV medidas pelo experimento ALICE no acelerador LHC. São resentadas medidas inéditas das taxas de produção em rapidez central do méson (K) também dos bárions ?,? e O- e suas antipartículas correspondentes. As taxas totais de produção excedem significativamente as previsões de modelos que utilizam a Cromodinâmica Quântica Perturbativa (pQCD). Em particular, comparamos as medidas com previsões do simulador de eventos PYTHIA, onde é observado que as previsões concordam com as medidas apenas para momentos transversais (Pt) acima de 6- 7 GcV / c. Este resultado indica que os processos de primeira ordem não-nula implementados no PYTHIA estão bem ajustados, mas a implementação de um cálculo que descreva os dados em baixo Pt ainda é um problema em aberto. Os resultados aqui apresentados devem contribuir para melhorar o entendimento dos mecanismos de produção de partículas na região de baixo Pt¿ Colisões pp são também utilizadas como referência para colisões nucleares de diferentes multiplicidades no estudo da formação do Quark-Gluon Plasma (QGP). Em particular, o aumento da produção de estranheza é considerado um importante observável do QGP. Neste contexto, estudamos a dependência da produção de partículas estranhas com a multiplicidade de partículas carregadas e comprovamos que não há indício de aumento da produção relativa de estranheza cm eventos pp. Isto é um importante resultado, pois complementa os estudos que consideram a possibilidade de formação de QGP em colisões pp
Abstract: In this work, we study the production of strange hadrons in proton-proton (pp) collisions at energies of (s)^(1/2)= 7 TeV measured by the ALICE experiment at the LHC. We present the first measurements of particle yields at central rapidities for the (K)meson as well as for the ?, ? e O- baryon and their antiparticles. Total particle production rates exceed predictions by models that use Perturbative Quantum Chromodynamics (pQCD). In particular, we compare our measurements to predictions by the PYTHIA event generator and find that predictions agree with data only at transverse momenta (Pt) higher than 6- 7 GeV / c. This result indicates that the leading order processes implemented in PYTHIA are well adjusted, but the implementation of a calculation that describes the data at low Pt is still an open issue. The results presented here should contribute to improve our understanding of particle production mechanisms at low Pt ¿ Proton-proton collisions are also used as a reference for nuclear collisions of different multiplicities in the study of Quark-Gluon Plasma (QGP) formation. The increase of strangeness production is considered an important observable of the QGP. In this context, we study the dependence of strange particle production with charged particle multiplicity and find that there is no indication of an increase strangeness production rate in pp collisions. This is an important result that contributes to the studies that consider the possibility of QGP formation in pp collisions
Doutorado
Física
Doutor em Ciências

In this thesis we present several studies on the origin of masses at the LHC. First we study the indirect effects of new physics on the couplings of the recently discovered Higgs boson and on the electroweak symmetry breaking (EWSB) sector interactions. In a model independent framework these effects can be parametrized in terms of an effective Lagrangian at the electroweak scale. In the first Chapter we present the effective Lagrangian description based on the linear realization of the electroweak symmetry, where the Higgs particle is assumed to be part of an SU(2)L doublet. We discuss a choice of dimension–six operators guided by the existing data, and we study the phenomenology of the operators. We perform a global analysis to the existing Higgs, triple gauge boson vertex and electroweak precision data, coming from LHC, Tevatron, LEP and low energy observables. Finally we exploit the interesting complementarity between the studied Higgs and triple gauge boson vertex measurements in order to test the linear realization. In the second Chapter we present two alternative Lagrangian descriptions. First, we study the non–linear or chiral effective Lagrangian, where now the Higgs is not part of an SU(2)L doublet. We describe the chiral operators and, while focusing on the phenomenological di.erences with respect to the linear realization, we also perform the first global analysis of the non–linear basis. Second, we present the Lagrangian parametrization commonly used to measure and describe the triple gauge boson vertex WWZ. We perform a collider analysis where we optimize the LHC capability to measure this vertex, obtaining an impressive LHC potential to improve the current sensitivity on anomalous interactions. We focus on their relation with the disentanglement of the Higgs nature. In the second part of the thesis we study the origin of masses at the LHC by the direct exploration of new resonances related to several beyond the Standard Model descriptions. In the third Chapter we study new vector resonances that couple to electroweak gauge boson pairs, which are common resonances on several EWSB extensions of the Standard Model. We analyze the LHC potential to determine the spin of these resonances, and furthermore we use the present available LHC public analyses to constrain the existence of the new neutral vector resonances, Z’, obtaining the strongest exclusion bounds on their existence. In the last Chapter we analyze the LHC potential to access the mechanism related to the origin of the neutrino masses. We study the different characteristics of a model that, while generating the observed pattern of neutrino masses and mixing, can lead to observable TeV signatures. We describe the phenomenology of the model and the new heavy leptons that are introduced, to finally optimize and analyze the LHC potential to observe these new partners, finding again very promising results.
En esta tesis presentamos varios estudios sobre el origen de masas en el LHC. Primero estudiamos los efectos indirectos de nueva física en las interacciones del recientemente descubierto bosón de Higgs y del resto del sector de rotura de la simetría electrodébil. Independientemente del modelo estos efectos se pueden caracterizar por medio de Lagrangianos efectivos en la escala electrodébil. En el primer Capítulo presentamos el Lagrangiano efectivo en la realización lineal de la simetría, donde la partícula de Higgs se introduce como parte de un doblete de SU(2)L. Describimos una elección de la base de operadores de dimensión–seis guiada por los datos existentes y estudiamos la fenomenología de los operadores. Realizamos un análisis global con todos los datos existentes de producción del Higgs, de medidas del vértice triple de bosones de gauge y de medidas de alta precisión electrodébiles, que provienen de LHC, Tevatron, LEP y otras observaciones a bajas energías. Finalmente estudiamos cómo la complementariedad entre las medidas de las interacciones del Higgs y del vértice con tres bosones de gauge sirve para testear la realización lineal. En el segundo Capítulo presentamos dos Lagrangianos alternativos. Primero el Lagrangiano efectivo no–lineal o quiral, donde ahora el bosón de Higgs no es parte de un doblete de SU(2)L. Describimos los operadores quirales, centrándonos en las diferencias fenomenológicas respecto a la expansión lineal y, además, realizamos el primer análisis global en la base quiral. En segundo lugar describimos el Lagrangiano utilizado históricamente para estudiar el vértice WWZ. Realizamos un análisis optimizando el potencial del LHC para medir anomalías en este vértice, obteniendo previsiones que superan la precisión actual. En la segunda parte de la tesis estudiamos el origen de masas en el LHC buscando directamente nuevas resonancias relacionadas con extensiones del modelo estándar. En el tercer Capítulo analizamos resonancias vectoriales que interaccionan con pares de bosones de gauge electrodébiles, estados comunes en varias extensiones teóricas que explican la rotura de la simetría electrodébil. Estudiamos primero el potencial del LHC para determinar el espín de estas nuevas partículas y después utilizamos los datos públicos disponibles del LHC para constreñir la existencia de nuevas resonancias vectoriales neutras, Z’, obteniendo los límites más fuertes sobre su existencia. En el último capítulo analizamos el potencial que tiene el LHC para acceder al mecanismo relacionado con el origen de las masas de los neutrinos. Estudiamos las características de un modelo que consigue explicar el patrón de masas y mezclas observado para los neutrinos, dando lugar a la vez a nuevas señales en la escala del TeV. Describimos la fenomenología del modelo y de los nuevos leptones pesados que se introducen, para .nalmente analizar la capacidad que tiene el LHC para observar estos estados, dando lugar, otra vez, a resultados muy prometedores.

Cette thèse a été réalisée dans le cadre du LHC (Large Hadron Collider). Le LHC produit des collisions proton-proton avec une énergie dans le centre de masse de 7 TeV au CERN depuis mars 2010. C'est le collisionneur doté de la plus grande énergie dans le centre de masse et de la plus grande intensité de faisceaux jamais construit ;ce qui permet de pouvoir rechercher des processus physiques très rares associés à des énergies encore jamais atteintes. L'expérience CMS (Compact Muon Solenoid), au LHC, est un détecteur généraliste pour de nombreuses études de physique. CMS va fournir un outil très précieux pour tester la physique à l'échelle du TeV. Il possède des caractéristiques qui en font un excellent détecteur pour la reconstruction et la mesure des leptons.

Le Modèle Standard de la physique des particules élementaires décrit les particules élementaires et trois des quatre interactions fondamentales (l'électromagnétisme, la force faible et la force forte). La volonté de décrire les quatre forces fondamentales en une seule et même théorie, ainsi que des insuffisances du Modèle Standard, ont mené les physiciens à élaborer de nouvelles approches théoriques. Plusieurs de ces théories prédisent l'existence de nouveaux bosons massifs, pouvant se désintégrer en une paire de leptons chargés. L'objet de cette thèse est la recherche de tels bosons massifs se désintégrant en une paire ee ou e-mu dans le détecteur CMS, en utilisant les données prises au LHC en 2010 (35 pb-1) et en 2011 (3.35 fb-1).

A priori, les canaux dileptoniques sont appropriés pour rechercher des signaux de nouvelle physique dans la phase de démarrage d'un collisionneur hadronique. Néanmoins, il est important de vérifier que la réponse du détecteur, décrite par les simulations détaillées de celui-ci, est conforme aux attentes. Ceci constitue une partie importante de ma thèse.

J'ai mis en évidence au cours de ce travail, au moyen de simulations par Monte Carlo, que le processus t-tbar contribuait de façon significative au bruit de fond pour les paires de leptons de même saveur, juste après le processus de Drell-Yan par ordre d'importance. Je me suis alors consacré à l'étude de ce bruit de fond et à la vérification des prédictions des simulations à partir des données.

J'ai mis en place une méthode originale de mesure du bruit de fond dileptonique, incluant le t-tbar, en exploitant le taux d'embranchement des processus dileptoniques en paires e-mu, deux fois plus important que le taux d'embranchement en paires de leptons de mêmes saveurs. Cette méthode, appelée la méthode e-mu, a permis de vérifier avec précision la prédiction des générateurs pour la simulation des processus dileptoniques dans le canal e-mu en 2010.

Enfin, cette expertise m'a permis de rechercher une résonance dans le spectre de masse des paires e-mu en 2011 et de mettre des limites sur un modèle particulier à dimensions supplémentaires prédisant la coexistence de nouveaux bosons massifs dont certains se désintègrent sans conserver les nombres leptoniques. Pour ce modèle particulier, des limites supérieures à 95% C.L. sur les sections efficaces ont été placées respectivement à 1.37 x 10^{-3}, 1.33 x 10^{-3} et 1.32 x 10^{-3} pb pour des résonances de 1, 1.5 et 2 TeV. Pour un modèle généraliste de résonance Z' se désintégrant en une paire e-mu de charges opposées, des limites supérieures à 95% C.L. de 3.0, 3.0 et 3.0 événements ont aussi été placées pour les trois points de masse.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Apresentamos um estudo preliminar da produção difrativa de mésons utilizando dados obtidos da colisão próton-próton, a energias de centro de massa de 7 TeV, com o experimento CMS-LHC. O trabalho inclui o desenvolvimento do algoritmo de reconstrução dos mésons D* através do canal de decaimento D*->D0 + pion (lento) ->K+pion, a medida da eficiência de detecção e reconstrução, e uma análise do comportamento de variáveis cinemáticas na produção difrativa dessas partículas, particularmente, das lacunas de rapidez. Para isso, foi utilizada uma luminosidade integrada de 3,171pb^(-1) de dados coletados no ano de 2010. As análises com os dados experimentais foram comparadas com os resultados obtidos com geradores de Monte Carlo PYTHIA6, PYTHIA8 e POMPYT.
In this work we present a very preliminary study of the diffractive production of mesons D* from proton-proton collisions, at center of mass energy of 7 TeV, with the CMS-LHC experiment. The decay channel where D*->D0 + pion (slow) ->K+pion was reconstructed and the efficiency was computed. An analysis of the dependence of the kinematical variables,particularly of the pseudo rapidity gap Δη, on the meson production was done. A total luminosity of 3.171pb^(-1) of data collected in the year of 2010 was analyzed, and the reconstructed data were compared to the ones obtained with PYTHIA6, PYTHIA8 and POMPYT generators.

Le sujet de la présente thèse porte sur la recherche de nouvelles particules très massives se désintégrant en une paire électron-positron avec le détecteur CMS.

Le démarrage en 2010 du Large Hadron Collider au CERN marque le début d'une nouvelle ère en physique des particules. L'énergie et l'intensité de ses faisceaux de protons, inégalées à ce jour, offre en effet la possibilité d'étudier les lois décrivant les constituants ultimes de la matière et leurs interactions à des énergies jusqu'alors inaccessibles et d'étudier des processus rares.

La découverte récente par les expériences ATLAS et CMS du boson scalaire prédit par la théorie de la brisure de symétrie électro-faible constitue ainsi la première percée du programme de recherche du LHC et confirme la théorie actuelle décrivant la physique subatomique, le Modèle Standard.

Il est cependant largement admis que cette théorie, bien que hautement prédictive et jamais mise en défaut expérimentalement jusqu'à présent, ne constitue qu'une approximation à basse énergie d'une théorie plus fondamentale.

Cette thèse décrit la recherche de nouvelles particules, prédites par plusieurs modèles au delà du Modèle Standard, via leur désintégration en une paire électron-positron de haute énergie.

La reconstruction et la sélection des électrons de haute énergie par le détecteur CMS sont des éléments centraux de cette analyse et sont étudiées en détail. Divers critères sont développés afin de distinguer les électrons des autres types d'objets physiques produits lors de collisions de protons, tels que les jets. L'intensité des faisceaux du LHC est telle que plusieurs collisions ont lieu simultanément dans le détecteur et il est montré que l'efficacité de sélection des électrons dépend fortement du nombre de ces interactions. Une technique est donc mise au point pour corriger cet effet.

Une méthode pour mesurer l'efficacité de la sélection directement sur les données est également développée. Celle-ci permet de confirmer les mesures obtenues à partir de simulations, jusqu'à des impulsions transverses de plusieurs centaines de GeV.

Le spectre de masse des paires diélectron est établi pour les données enregistrées en 2012 à une énergie dans le centre de masse des protons de 8 TeV, et un excès localisé d'événements est recherché. Aucune déviation significative par rapport au bruit de fonds attendu n'est observée et des limites très contraignantes sont établies sur le rapport de la section efficace de production d'une nouvelle résonance diélectronique et de celle mesurée au pic du boson Z. Ces résultats sont utilisés pour fixer des limites inférieures sur la masse de nouvelles particules prédites par certains modèles.

Le redémarrage du LHC en 2015 avec une énergie de 6.5 TeV par faisceau de proton élargira fortement le potentiel de découverte de ces résonances. En cas de découverte d'un signal, ses propriétés (telles que le spin ou l'asymétrie avant-arrière) seront étudiées avec attention. Des projections sur la précision qui pourrait alors être atteinte pour ces mesures sont donc finalement présentées en fonction de la luminosité intégrée collectée.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

This work presents the search for Dark Matter particles associated with the Higgs Boson decaying into a b b-bar quark pair. The dark matter search result is based on proton-proton collision data collected at a center-of-mass energy of 13 TeV by the ATLAS detector during Run II. The results are interpreted in the context of a simplified model (Z’-2HDM) which describes the interaction of dark matter and standard model particles via new heavy mediator particles. The new powerful Higgs tagging techniques, which exploit the jet substructure and heavy flavor information to a large extent, are developed to improve the search sensitivity of the search. The target physics signals are signature with an optimized search region and interpreted with background estimation result statistically.

A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016.
Plastic scintillators play a key role in reconstructing the energy and tracks of hadronic particles that impinge the Tile Calorimeter of the ATLAS detector as a result of high energy particle collisions generated by the Large Hadron Collider of CERN. In the detector, plastic scintillators are exposed to harsh radiation environments and are therefore susceptible to radiation damage. The radiation damage effects to the optical properties and structural damage were studied for PVT based commercial scintillators EJ200, EJ208, EJ260 and BC408, as well as PS based UPS923A and scintillators manufactured for the Tile Calorimeter. Samples of dimensions 5x5x0.3 mm were subjected to irradiation using 6 MeV protons to doses of approximately 0.8 MGy, 8 MGy, 25 MGy and 25 MGy using the 6 MV tandem accelerator of iThemba LABS. Results show that damage leads to a reduced light output and loss in transmission character. Structural damage to the polymer base and the formation of free radicals occur for doses ≥ 8 MGy leading to reduced scintillation in the base and re-absorption of scintillation light respectively. Scintillators containing a larger Stokes shift, i.e. EJ260 and EJ208 exhibit the most radiation hardness. EJ208 is recommended as a candidate to be considered for the replacement of Gap scintillators in the TileCal for the 2018 upgrade.
LG2017

A dissertation submitted to the Faulty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. April 2017.
Plastic scintillators, situated in the Tile Calorimeter (TileCal) of the ATLAS detector at the Large Hadron Collider (LHC), play an integral part in the detection of diffractive, energetic hadronic particles that result from high energy proton-proton collisions. As these particles traversetheplastic, theresultantdecayofthepolymerbaseemitsluminescentlightwhichacts as a signature of this interaction. However, the deleterious radiation environment in which the plastics are situated ultimately degrade the plastic through the formation of free radicals which initiate chemical reactions and alter the structure of the plastic. Radical formation was studied using electron paramagnetic resonance (EPR) spectroscopy in six plastic scintillator types of either polystyrene (PS) or polyvinyl toluene (PVT) base, and computational EPR studies were conducted on two small monomer structures and two large polymer, plastic-like computer models. Damage was simulated in the computational models by removing an increasing number of hydrogen atoms. Plastic samples, of volume 500 ⇥500 ⇥ 250 µm, were to subjected protons accelerated to 6 MeV using the tandem accelerator at iThemba LABS, Gauteng, to increasing target doses of 0.8 MGy, 2.5 MGy, 8.0 MGy, 25 MGy, 50 MGy, and 80 MGy. The experimental EPR data taken after two weeks of the sample exposure to air indicate the presence of peroxy-type radicals that initiate chemical reactions, discolour the plastic, and decrease the efficiency of the plastic. Furthermore, the data suggests that damaged PS and PVT samples are susceptible to different mechanisms of radiation damage. However, results pertaining to the decrease in the g-factor and the increase in normalised EPR intensity suggest that all plastics behave similarly using an EPR analysis as a function of dose. Thus, the EPR analysis could not identify a specific plastic that would perform better than the existing plastics used in the TileCal. The computational chemical potential results indicate that electron transfer between damaged pristine and damaged models is possible. In the two small damaged models, the computational EPR data indicate the presence of a various stable akyl-like radicals depending on the site from which the hydrogen atoms are removed. In the two large damaged models, these results indicate a number of alkyl-, benzyl-, and cyclohexadienyl-like radicals.
LG2017

Dissertation presented in ful lment of the requirements for the degree of: Master of Science in Physics 2016
The Phase-II upgrade of the Large Hadron Collider at CERN in the early 2020s will enable an order of magnitude increase in the data produced, unlocking the potential for new physics discoveries. In the ATLAS detector, the upgraded Hadronic Tile Calorimeter (TileCal) Phase-II front end read out system is currently being prototyped to handle a total data throughput of 5.1 TB/s, from the current 20.4 GB/s. The FPGA based Super Read Out Driver (sROD) prototype must perform an energy reconstruction algorithm on 2.88 GB/s raw data, or 275 million events per second. Due to the very high level of pro ciency required and time consuming nature of FPGA rmware development, it may be more e ective to implement certain complex energy reconstruction and monitoring algorithms on a general purpose, CPU based sROD co-processor. Hence, the feasibility of a general purpose ARM System on Chip based co-processing unit (PU) for the sROD is determined in this work. A PCI-Express test platform was designed and constructed to link two ARM Cortex-A9 SoCs via their PCI-Express Gen-2 x1 interfaces. Test results indicate that the latency of the PCI-Express interface is su ciently low and the data throughput is superior to that of alternative interfaces such as Ethernet, for use as an interconnect for the SoCs to the sROD. CPU performance benchmarks were performed on ve ARM development platforms to determine the CPU integer, oating point and memory system performance as well as energy e ciency. To complement the benchmarks, Fast Fourier Transform and Optimal Filtering (OF) applications were also tested. Based on the test results, in order for the PU to process 275 million events per second with OF, within the 6 s timing budget of the ATLAS triggering system, a cluster of three Tegra-K1, Cortex-A15 SoCs connected to the sROD via a Gen-2 x8 PCI-Express interface would be suitable. A high level design for the PU is proposed which surpasses the requirements for the sROD co-processor and can also be used in a general purpose, high data throughput system, with 80 Gb/s Ethernet and 15 GB/s PCI-Express throughput, using four X-Gene SoCs.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, School of Physics. Johannesburg. February, 2017
The discovery of the Higgs boson at the Large Hadron Collider in Switzerland marks the beginning of a new era: Physics beyond the Standard Model (SM). A model is proposed to describe numerous Run I features observed with both the ATLAS and CMS experiments. The model introduces a heavy scalar estimated to be around 270 GeV and an intermediate scalar which can decay into both dark matter and SM particles. Three different final state searches, linked by the new hypothesis, are presented. These are the hh → γγb¯ b, γγ + Emiss T and high mass diphoton channels. No significant excesses were observed in any channel using the available datasets and limits were set on the relevant cross sections times branching ratios. The lack of statistics in the γγb¯ b analysis prevents any conclusive statement in regard to the excess observed with Run I data. Observing no excess in the γγ + Emiss T channel with the current amount of data is also consistent with the intermediate scalar decaying to SM particles. This could explain the excess of Higgs bosons produced in associations with top quarks in the multilepton final states observed in ATLAS and CMS in Run I and Run II. The work presented provides a deeper understanding on the underlying phenomenology of the hypothesis and provides a foundation for future work. The ATLAS detector underwent a stringent consolidation and validation effort before data taking could commence in 2015. A high voltage board was designed and implemented into a portable test-bench used in the certification and validation process. In addition to these efforts, the electronics on the ATLAS detector are being improved for the Phase-II upgrade program in 2024. A software tool has been designed which integrates the envisioned Phase-II backend infrastructure into the existing ATLAS detector control system. This software is now an ATLAS wide common tool used by multiple sub-detectors in the community.
XL2017

A dissertation submitted in fulfillment of the requirements for the degree of Master of Physics in the School of Physics November 1, 2017.
The ATLAS detector at the Large Hadron Collider (LHC) at CERN is undergoing upgrades to its instrumentation, as well as the hardware and software that comprise its Trigger and Data Acquisition (TDAQ) system. The increased energy will yield larger cross sections for interesting physics processes, but will also lead to increased artifacts in on-line reconstruction in the trigger, as well as increased trigger rates, beyond the current system’s capabilities. To meet these demands it is likely that the massive parallelism of General-Purpose Programming with Graphic Processing Units (GPGPU) will be utilised. This dissertation addresses the problem of integrating GPGPU into the existing Trigger and TDAQ platforms; detailing and analysing GPGPU performance in the context of performing in a high-throughput, on-line environment like ATLAS. Preliminary tests show low to moderate speed-up with GPU relative to CPU, indicating that to achieve a more significant performance increase it may be necessary to alter the current platform beyond pairing suitable GPUs to CPUs in an optimum ratio. Possible solutions are proposed and recommendations for future work are given.
LG2018

Two-charged-particle correlations are measured as a function of pseudorapidity and azimuthal angle difference in pp collisions at √s = 13, 2.76 and 5.02 TeV with the ATLAS detector at the Large Hadron Collider. A long-range structure in the two-dimensional function centered at ∆φ = 0 and extending over a large range of ∆η referred to as the “ridge” is seen in the three data sets. A template fitting method is implemented to extract the Fourier harmonics of the flow and gives the dependence of the harmonics on the charged-particle multiplicities. In this method a rescaled correlation function from peripheral events representing the recoil component plus a cosine modulation representing the ridge is used to describe the whole one-dimensional correlation function. Different multiplicity intervals for the peripheral events are used to extract the harmonics. The results presented show that vn,n from correlation functions can be factorized into the products of single particle vn. Significant contributions from v₂, v₃ and v₄ are obtained and their dependences on multiplicity and transverse momentum are studied. It is also shown that there is significant vn even in the lowest multiplicity bins. In addition, the second harmonics v₂ in pp do not have a significant dependence on both the multiplicity and collision energy. Results of pp and pPb at the same energy are compared with each other in both multiplicity and pT distributions. Both chᵗʳᵏ−chᵗʳᵏ and chᵗʳᵏ-muon correlations are measured in pPb collisions at √sNN = 8.16 TeV. Long-range correlations are studied through template fitting procedure. chᵗʳᵏ-v₂ increases with the number of reconstructed charged tracks at low multiplicity and saturates at high multiplicity. Muon-v₂ is considerably smaller than chᵗʳᵏ-v₂ and only has a weak dependence on event multiplicity. Factorization in both cases works pretty well. Two-charged-particle correlation functions are also measured in Xe+Xe events at √sNN = 5.44 TeV. In the most central collisions direct Fourier decomposition is preferred to avoid negative recoil component that might appear in the template fitting method. vn reaches its maximum value in the mid-centrality region and becomes smaller at both low and high centralities. Results are compared with Pb+Pb events at √sNN = 5.02 TeV showing that vn obtained from these two systems have similar values and behaviors.

A search for Higgs boson pairs produced in the decay of high mass exotic resonances is presented. The search uses the bb̄bb̄ final state, analyzing 139 fb⁻¹ of proton–proton collision data at √s = 13 TeV collected with the ATLAS detector. Spin-0 and spin-2 benchmark signal models are considered and no significant deviation from the Standard Model prediction is observed. The search is combined with a complementary analysis for lower mass resonances to set upper limits on the production cross-section times HH branching ratio of new resonances in the mass range of 251 GeV to 5 TeV. In addition, the methodology for the in-situ calibration of a novel double-b-tagging algorithm (Xbb2020) using gluon to bb̄ decays is presented. Preliminary scale factors for Monte-Carlo simulation are computed using 139 fb⁻¹ of √s = 13 TeV pp collision data collected with the ATLAS detector. The completed calibration will allow the Xbb2020 algorithm to be used in future ATLAS searches for H → bb̄ decays.

This thesis describes the measurement of inclusive heavy flavor jet suppression in collisions between two lead nuclei with the center of mass energy per nucleon-nucleon pair of 2.76 TeV with the ATLAS detector at the Large Hadron Collider (LHC). The measurement of the heavy flavor jets and b-jet quenching at the LHC is important in the path towards the understanding of QGP. Parton showers initiated by heavy quarks are expected to be sensitive to the medium in a different way as the large quark mass suppresses the medium-induced radiation. This results in a different interplay between radiative and collisional energy loss. Therefore the analysis of the properties of jet associated with b hadrons (b-jet) is useful in understanding energy loss in the QGP. The inclusive b-jet suppression R AA has been measured using muons in jets, where a b-jet corresponds to a jet with at least one muon clustered with the anti-k t algorithm with parameter R = 0.2. The b-jets of p T between 30 GeV - 150 GeV are identified by the semileptonic decay of beauty hadrons. Muons originating from background sources, primarily Charm hadrons, pion and kaon decays, have been removed from the analysis using template fits to the distribution of a quantity(p T^rel) capable of statistically distinguishing between signal and background. The measured nuclear modification factor R AA has been presented in different centrality bins as a function of the b-jet transverse momentum p T.The results of R AA indicate that the yield of the most central event (0-10%) experiences more suppression compared to the most peripheral event (60-80%) by a factor of approximate 2.