This thesis presents the first search for a neutral Minimal Supersymmetric Standard Model Higgs boson decaying to a pair of hadronic taus in proton-proton collisions with a center-of-mass energy of √s = 7 TeV. Results are presented for an integrated lumi- nosity of 1.056 fb−1 using data from the ATLAS detector at the Large Hadron Collider. After signal selection, 245 events are observed. The number of events is consistent the background estimate of 256 ± 26. An exclusion limit for the Higgs boson production is derived as a function of (cross-section)×(branching ratio). This limit is presented as a function of two parameters: mA and tanβ in the mmax scenario. A point in the (mA, h tanβ) phase space is excluded if the signal hypothesis is rejected at the 95% confidence level.

This thesis presents the first search for a neutral Minimal Supersymmetric Standard Model Higgs boson decaying to a pair of hadronic taus in proton-proton collisions with a center-of-mass energy of √s = 7 TeV. Results are presented for an integrated lumi- nosity of 1.056 fb−1 using data from the ATLAS detector at the Large Hadron Collider. After signal selection, 245 events are observed. The number of events is consistent the background estimate of 256 ± 26. An exclusion limit for the Higgs boson production is derived as a function of (cross-section)×(branching ratio). This limit is presented as a function of two parameters: mA and tanβ in the mmax scenario. A point in the (mA, h tanβ) phase space is excluded if the signal hypothesis is rejected at the 95% confidence level.

This thesis presents a Monte Carlo study of the search for charged Higgs bosons heavier than the mass of the top quark within the Minimal Supersymmetric Standard Model (MSSM) at the ATLAS experiment at the CERN Large Hadron Collider. Charged Higgs bosons are predicted in many non-minimal Higgs extensions of the Standard Model. Their discovery would be a clear signal for the existence of New Physics beyond the Standard Model, possibly the first experimental evidence to be found if the MSSM is realized in nature. The feasibility of detecting the heavy charged Higgs boson with a mass up to 600 GeV is studied in the decay channel H->tb for an integrated luminosity of 30 inverse femtobarns at a center-of-mass energy of 10 TeV. A major difficulty for charged Higgs boson reconstruction is the combinatorial background in the complex multi-jet final state environment. It is shown that this can be overcome by applying a kinematic fitting procedure and by a subsequent cut-based event and candidate selection. An iterative solution of the developed kinematic fit with non-linear constraints is presented. In addition, the reconstruction of charged Higgs bosons makes high demands on the ability to identify jets containing b-hadrons. A special b-tagging algorithm is introduced and a comparison with first ATLAS data is presented showing good agreement of the expected performance. This study is performed with a realistic simulation of the ATLAS detector and takes into account all dominant experimental uncertainties and statistical uncertainties arising from limited Monte Carlo statistics. The result is given in terms of discovery and exclusion contours in the (m(H), tan(beta)) parameter space in the mhmax scenario of the MSSM. This study indicates that the heavy charged Higgs boson can be discovered in this decay channel only for large values of tan(beta). It, however, can contribute to a combined H sensitivity including other decay channels. Compared to previous studies the discovery reach could be improved. / Die vorliegende Arbeit präsentiert eine Monte Carlo Studie zur Suche nach geladenen Higgs Bosonen mit einer Masse größer als die des top Quarks im Kontext des Minimal Supersymmetrischen Standardmodells (MSSM) am ATLAS Experiment am CERN Large Hadron Collider. Geladene Higgs Bosonen werden von vielen Erweiterungen des Standardmodells vorhergesagt. Ihre Entdeckung wäre ein klares Indiz für neue Physik jenseits des Standardmodells, möglicherweise der erste experimentelle Befund für das MSSM. Eine Machbarkeitsstudie zur Entdeckung geladener Higgs Bosonen mit Massen bis zu 600 GeV im Zerfallskanal H->tb für eine integrierte Luminosität von 30 inversen femtobarn und einer Schwerpunktsenergie von 10 TeV wird vorgestellt. Eine der Hauptschwierigkeiten bei der Rekonstruktion geladener Higgs Bosonen ist der durch den komplexen Endzustand mit vielen Jets auftretende kombinatorische Untergrund. Es wird gezeigt, dass die Rekonstruktion durch Anwendung eines kinematischen Fits mit anschließender schnittbasierter Ereignis- und Kandidatenselektion möglich ist. Eine iterative Lösung des entwickelten kinematischen Fits mit nicht-linearen Zwangsbedingungen wird vorgestellt. Des Weiteren stellt die Rekonstruktion geladener Higgs Bosonen hohe Ansprüche an die Möglichkeit, Jets, welche b-Hadronen beinhalten, zu identifizieren. Ein spezieller b-tagging Algorithmus wird vorgestellt und ein Vergleich mit den ersten ATLAS Daten zeigt eine gute Übereinstimmung mit den erwarteten Eigenschaften. Die Studie wurde mit einer realistischen ATLAS Detektorsimulation durchgeführt und beinhaltet alle dominanten experimentellen Unsicherheiten sowie die statistische Unsicherheiten aufgrund begrenzt vorhandener simulierter Ereignisse. Die erwarteten Entdeckungs- und Ausschlusskonturen im mhmax Szenario des MSSM werden als Funktion der geladenen Higgs Boson Masse und tan(beta) gegeben. Die durchgeführte Studie zeigt, dass geladene Higgs Bosonen in diesem Zerfallskanal nur bei großen Werten von tan(beta) entdeckt werden können. Dennoch kann dieser Zerfallskanal zur H Sensitivität beitragen, wenn andere Zerfallskanäle mit in Betracht gezogen werden. Im Vergleich zu vorherigen Studien konnte die Sensitivität erhöht werden.

This thesis presents a Monte Carlo study of the search for charged Higgs bosons heavier than the mass of the top quark within the Minimal Supersymmetric Standard Model (MSSM) at the ATLAS experiment at the CERN Large Hadron Collider. Charged Higgs bosons are predicted in many non-minimal Higgs extensions of the Standard Model. Their discovery would be a clear signal for the existence of New Physics beyond the Standard Model, possibly the first experimental evidence to be found if the MSSM is realized in nature. The feasibility of detecting the heavy charged Higgs boson with a mass up to 600 GeV is studied in the decay channel H->tb for an integrated luminosity of 30 inverse femtobarns at a center-of-mass energy of 10 TeV. A major difficulty for charged Higgs boson reconstruction is the combinatorial background in the complex multi-jet final state environment. It is shown that this can be overcome by applying a kinematic fitting procedure and by a subsequent cut-based event and candidate selection. An iterative solution of the developed kinematic fit with non-linear constraints is presented. In addition, the reconstruction of charged Higgs bosons makes high demands on the ability to identify jets containing b-hadrons. A special b-tagging algorithm is introduced and a comparison with first ATLAS data is presented showing good agreement of the expected performance. This study is performed with a realistic simulation of the ATLAS detector and takes into account all dominant experimental uncertainties and statistical uncertainties arising from limited Monte Carlo statistics. The result is given in terms of discovery and exclusion contours in the (m(H), tan(beta)) parameter space in the mhmax scenario of the MSSM. This study indicates that the heavy charged Higgs boson can be discovered in this decay channel only for large values of tan(beta). It, however, can contribute to a combined H sensitivity including other decay channels. Compared to previous studies the discovery reach could be improved. / Die vorliegende Arbeit präsentiert eine Monte Carlo Studie zur Suche nach geladenen Higgs Bosonen mit einer Masse größer als die des top Quarks im Kontext des Minimal Supersymmetrischen Standardmodells (MSSM) am ATLAS Experiment am CERN Large Hadron Collider. Geladene Higgs Bosonen werden von vielen Erweiterungen des Standardmodells vorhergesagt. Ihre Entdeckung wäre ein klares Indiz für neue Physik jenseits des Standardmodells, möglicherweise der erste experimentelle Befund für das MSSM. Eine Machbarkeitsstudie zur Entdeckung geladener Higgs Bosonen mit Massen bis zu 600 GeV im Zerfallskanal H->tb für eine integrierte Luminosität von 30 inversen femtobarn und einer Schwerpunktsenergie von 10 TeV wird vorgestellt. Eine der Hauptschwierigkeiten bei der Rekonstruktion geladener Higgs Bosonen ist der durch den komplexen Endzustand mit vielen Jets auftretende kombinatorische Untergrund. Es wird gezeigt, dass die Rekonstruktion durch Anwendung eines kinematischen Fits mit anschließender schnittbasierter Ereignis- und Kandidatenselektion möglich ist. Eine iterative Lösung des entwickelten kinematischen Fits mit nicht-linearen Zwangsbedingungen wird vorgestellt. Des Weiteren stellt die Rekonstruktion geladener Higgs Bosonen hohe Ansprüche an die Möglichkeit, Jets, welche b-Hadronen beinhalten, zu identifizieren. Ein spezieller b-tagging Algorithmus wird vorgestellt und ein Vergleich mit den ersten ATLAS Daten zeigt eine gute Übereinstimmung mit den erwarteten Eigenschaften. Die Studie wurde mit einer realistischen ATLAS Detektorsimulation durchgeführt und beinhaltet alle dominanten experimentellen Unsicherheiten sowie die statistische Unsicherheiten aufgrund begrenzt vorhandener simulierter Ereignisse. Die erwarteten Entdeckungs- und Ausschlusskonturen im mhmax Szenario des MSSM werden als Funktion der geladenen Higgs Boson Masse und tan(beta) gegeben. Die durchgeführte Studie zeigt, dass geladene Higgs Bosonen in diesem Zerfallskanal nur bei großen Werten von tan(beta) entdeckt werden können. Dennoch kann dieser Zerfallskanal zur H Sensitivität beitragen, wenn andere Zerfallskanäle mit in Betracht gezogen werden. Im Vergleich zu vorherigen Studien konnte die Sensitivität erhöht werden.

This thesis describes a measurement of the Standard Model Higgs boson coupling to fermions in decays to two τ-leptons, a search for charged Higgs bosons in decays to a τ-lepton and a neutrino, as well as the reconstruction, identification and triggering of hadronically decaying τ-leptons. The data considered are collected by the ATLAS experiment at the Large Hadron Collider. The reconstruction and identification of hadronically decaying τ-leptons in the ATLAS experiment during Run 2 of the Large Hadron Collider are described, and the performance of the τ trigger is measured in events with top–antitop-quark pairs using data from 13 TeV proton–proton collisions, corresponding to an integrated luminosity of 3.2 fb-1, collected in 2015, and 11.5 fb-1, collected in 2016. Hadronically decaying τ-leptons are of importance to many physical processes involving Higgs bosons. The coupling of the Standard Model Higgs boson to fermions is measured in decays to two τ-leptons using 7 TeV data corresponding to an integrated luminosity of 4.5 fb-1, collected in 2011, and 8 TeV data corresponding to an integrated luminosity of 20.3 fb-1, collected in 2012. The signal strength is measured to be μ = 1.4, corresponding to an excess over the background-only model of 4.5σ. Charged Higgs bosons are searched for in decays to a τ-lepton and a neutrino, where the τ-lepton decays hadronically. 13 TeV data corresponding to an integrated luminosity of 14.7 fb-1, collected in 2015 and 2016, are used. No excess over the Standard Model background is observed, and the 95 % confidence-level exclusion limits on σ(pp → [b]tH+) × (H+ → τν) are set to 2.0 pb–8 fb in the range 200–2000 GeV.

La observació de un o varis bosons de Higgs és fonamental a l'hora d'entendre el trencament espontani de simetria del Model Estàndard (SM). En aquest model, un doblet escalar és el que proporciona el trencament espontani de simetria donant un bosó de Higgs.No obstant, el Model Estàndard té alguns problemes que són difícils de solucionar. Aquest tipus de problemes troben solució en les teories Supersimètriques (SUSY). El Minimal Supersymmetric Standard Model (MSSM) és l'extensió supersimètrica més simple del Model Estàndard. El sector de Higgs del MSSM està format per 2 doblets de Higgs que donen 5 bosons de Higgs després del trencament espontani de simetria: 3 bosons neutres (h, H, A) i un parell carregat (H+ ,H-).El descobriment d'un bosó de Higgs carregat (H±) seria una senyal clara de nova física més enllà del Model Estàndard. Els bosons de Higgs carregats es podrien produir en grans quantitats al LHC (Large Hadron Collider) en el decaiment del quark top. En aquesta tesi, la sensitivitat del detector ATLAS en condicions de baixa lluminositat al descobriment del Higgs carregat és estudiada en detall fent ús del canal: amb i . Després d'aplicar talls cinemàtics a les diferents mostres (senyal i fons) d'esdeveniments, el fons més rellevant és el format per amb un bosó W desintegrant-se en tn i l'altre desintegrant-se hadrònicament donant un parell de jets lleugers. Aquest fons va poder ser reduït imposant un tall a la massa transversa i en el moment transvers del leptó t: degut a la polarizació del t, en els casos en que el t decau hadrònicament en una partícula carregada, aquells jets de t que provenen de la desintegració del H± tenen un moment més gran que els que provenen de la desintegració del W±.El potencial de descoberta del detector ATLAS per a un Higgs carregat produït en la desintegració del quark top ja va ser estudiat en ATLAS amb W± desintegrant-se leptònicament (mnm o ene). Els estudis de significància presentats en aquesta tesi completen els anàlisis efectuats en el pla del MSSM (mA, tanb) per la descoberta del Higgs carregat. En la regió fins ara no coberta al voltant de tanb 5-10, s'obté una significància superior a 5s per 10 fb-1 de dades (que correspon a un any funcionant a baixa lluminositat) amb el canal estudiat en aquest treball. Amb això, la combinació els estudis de potencial de descoberta dels detectors ATLAS i CMS pels diferents bosons de Higgs (h, H, A, H+, H-) cobreixen tot l'espai (mA, tanb) del MSSM. A més a més, una característica important d'aquest canal és permet mesurar la massa. La massa del Higgs cargado del MSSM ha sigut obtinguda a partir de la distribució de la massa transversa mitjançant un mètode de màxima versemblança. L'error associat a aquesta mesura està completament dominat pels efectes sistemàtics i la precisió relativa obtinguda amb aquest mètode per la mesura de la massa és d'un 4% per una lluminositat integrada de 10 fb-1. / La observación de uno o varios bosones de Higgs es fundamental para entender la rotura espontánea de simetría del Modelo Estándar (SM). En este modelo, un doblete escalar es el que proporciona la rotura espontánea de simetría dando lugar a un bosón de Higgs.Sin embargo, el Modelo Estándar tiene algunos problemas difíciles de solucionar. Esta clase de problemas hallan solución en teorías Supersimétricas (SUSY). El Minimal Supersymmetric Standard Model (MSSM) es la extensión supersimétrica más simple del Modelo Estándar. El sector de Higgs del MSSM está formado por dos dobletes de Higgs dando lugar a 5 bosones de Higgs después de la rotura espontánea de simetría: 3 bosones neutros (h, H, A) y un par cargado (H+, H-).El descubrimiento del bosón de Higgs cargado (H±) sería una señal clara de nueva física más allá del Modelo Estándar. Los bosones de Higgs cargados podrían producirse copiosamente en el LHC (Large Hadron Collider) en la desintegración del quark top. En esta tesis, la sensitividad del detector ATLAS en condiciones de baja luminosidad al descubrimento del Higgs cargado es estudiada en detalle mediante el canal: con y . Después de aplicar cortes cinemáticos a las distintas muestras de señal y fondo, los sucesos de fondo de tipo con un bosón W desintegrándose en tn y el otro desintegrándose hadrónicamente són todavía importantes. Este fondo pudo ser reducido imponiendo cortes en la masa transversa del H± y en el momento transverso del leptón t: debido a la polarización del t, cuando éste se desintegra hadrónicamente en una partícula cargada, los jets de t que provienen de la desintegración del H± tienen un momento superior a los que provienen de la desintegración del W±.El potencial de descubrimiento del detector ATLAS para un Higgs cargado producido en la desintegración del quark top ya fue estudiado en ATLAS con W± desintegrándose leptónicamente (mnm o ene). Los estudios de significancia presentados en esta tesis completan los análisis efectuados en el plano del MSSM (mA, tanb) para el descubrimiento del Higgs cargado. En la región no cubierta hasta ahora alrededor de tanb 5-10, se obtiene una significancia superior a 5s para 10 fb-1 de datos (que corresponde a un año funcionando a baja luminosidad) con el canal estudiado en esta tesis. Es por eso que la combinación de los estudios de potencial de descubrimiento de los detectores ATLAS y CMS para los distintos bosones de Higgs (h, H, A, H+, H-) cubren todo el espacio (mA, tanb) del MSSM.Además, una característica importante de este canal es que permite medir la masa. La masa del Higgs cargado del MSSM se ha obtenido a partir de la distribución de la masa transversa utilizando un método de máxima verosimilitud. El error asociado a esta medida está completamente dominado por los efectos sistemáticos y la precisión relativa para la medida de la masa que se obtiene con este método es de un 4% para una luminosidad integrada de 10 fb-1. / The observation of one or several Higgs bosons will be of capital importance in order to understand the electroweak symmetry breaking mechanism. In the Standard Model (SM), one scalar doublet is responsible for the electroweak symmetry breaking giving as a result only one Higgs boson.However, the SM has serious drawbacks which are difficult to solve within the theory itself. This problems find a solution in Supersymmetric (SUSY) models. The Minimal Supersymmetric Standard Model (MSSM) is the simplest SUSY extension of the SM. The MSSM Higgs sector is made of two Higgs doublets which lead to five Higgs particles after electroweak symmetry breaking: three neutral (h, H, A) and a charged pair (H+ ,H-).The discovery of the MSSM charged Higgs (H±) would be a clear sign of new physics beyond the SM. MSSM charged Higgs could be copiously produced at the LHC (large Hadron Collider) in top decays. In the present thesis, the sensitivity of the ATLAS detector to the discovery of the charged Higgs in the low luminosity running conditions is studied in detail in the channel with and .After event selection, the most relevant remaining background is with one W boson decaying into tn and the other decaying hadronically in two light jets. That background was further reduced by imposing a cut on the transverse mass and a harder cut on the t jet transverse momentum: due to t polarization effects, in one-prong t hadronic decays, t jets from H± decay are harder than the ones produced in W± decay.The discovery potential of the ATLAS detector for a charged Higgs produced in top decays was already studied in ATLAS with the W± decaying leptonically (mnm or ene). The significance studies presented in this thesis complete the (mA, tanb) survey for the MSSM charged Higgs. Furthermore, in the so far uncovered 5-10 tanb region a significance greater than 5s is obtained for 10 fb-1 of data (corresponding to one year of running at low luminosity) with the channel studied in this work. Therefore, the discovery potential studies of ATLAS standalone and ATLAS+CMS combined for any of the different MSSM Higgs bosons (h, H, A, H+, H-) cover the whole MSSM (mA, tanb) space.Moreover, an important aspect of this channel is that it gives the possibility to measure the mass. The light MSSM charged Higgs mass was obtained from the transverse mass distribution (since in the channel there is no resonance peak) by means of a maximum likelihood method and the expected precision on this mass was also given. The error was completely dominated by systematics and a precision close to 4% could be achieved for an integrated luminosity of 10 fb-1.

The Minimally Supersymmetric Standard Model (MSSM) provides us with a WIMP dark matter candidate particle, the neutralino. Neutralinos from the dark matter halo can potentially become captured by the sun and concentrated in the core, where they can undergo self-annihilation and so produce a distinct neutrino signal. The IceCube Neutrino Observatory has the potential to detect this neutrino signal and thus give indirect evidence of the presence and properties of neutralino dark matter. Although the full, unconstrained MSSM has 105 parameters this can be reduced to 25 parameters by the application of physically motivated assumptions. Scans of this MSSM-25 parameter space are conducted using the DarkSUSY software package and an adaptive scanning technique based on the Monte-Carlo VEGAS algorithm. The IceCube exclusion confidence level is then calculated for a set of points produced by these scans. Results indicate that the detection capability of IceCube exceeds that of current direct detection methods in certain regions of the parameter space. The use of a 25 dimensional parameter space reveals that there are new regions of observables with high exclusion confidence levels compared to earlier simulations performed with a seven dimensional parameter space.

This thesis presents searches for a charged Higgs boson (H±) in proton-proton collisions with center-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. Multiple search channels are used with the common characteristic of at least one charged lepton (electron or muon) that effectively reduces the multi-jet background and is used for efficient triggering. Charged Higgs bosons decaying to a tau lepton and a neutrino are searched for using final states with two charged leptons, or one charged lepton and a hadronically decaying tau. A significant background originates from quark- or gluon-initiated jets that may be misidentified as hadronic tau decays. Methods to estimate this background are presented, including a largely data-driven matrix method. Signal processes with a charged Higgs boson mass below or above that of the top quark are considered. With the dataset collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb-1, upper limits at 95% confidence level are placed on the branching fraction B(t→bH±)×B(H±→τν) in the range 1.1–0.3% for charged Higgs boson masses between 80 GeV and 160 GeV, and on the top-quark associated H± production cross section in the range 0.93 to 0.03 pb for charged Higgs boson masses between 180 GeV and 1 TeV.

The three main parts of this thesis demonstrate our current understanding of certain physics but mostly go beyond our understanding and present novel approaches, both technically and physically. The first part concentrates on the scalar mesons and presents search methodology to enable a better understanding of their existence and structures. The second part discusses one step further on beyond the standard model physics searches. Emphasis is given to discriminating factors between the MSSM and the U(1)' gauge extended models. The last part discusses a specific readout problem in calorimetry together with its solution and presents the digital hadron calorimetry, which will be an essential part of calorimeter systems of future colliders.

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