This thesis considers the search of charged Higgs bosons, which are predicted by several extensions of the Standard Model of particle physics. Light charged Higgs bosons (below the top quark mass) can appear in top quark decays and are assumed to decay exclusively to tauons. Two discriminating variables are presented that are sensitive to such a process taking place in top-antitop events with two final state leptons. Distributions of these variables are computed for Monte Carlo simulations and for 35/pb of data from 7 TeV proton-proton collisions recorded in 2010 by the ATLAS experiment at the LHC. Monte Carlo simulations are found to agree well with data, validating the use of these discriminating variables, but no conclusions about the existence of charged Higgs bosons can be made at this point.

<p>Conventional searches for the charged Higgs boson using its production in association with Standard Model (SM) quarks is notoriously weak in the mid-tanB range. Hoping to find an alternate channel to fill this gap, the production of the charged Higgs boson in association with supersymmetric squarks is studied. Using Monte Carlo generators the production at the LHC is simulated within the non universal Higgs mass model (NUHM). If the six parameters of the model (m₀, m_1/2, A₀, tanB, u, m_A) induce small masses of the stop, sbottom and charged Higgs, the production cross section can be of the order pb. Through scans of the input parameter the cross section is maximized, with the requirement that the stop decays directly to a neutralino - simplifying detection, in the point (m₀, m_1/2, A₀, tanB, u, m_A) = (190, 187, -1147, 179, 745, 13.2) where the cross section is 559 fb.</p><p>The production is compared to the irreducible backgrounds stop, stop, t, tbar and t, tbar + 2 jets. The former poses no severe constraints and can be easily removed using appropriate cuts. The latter, SM background, has a cross section almost 1000 times larger and strong cuts must be imposed to suppress it. Neglecting hadronization and systematic effects, we show that a 5 sigma discovery is possible at 133 fb^-1. In this range, mH+ = 194 GeV and tanB = 13.2, other channels have little or no prospects of detecting the charged Higgs and the studied process shows good prospects for complementing charged Higgs searches at the LHC in the mid-tanB range.</p>

Conventional searches for the charged Higgs boson using its production in association with Standard Model (SM) quarks is notoriously weak in the mid-tanB range. Hoping to find an alternate channel to fill this gap, the production of the charged Higgs boson in association with supersymmetric squarks is studied. Using Monte Carlo generators the production at the LHC is simulated within the non universal Higgs mass model (NUHM). If the six parameters of the model (m0, m1/2, A0, tanB, u, mA) induce small masses of the stop, sbottom and charged Higgs, the production cross section can be of the order pb. Through scans of the input parameter the cross section is maximized, with the requirement that the stop decays directly to a neutralino - simplifying detection, in the point (m0, m1/2, A0, tanB, u, mA) = (190, 187, -1147, 179, 745, 13.2) where the cross section is 559 fb. The production is compared to the irreducible backgrounds stop, stop, t, tbar and t, tbar + 2 jets. The former poses no severe constraints and can be easily removed using appropriate cuts. The latter, SM background, has a cross section almost 1000 times larger and strong cuts must be imposed to suppress it. Neglecting hadronization and systematic effects, we show that a 5 sigma discovery is possible at 133 fb-1. In this range, mH+ = 194 GeV and tanB = 13.2, other channels have little or no prospects of detecting the charged Higgs and the studied process shows good prospects for complementing charged Higgs searches at the LHC in the mid-tanB range.

In the ATLAS experiment software plays a central role in data taking, reconstruction, and analysis. This thesis presents a set of analysis algorithms written for the ATLAS software framework Athena, towards H+ searches in the τ+jets channel. Data recorded by the ATLAS experiment for 900 GeV proton-proton collisions from 2009 and 7 TeV proton-proton collisions from 2010 are analyzed with these algorithms and compared to Monte Carlo. A search for top quarks is performed on 36 pb-1 of data, and the results are found to agree well with Monte Carlo.

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

The ATLAS experiment at the LHC is searching for evidence of charged Higgs bosons, without yet finding it. This report investigates the possibility of reconstructing charged Higgs bosons using multi-jet final states (pp -&gt; [b]tH+ -&gt; [b]bbbql\nu) with focus on fully hadronic H^+ -&gt; hW decays using a simulation of 500k events with m_{H+} = 250 GeV. After optimizing TMVA (Toolkit for Multivariate Analysis) input variables for BDT (Boosted Decision Trees), the mass of H+ was recreated with an efficiency of up to 26,7% (correct combinations).

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.

In this thesis I explore the possibilities of detecting doubly charged Higgs bosons from the Higgs Triplet Model (HTM) at the Large Hadron Collider (LHC) at CERN. Higgs bosons are included into the Standard Model (SM) of particle physics in order to explain the origin of mass of the elementary particles. Even though the SM is considered to be a reliable starting point for any particle theory, no Higgs particles have to this day been found. There are therefore high expectations for the record-breaking energies of the LHC to lead to a Higgs discovery. The HTM produces seven different Higgs bosons, and among these we find the doubly charged ones. Because of their exotic charge, I focus on studying these Higgs particles in particular. To this purpose, I implement the full HTM theory as an alternative model option in standard particle physics Monte-Carlo software and then simulate LHC proton-proton collisions at a center of mass energy of 14 TeV. The investigated signal is defined as two like-signed leptons, four hard jets and missing energy in the final state. The main production mechanisms are pair-production and associated production with a singly charged Higgs. Since I choose to study a region of parameter space where the triplet vacuum expectation value is relatively large, the doubly charged Higgs decays into W's and the singly charged Higgs into WZ or tb. The results of the simulations show that the LHC could probe Higgs masses up to at least 300 GeV with an integrated luminosity of about 300 fb.1.

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.