A search for the neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM) is presented. The analysis is performed on data from proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV at the Large Hadron Collider recorded by the ATLAS detector. The data were collected in 2012, correspond to an integrated luminosity of 20.3 fb-1 and represent the full 8 TeV dataset. This search is performed in the tau-tau final state, with one tau lepton decaying leptonically and the other hadronically. The analysis is optimized in three categories, addressing low-mass Higgs bosons which are accompanied by a bottom quark induced jet or not and high-mass Higgs bosons. No deviations from Standard Model predictions are observed, and therefore no evidences of new Higgs bosons are found. Exclusion limits are set on the cross-section times branching fraction of the Higgs bosons and for parameters m_A and tan{beta} of MSSM benchmark scenarios. / In dieser Arbeit wird die Suche nach neutralen Higgs-Bosonen im Rahmen des Minimalen Supersymmetrischen Standardmodells (MSSM) vorgestellt. Die Analyse basiert auf dem kompletten Datensatz von Proton-Proton-Kollisionen des Large Hadron Colliders, LHC, die mit dem ATLAS-Detektor bei einer Schwerpunktsenergie von sqrt{s}=8 TeV im Jahr 2012 aufgenommen wurden. Die Daten entsprechen einer Luminosität 20.3 fb-1. Die Suche fokussiert auf den Tau-Tau Endzustand, wobei ein Tau-Lepton hadronisch zerfällt und das andere leptonisch, entweder in ein Elektron oder in ein Muon. Die Analyse wurde in drei Kategorien optimiert, um sowohl für Higgs-Bosonen im niedrigen Massenbereich von 90-200 GeV als auch im hohen Massenbereich von 200 GeV bis 1 TeV sensitiv zu sein. Der niedrige Massenbereich wurde dabei in zwei Unterkategorien geteilt, entsprechend der Anwesenheit bzw. Abwesenheit von zusätzlichen, durch b-Quarks induzierten Jets, um die Empfindlichkeit für spezielle Higgs-Boson-Produktionsmodi zu erhöhen. Im Rahmen dieser Analyse wurden keine Hinweise auf neue Higgs-Bosonen gefunden und daher Ausschlussgrenzen auf den Wirkungsquerschnitt x Verzweigungsverhältnis gesetzt. Ebenfalls wurden die Resultate in MSSM-Benchmark-Szenarien interpretiert.

In dieser Dissertation wird die Suche nach einzeln erzeugten vektorartigen T-Quarks mit einer elektrischen Ladung +2/3 und Y-Quarks mit einer elektrischen Ladung von -4/3 vorgestellt. Diese exotischen Teilchen werden von verschiedenen Modellen jenseits des Standardmodells vorhergesagt, welche versuchen das Verständnis der elektorschwachen Symmetriebrechung zu erweitern und somit das Naturalness Problem zu lösen. Hierfür werden die in 2015 und 2016 bei einer Schwerpunktsenergie von 13 TeV und einer integrierten Luminosität von 36.1 inversen fb vom ATLAS Detektor am LHC aufgezeichneten Proton-Proton-Kollisionen ausgewertet. Die Ereignisselektion ist auf leptonisch zerfallende W-Bosonen der Y/T nach Wb Zerfälle ausgerichtet, weshalb ein hochenergetischer, von einem Bottom-Quark stammender Jet, ein einzelnes Lepton, ein in Vorwärtsrichtung emittierter Jet sowie fehlende transversale Energie, neben verschiedenen Isolationskriterien, verlangt wird. Es konnte keine signifikante Abweichung zur Vorhersage des Standardmodells gemessen werden. Aus diesem Grund wurden obere Grenzen for die Kopplungsstärke und den Mischungswinkel von T-Quarks in einem Singulett-Modell sowie Y-Quarks eines (B,Y)-Dublett bzw. (T,B,Y)-Triplett-Modells mit Quarks der dritten Generation des Standard Modells bestimmt. Hierbei wurden erstmalig Interferenzeffekte mit Standardmodellprozessen berücksichtigt. Die obere Grenze für den Mischungswinkel wurden für ein T-Singulett-Modell bei einer Masse von 800 GeV zu |sin(theta_L)|= 0.18, für ein Y eines (B,Y)-Dublett-Modells zu |sin(theta_R)|= 0.17 sowie für ein Y eines (T,B,Y)-Triplett-Modells zu |sin(theta_L)|= 0.16 bestimmt. Die ermittelten Grenzen innerhalb des (B,Y)-Dublett-Modells im Massenbereich 800 GeV bis 1250 GeV sind zu indirekten, aus elektroschwachen Präzisionsvariablen abgeleiteten Schranken kompetitiv. / This thesis presents a search for singly produced vector-like T quarks with charge +2/3 or Y quarks with charge -4/3. These exotic particles are predicted by several beyond-the-standard-model theories which try to shed light on the mechanism of electro-weak symmetry breaking, thus addressing the Naturalness Problem. The analysis is performed on a proton-proton collision data set with an integrated luminosity of 36.1 inverse fb at a centre-of-mass energy of 13 TeV recorded in 2015 and 2016 by the ATLAS detector at the LHC. The focus is on leptonically decaying W bosons from Y/T to Wb decays of the heavy quarks and the event selection requires a jet leading in transverse momentum and being b-tagged, exactly one lepton, a forward jet as well as missing transverse energy beside several isolation requirements. No significant deviation from the expected Standard Model background is observed. Therefore, upper limits on the coupling strength and mixing angle between third generation Standard Model quarks and a T from a singlet model as well as a Y from a (B,Y) doublet or a (T,B,Y) triplet model are computed using the CLs method. This search represents the first analysis in which the interference effects with the Standard Model background are taken into account. The smallest upper limits are found to be |sin(theta_L)|= 0.18 for a T singlet for a mass of 800 GeV, |sin(theta_R)|= 0.17 in the (B,Y) doublet case at 800 GeV and |sin(theta_L)|= 0.16 for the (T,B,Y) triplet model. The observed limits on the mixing parameter |sin(theta_R)| are competitive with indirect bounds from electroweak precision observables in the mass range between about 800 GeV and 1250 GeV.

This thesis presents an analysis of misidentified leptons in the Higgs boson decaychannel H → W W ∗ → lνlν. Misidentified leptons, resulting from jets misidentifiedas leptons, mimic the signal of a Higgs boson decay, resulting in a backgroundcontribution to the signal. The analysis is performed on proton-proton collisions ata center-of-mass energy of 13.6 TeV recorded by the ATLAS experiment at CERN’sLarge Hadron Collider. The estimation of misidentified leptons is done using theso-called Fake Factor method, which is used to assess the contributed backgroundwith misidentified leptons in the signal region. The Fake Factor values increasewith increasing momenta for misidentified electrons while it remains constant formisidentified muons. An analysis of the impact parameters d0 and z0 show thatthe accuracy of the Monte Carlo simulation to correctly predict the contribution ofmisidentified leptons from heavy quarks is high. / Denna avhandling presenterar en analys av felaktigt identifierade leptoner i Higgsbosonens sönderfallskanal H → W W ∗ → lνlν. Felaktigt identifierade leptoner frånhadroner imiterar signalen av ett sönderfall från en Higgs boson, vilket resulterar ibakgrund till signalen. Analysen utförs på proton-proton kollisioner med energier på13.6 TeV som samlats in av ATLAS-experimentet vid CERNs Large Hadron Collider.Denna avhandling fokuserar främst på uppskattningen av felaktigt identifieradeleptoner genom användningen av Fake Factor-metoden, vilket används för att bedömaden bidragande bakgrunden med felidentifierade leptoner i signalregionen. Värdenaför Fake Factors ökar med ökande rörelsemängd för felidentifierade elektroner medanför det förblir konstant för felidentifierade muoner. En analys av ”impact parameters”d0 och z0 visar att Monte Carlo-simuleringens noggrannhet att uppskatta antaletfelidentifierade leptoner från tunga kvarkar är hög.

The Standard Model predicts existence of one Higgs boson with combined parity CP = +1. In MSSM there exist Higgs boson with CP = −1 in addition. The work develops one method of Higgs boson CP determination on the basis of angular correlations of pions and ρ-mesons born in cascade decay H/A → τ− τ+ → ρ− /π− ντ ρ+ /π+ ¯ντ . The calculations are done in the leading order of perturbation theory. Further, the possibility of signal (Higgs boson decay) and background (Z boson decay) differentiation is studied. The processes in question are simulated using Monte Carlo generators Pythia and Tauola. Simulation outputs are compared with calculated theoretical results. 1

Semileptonic B decays of the type Bq⁰→Dq⁻μ⁺ν (where Dq⁻→K⁻K⁺pie⁻) are selected and their lifetimes are corrected using a statistical simulation-based correction called the k-factor. Using 1 fb⁻¹ of LHCb data the B⁰ and Bs⁰ mixing frequencies are measured to be Deltamd = ( 0.503 ± 0.011 (stat) ± 0.013 (syst) ) ps⁻¹ and Deltams = ( 17.93 ± 0.22 (stat) ± 0.15 (syst) ) ps⁻¹. We exclude the null hypothesis of no mixing for the B⁰ and Bs⁰ by 5.8 and 13.0 standard deviations respectively. This is the first observation of Bs Bsbar mixing using only semileptonic B decays. The lepton flavour violating decay D⁰→eμ is searched for, using tagged D⁰ decays from D*→D⁰pie, and the measurement is normalised using D⁰→K⁻pie⁺ decays. No evidence is seen of an excess over the expected background and so a limit is placed B(D⁰→eμ) < 1.3×10⁻⁸ at a 90% confidence level using 3 fb⁻¹ of LHCb data. This improves the previous measurement by a factor of 20 and is the world's best measurement. Possible upgrades to the LHCb VELO detector are simulated and aspects of the upgraded detector are optimised to ensure that all tracks within the angular acceptance can be detected with high precision. Finally the simulated performance of the current and upgraded VELO detectors are compared.

Enquanto todos os modelos com neutrinos massivos de Majorana levam ao mesmo operador efetivo de dimensão d = 5, que não conserva número leptônico, os operadores de dimensão d = 6, obtidos a baixas energias, conservam número leptônico e são diferentes dependendo do modelo de alta energia da nova física. Derivamos os operadores de dimensão d = 6 que são característicos de modelos Seesaw genéricos, no qual a massa do neutrino resulta do intercâmbio de campos pesados que podem ser tanto singletos fermiônicos, tripletos fermiônicos ou tripletos escalares. Os operadores resultantes podem conduzir a efeitos observáveis no futuro próximo, se os coeficientes dos operadores de dimensão d = 5 e d = 6 são desacoplados. Neste trabalho apresentamos o modelo violação mínima de sabor no contexto do modelo seesaw tipo III, no qual é possível obter tal desacoplamento. Isto permite reconstruir a estrutura de sabor a partir dos valores das massas dos neutrino leves e dos parâmetros de mistura, mesmo na presença de fases de violação CP. / While all models of Majorana neutrino masses lead to the same dimension five effective operator, which does not conserve lepton number, the dimension six operators induced at low energies conserve lepton number and differ depending on the high energy model of new physics. We derive the low-energy dimension six operators which are characteristic of generic Seesaw models, in which neutrino masses result from the exchange of heavy fields which may be either fermionic singlets, fermionic triplets or scalar triplets. The resulting operators may lead to effects observable in the near future, if the coefficients of the dimension five and six operators are decoupled. In this work we present the model of minimal avor violation in the context of the type III seesaw model, in which it is possible to obtain the decoupling mentioned before. This allows to reconstruct the avour structure of the model from the values of the light neutrino masses and mixing parameters, even in the presence of CP-violating phases.

La désintégration $B_{ (s)}$ → τμ est supprimée dans le SM, où le nombre leptonique est conservé. Son observation serait donc une preuve non ambiguë de la physique au-delà du SM. Des résultats récents [1,2] ont ravivé l'intérêt pour la recherche de tels processus [3]. Cette thèse présente la recherche des désintégrations $B_{ (s)}$→ τμ dans l'expérience LHCb, l'une des 4 grandes expériences menées au Large Hadron Collider (LHC) du CERN.Le lepton τ se désintègre avant d'atteindre le détecteur LHCb et est reconstruit en utilisant le canal τ → πππν. Le neutrino provenant de la désintégration du τ échappe à la détection. Une technique de reconstruction spécifique est utilisée pour déduire l'énergie du neutrino et donc la masse invariante du méson B qui s'est désintégré.Afin de séparer le signal du bruit de fond, une sélection hors ligne composée de différentes étapes est appliquée. Des techniques d'analyse multivariées, telles que les arbres de décision boostés (BDT), sont utilisées pendant le processus de sélection.La stratégie d'analyse est complétée par un ajustement simultané à la distribution de masse invariante des mésons B dans différentes régions d'un BDT final. Selon les prédictions du SM, aucun événement de signal n'est attendu. Dans ce cas, la méthode CLs sera utilisée pour extraire les limites supérieures des rapports de branchement BR($B_{ (s)}$→ τμ).[1] Test of lepton universality using $B^+$→$K^+$$l ^+$$l ^-$ decaysPhys. Rev. Lett. 113, 151601 (2014)[2] Measurement of the ratio of branching fractions BR(B → D*τ ν)/BR(B → D*μν)Phys. Rev. Lett. 115 (2015) 111803[3] Lepton Flavour Violation in B Decays ? Phys. Rev. Lett. 114, 091801 (2015) / The decay $B_{ (s)}$→τμ is suppressed in the SM, in which lepton flavour is conserved. Its observation would therefore be an unambiguous evidence of physics beyond the SM. Recent results [1,2] revived the interest for the search of such processes [3]. This thesis presents the search for the $B_{ (s)}$→τμ decays within the LHCb experiment, one of the 4 large experiments operated at the CERN Large Hadron Collider (LHC).The τ lepton decays before reaching the LHCb detector and is reconstructed using the τ→πππν channel. The neutrino from the τ decay escapes detection. A specific reconstruction technique is used in order to infer the energy of the ν and thus the invariant mass of the decaying B meson. This way, the complete kinematics of the process can be solved up to a two fold ambiguity.In order to disentangle signal from background, an offline selection consisting of different steps is applied. Data driven and multivariate analysis techniques, such as Boosted Decision Trees (BDT), are used during the selection process. The analysis strategy is completed by a simultaneous fit to the B meson invariant mass distribution over the different bins of a final BDT. According to the SM expectations, no signal events should be observed. In this case, the CLs method will be used to extract the upper limits on the branching fractions.[1] Test of lepton universality using $B^+$→$K^+$$l ^+$$l ^-$ decaysPhys. Rev. Lett. 113, 151601 (2014)[2] Measurement of the ratio of branching fractions BR(B → D*τ ν)/BR(B → D*μν)Phys. Rev. Lett. 115 (2015) 111803[3] Lepton Flavour Violation in B Decays ? Phys. Rev. Lett. 114, 091801 (2015)

The spin of the proton is thought to be produced by several constituents including quarks, antiquarks, and gluons. It has been the goal of the spin physics program at RHIC (Relativistic Heavy Ion Collider) to measure the contribution from gluons and various sea quarks to the proton spin. The Forward GEM Tracker (FGT) is a detector in STAR (Solenoidal Tracker at RHIC), which is one of two large experiments and is located at the RHIC collider at Brookhaven National Laboratory. The FGT serves to locate the position of forward-going charged particles, and it will be instrumental in observing the trajectories of decay leptons from W bosons, created by the interaction of the quark of one proton and the antiquark of another in a proton-proton collision. This position information will be helpful in determining the contribution of the quarks and antiquarks to the spin of the proton. This analysis investigates the initial performance of the FGT. Studies were conducted using cosmic-ray test data collected during the detector’s construction and more recent data from polarized-proton collisions within STAR. The goals of these studies were: to see if the detector works, see how well it works, check for any odd behavior, and optimize the performance of the detector. / Department of Physics and Astronomy

We investigate the decay mode τ − → π − π + π − ηντ , where the η subsequently decays to π + π − π 0 using 232 fb−1 data acquired by the BABAR detector. The branching fraction of τ − → π − π + π − ηντ is found to be (1.88 ± 0.14 ± 0.11) × 10−4 . The first error on the is measurement is purely statistical and the second error is estimated systematic error. This measurement is consistent with the prior experimental mesaurements at CLEO and BABAR.

A search is done on the entire BaBar data set for the neutrino-less tau decay tau->e gamma. No evidence for the decay is found and a 90% confidence level upper limit is determined to be 3.3 * 10^(-8) including systematic uncertainty. This measurement is a factor of three improvement from the previous best result from BaBar.