Cette thèse est composée de deux analyses principales: la recherche du boson the Higgs (H) produit en association avec an boson vecteur (VH) dans le canal de désintégration en deux quark-b, dans le Modèle Standard (SM), avec les données du LHC Run-1 (jusqu’à Juillet 2016); la recherche d’un boson d’haute masse A, CP-odd et pseudo-scalar, dans la désintégration dans une paire ZH, avec la désintégration du H en deux quark-b, dans les données du LHC Run-2 (jusqu’à Décembre 2015). Le document est constitué de quatre Chapitres générales et trois Chapitres d’analyse: on donne une description théorétique de la physique du particule, une présentation du LHC et du détecteur ATLAS, un exposé des méthodes statistiques utilisés et des techniques de reconstruction des évènements dans ATLAS. Les Chapitres d’analyse présentent: la recherche des processus VH SM avec les données du Run-1 et du Run-2, dans le canal de désintégration en deux quark-b. En plus on présente la recherche du boson d’haute masse A, avec les données du LHC Run-2. On n’observe pas une mis en évidence du boson de Higgs SM dans ce canal de désintégration, avec une sensibilité de 1.4(0.42) déviations standard pour l’analyse des données du Run-1(Run-2). L’analyse des données du Run-1 obtient la plus hautes sensibilité attendue (2.6 déviations standard) pour ce type de recherche dans un collisionneur hadroniques. L’analyse des données du Run-2 est la première recherche de ce processus a une énergie de 13 TeV. Aucune évidence d’un boson d’haute-mass A est trouvée dans l’analyse des données du Run-2: on donné une interprétation des limites su sa production et désintégration dans le contexte des Two Higgs Double Models. / This thesis is composed of two main separate analyses: the search for the Standard Model (SM) Higgs boson in the associated production mode (VH), decaying to b-quark pairs, with data collected during the LHC Run-1 and Run-2 (until July 2016); the search for a high-mass CP-odd pseudoscalar A boson decaying to ZH pairs, with the H decaying to bottom-quark pairs, with data collected during the LHC Run-2 (until December 2015). The document is divided into four general Chapters, and three analysis Chapters: starting from an overview of the theoretical aspects of particle physics, to a description of the LHC and the ATLAS detector, an outline of the statistical methods used for data analysis and a summary of event reconstruction techniques. The analysis Chapters cover separately: the analysis of the LHC Run-1 data for the search of the VH production of the Higgs boson in the bottom-quark pair decay channel, and the same analysis performed with the LHC Run-2 data. Furthermore the search for a high-mass CP-odd pseudoscalar A boson, performed with LHC Run-2 data, is described. No evidence for a SM Higgs boson decaying to bottom-quark pairs is observed, with a significance of 1.4 (0.42) standard deviations from the analysis of LHC Run-1 (Run-2) data. The Run-1 data analysis provides the highest expected sensitivity (2.6 standard deviations) among Higgs searches in this channel at hadron colliders, while the analysis of Run-2 data is the first search performed at a center of mass energy of 13 TeV. No evidence for a massive A boson is found in the analysis of the Run-2 data, the limits set on its production and decay are interpreted in the framework of Two Higgs Doublet Models.

Searches are presented for narrow heavy resonances decaying to a top quark and a bottom quark using data collected by the CMS experiment at a center-of-mass energy of 13 TeV. Final states that include a single lepton (electron, muon), multiple jets, and missing transverse momentum are analyzed. No evidence is found for the production of a W' boson, and the production of right-handed W' bosons is excluded at 95% confidence level for masses up to 3.6 TeV depending on the scenario considered. Exclusion limits for W' bosons are also presented as a function of their coupling strength to left- and right-handed fermions. These limits on a W' boson decaying via a top and a bottom quark are the most stringent published to date. Projections for future searches with an integrated luminosity of up to 3 inverse attobarns are also presented, and suggest that W' boson masses above 4 TeV could be excluded.

Despite making up over 80% of the matter in the universe, very little is known about dark matter. Its only well-established property is that it interacts gravitationally, but does not interact with ordinary matter through any of the other known forces. Specific details such as the number of dark matter particles, their quantum properties, and their interactions remain elusive and are only loosely constrained by experiments. In this dissertation I describe a novel search for a particular type of dark matter that couples preferentially to heavy quarks, using LHC proton-proton collisions at ATLAS. With a model-independent framework, comparisons are made to results obtained from other dark matter searches, and new limits are set on various interaction strengths.

The Standard Model of particle physics is currently the most complete theory of subatomic particles. The discovery of the Higgs boson with a mass of 125 GeV in 2012 further validated the Standard Model, providing evidence for the theory that vector bosons obtain non-zero masses through the Higgs mechanism. Studies are ongoing to determine the exact nature and properties of the Higgs boson. A Higgs boson of this mass is predicted to decay to a pair of b-b quarks with a branching ratio of 58%, however this decay mode has not yet been observed. This thesis presents a search for the associated production of a Higgs boson with a leptonically decaying W boson, WH → ℓvb-b, using 20.3 fb-1 of Run 1 data collected by ATLAS at the LHC from pp collisions at a centre-of-mass energy of ps = 8 TeV. The observed (expected) significance of a Higgs boson with a mass of 125 GeV for the WH → ℓvb-b process is found to be 2:7σ (1:3σ). The measured cross section in units of the expected Standard Model cross section has a best-fit value of μ = μ/μSM = 2:2+0:67-0:64(stat:)+0:7-0:59(syst:) = 2:2+0:97-0:87. The results are combined with the search for ZH → v-vb-b and ZH → ℓ+ℓ-b-b to provide a best-fit value of μ = μ/μSM = 1:1+0:61-0:56. The start of Run 2 of the LHC in 2015 saw the collision energy being raised to √s = 13 TeV, increasing the probability of particles being produced with a large momentum boost. At these high energies there is also a possibility to discover new particles and interactions. An extension of the Standard Model, the Heavy Vector Triplet (HVT) model, describes new heavy vector bosons W¹ and Z¹, which can decay to pairs of heavy bosons (W, Z or Higgs bosons). If the W0 and Z0 bosons are sufficiently heavy, the hadronic decays of the diboson final states produce boosted jets. In this thesis, methods for identifying hadronically decaying boosted bosons are developed, based on techniques that examine the internal substructure of the jet. Multiple substructure variables are combined into a single discriminant using two machine learning techniques: boosted decision trees and deep neural networks. Simulated events of W¹→WZ → q-qq-q are used to develop these boosted W boson taggers. An improvement in the background rejection power, whilst keeping 50% of the signal, over previous boosted W boson taggers of up to 13%-when using deep neural networks-and 36%-when using boosted decision trees-is obtained. The performance of the new boosted W boson taggers are evaluated in a search for a narrow WW resonances from the decay of a Z¹ with boson-tagged jets in 3.2 fb-1 of pp collisions at √s = 13 TeV collected with the ATLAS detector.

Using 3 fbˉ1 of proton-proton collisions, collected at centre-of-mass energies of √s = 7 and 8 TeV by the LHCb detector, several measurements of charmless B⁰s meson decays are made. A search is also performed for a highly suppressed B⁰ decay. First, the branching fraction of the B0⁰s→ ØØ decay is measured to be B(B⁰s→ ØØ) = (1.84 ± 0.05(stat) ± 0.07(syst) ± 0.11(fs=fd) ± 0.12(norm))X10ˉ⁵; where the third and fourth uncertainties arise from the fragmentation fraction fs/fd and the branching fraction of the normalisation mode. This represents a factor of five reduction in the statistical uncertainty compared to the previous best measurement. An upper limit on the branching fraction of the mode B⁰→ ØØ is set at B(B⁰→ ØØ) < 2.8 X 10ˉ⁸ (90% CL). This is a factor of seven improvement over the previous best measurement. An amplitude analysis of the B⁰s→ ØK+Kˉ decay is performed, wherein first observations of the decay modes B⁰s→ Øf´2(1525) and B⁰s→ ØØ(1680) are made. The branching fraction of the B⁰s→ Øf´2(1525) decay is measured to be B(B⁰s→ Øf´2 (1525)) = (1.63 ± 0.18(stat) ± 0.12(syst) ± 0.29(model) ± 0.17(norm)) X 10ˉ⁶; where the 'model' uncertainty arises from the choice of amplitude model. The longitudinal polarisation fraction of the decay B⁰s→ Øf´2(1525) is measured to be F0 = (86.6 ± 3.4 ± 0.8 ± 2.0 (model))%.

BABAR is a particle physics experiment at the Stanford Linear Accelerator Center (SLAC). The purpose of BABAR is to study matter-antimatter asymmetry in the bottom quark system. At SLAC, electons and positrons collide, which annihilate and decay into a variety of daughters. An Upsilon(4S) meson is one of the possible daughters. An Upsilon(4S) decays into a B meson and an anti-B meson more than 96% of the time. A B meson has an anti-bottom quark and an anti-B meson has a bottom quark. The purpose of this thesis is to measure how many Upsilon(1S) originated from Upsilon(4S) in the entire BABAR data set. This thesis compares on-peak data and off-peak data. On-peak data was taken at center of mass energy 10.58GeV. One of the possible interactions is e+e− -> Upsilon(4S) since the mass of Upsilon(4S) is 10.58GeV/c^2. On-peak data, taken at center of mass energy 10.54GeV, is not enough to have any BB pairs because 10.54GeV is less than the mass of an Upsilon(4S). This thesis can be useful for BABAR physicist because it helps set an upper limit on how many BB pairs there are in the entire BABAR data set. In other words, it sets an upper limit on how much more than 96% does Upsilon(4S) decay to BB. Measurement of the decay of Upsilon(4S) -> Upsilon(1S) + X give evidence for non-BB decays of the Upsilon(4S). The final results of this study show that there were (110 +- 3) × 10^5 Upsilon(1S) on-peak, of which (10 +- 9) × 10^5 originated from an Upsilon(4S). Increasing the centre of mass energy from 10.54GeV to 10.58GeV increases the Upsilon(1S) production by (10 +- 8)%.

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