The electroweak standard model, as well-known, has been remarkably successful in describing a broad spectrum of high energy physics and has also given a satisfactory and consistent theoretical description of all the experimental data which has been obtained so far. The aim of the present work is to investigate some of the most important processes associated with the standard model which involve the charged intermediate bosons W(^±), the neutral intermediate boson Z, the pair production of weak gauge bosons and the Higgs boson. First of all we start with the study of the effect of the strong interaction on the direct production of weak gauge boson production at hadron colliders. We calculate the first order perturbative QCD corrections O(a(_s)) to the lowest order cross sections for photon and Z pair production at hadron colliders. The calculation contains divergences, represented in dimensional regularisation by poles of order O(1/ϵ) and O(1/ϵ(^2)), where ϵ = 4-n(_2). The order O(1/ϵ(^2)) terms are eliminated when real and virtual corrections are combined, and the remaining 0(1/ϵ) corrections will be absorbed into the quark momentum distribution functions. We then study the production rate of weak gauge boson pairs for proton- proton and proton-antiproton colliders using single and double parton scattering mechanisms. We extrapolate to the next generation of hadron colliders with centre-of-mass energy E(_cm)= 10-200 TeV. We find comparable values for the single and double scattering cross sections at very high energies. To expand our investigation, we study also the production rate of multiple gauge bosons at hadron supercolliders using the double parton scattering mechanism. These kinds of processes are important and represent a potential significant background for Higgs boson production. Finally, and for completeness, we study Higgs boson phenomenology, which now forms a substantial part of the physics program at the next generation of high-energy colliders. The calculation covers the production rate of single and double Higgs production at hadron colliders via gluon-gluon fusion, their branching ratios and decay widths.

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.

A search for a light charged Higgs boson (H^±) decaying into two jets (c¯s) using pp collisions at s = sqrt(7 TeV) is presented. A data sample corresponding to an integrated luminosity of 4.7 fb^−1 recorded by the ATLAS detector in 2011 has been analysed. The search is performed in the semi-leptonic t¯t channel, where one of the top quarks decays via t --> bH^±. The other top quark decays to bW, where the W boson decays to a lepton (e/μ) and a neutrino. The search is based on the invariant mass distribution of the two light jets in the final state as the discriminating variable. With no observation of an H^± signal, 95% confidence level(CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons. These limits are between 5% and 1% for charged Higgs boson masses between 90 and 150 GeV, and can be considered as model-independent limits on the decay branching ratio of top quarks to any charged boson beyond the Standard Model.

In the Standard Model of particle physics, the non-zero masses of the W and Z bosons and the fermions are generated through interactions with the Higgs field, excitations of which correspond to Higgs bosons. Thus, the experimental discovery of the Higgs boson is of prime importance to physics, and would confirm our understanding of fundamental mass generation. This thesis describes a search for the gg → H → WW → lνlν process of Higgs boson production and decay. It uses the LHC Run I dataset of pp collisions recorded by the ATLAS detector, which corresponds to an integrated luminosity of 4.5 fb⁻¹ at √s = 7 TeV and 20.3 fb⁻¹ at √s = 8 TeV. An excess of events is observed with a significance of 4.8 standard deviations, which is consistent with Higgs boson production. The significance is extended to 6.1 standard deviations when the vector boson fusion production process is included. The measured signal strength is 1.11^+0.23<sub style='position: relative; left: -2.1em;'>-0.21</sub> at m_H = 125 GeV. A cross section measurement of WW production, a major background to this search, is also presented using the √s = 7 TeV dataset only.

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Since the observation of a narrow mass resonance consistent with the Higgs boson by ATLAS and CMS collaborations in 2012, a number of important studies have been made in order to understand the properties of the newly discovered particle. The most fundamental precision measurements include the Higgs coupling to other particles and itself; properties that have a direct relation with the total decay width. The theoretical total width of the Standard Model (SM) Higgs boson is extremely small (4.2 MeV), making its direct measurement by the LHC experiments non feasible due to the finite experimental detector resolution. Recent publications have shown a novel way to set an indirect limit on the total Higgs boson width by using measurements of both off-shell and on-shell production. This thesis presents a determination of the off-shell Higgs boson coupling, and a further interpretation of the Higgs total width in the H → ZZ → 4l channel (l = e; μ). The results are based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb-1 at a collision energy of √s = 8 TeV. Using the CLs statistical method and assuming the same higher-order QCD corrections applied for both signal and background processes, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is 7.3 (with the yields normalized to the SM expectation). Similarly, the 95% CL upper limit on the Higgs total width is 24.7 MeV. The LHC will undergo its last big upgrade in 2021, in preparation for the high-luminosity LHC Run (HL-LHC), with a luminosity increase of approximately a factor of 5 beyond its nominal design rate. Raising the muon transverse momentum threshold becomes a necessity in order to maintain a low online selection rate with the existing trigger system, at the cost of a reduced efficiency for the electroweak scale physics. An alternative to this approach is a proposed design of a first-level hardware trigger that uses tracking information. Being able to use tracking information at the first level of the ATLAS trigger in the implementation of a muon isolation algorithm offers an extra handle for differentiating between signal and background. The second part of this thesis presents studies on the performance of tracking-based muon isolation designed for a first-level hardware trigger system. These studies demonstrate the improved trigger performance of the muon isolation algorithm when compared to an increase of the transverse momentum threshold of the muon candidates.

<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.

The rate of decay of the process W -> Wγ is well-defined by the Standard Model, so measured deviation is a strong but indirect indicator of non-Standard Model physics. Deviations of this coupling strength can be parameterized, in a model independent, general sense, into two parameters, Δk and λ.

The analysis of the Higgs boson decay into the pairs of tau leptons H → ττ is presented. In the scope of this thesis, only the tau leptons decaying purely lep- tonically are considered. This analysis is performed using data recorded from the proton-proton collisions at a center-of-mass energy √ s = 13 TeV by the ATLAS detector at the Large Hadron Collider and correspond to the integrated lumi- nosity of 36.1 fb−1 . The Boosted Decision Trees (BDT) multivariate technique was used in order to provide a classification of events. Datasets considered in this analysis consisted of the triggered, reconstructed and calibrated events. In addition, events had to pass the pre-selection criteria and were separated into four categories. Our main contribution to this analysis is based on developing and incorporating techniques used in the process of BDT training and evaluation. The performed analysis presented in this thesis is blinded. Therefore, the Monte Carlo simulation was used instead of real data in the Maximum Likelihood Fit in order to determine the expected significance. For the H → ττ leptonic channel, the expected signal significance was measured to be 1.16 σ. 1