Dynamical Symmetry Breaking is studied as an alternative to the Standard Model. A set of dynamical equations are derived for quark and vector particle masses. When the symmetry is broken, massless pseudoscalar boundstates appear. New interactions are necessary to explain the physical mass spectrum. A number of possible models for new physics are studied, and some conditions for new interactions are obtained. Numerical result for several interesting models are presented.

The switching on of the Large Hadron Collider (LHC) in March 2010 and its successful operation thereafter has opened doors in the search for new physics beyond the Standard Model. Supersymmetry (SUSY) is one of the leading theories that extends the Standard Model of particle physics. A search for new SUSY particles is presented requiring large numbers of hadronic jets and missing transverse momentum. Novel background estimation techniques were developed specifically for this final state, allowing the control of the backgrounds where the missing transverse momentum is dominated by jet miss-measurement. Other backgrounds are estimated from Monte-Carlo simulations validated and normalised in dedicated control regions. No significant excess was observed in the search. Model independent upper limits on the number of signal events passing the selection are given and the results are also interpreted in two planes of the parameter space. This is the most recent and sensitive incarnation of several searches developed by the author in this channel.

An angular analysis of B⁰d → K⁰(→K⁺π⁻)μ⁺μ⁻ is presented using 20.3 fb⁻¹ of pp collision data collected at √s = 8 TeV with the ATLAS experiment. The angular observables FL and S₃,₄,₅,₇,₈ were extracted in six bins of q², the invariant mass squared of the dimuon system, within the full range 0.04 < q² < 6.00 (GeV/c²)². The observables were determined from an unbinned maximum likelihood fit using four folded parameterisations of the full angular distribution. The fit results were used to obtain corresponding values for the optimised observables P₁ and P'₄,₅,₆,₈. The results presented are compatible with Standard Model predictions.

A first measurement of Electroweak W + 2 jets production at high dijet mass is performed using sqrt(s) = 7 TeV pp collision data from the Atlas experiment corresponding to 4.6 fb−1 of integrated luminosity. The background only hypothesis is excluded with a significance of 4.89-σ. A cross section of σ = 325 ± 6 (lumi) ±32 (stat) +63−70(syst) ±86 (theo) fb is extracted in the fiducial region.

The Large Hadron Collider at CERN completed its first data-taking phase in 2013, after three years of remarkable performance. The high-energy proton-proton collisions recorded by the ATLAS experiment provide a gateway to the world of subatomic particles. This thesis presents two analyses of the full 8 TeV dataset taken by ATLAS, inspired by two of the major physics goals of the experiment. The first analysis is a search for new phenomena that could explain the nature of Dark Matter and solve the hierarchy problem. In particular, the search is optimised to look for heavy supersymmetric particles decaying to large numbers (7 to &ge;10) of jets. The events are further classified according to the number of jets identified as originating from a b quark. No evidence is found for physics beyond the Standard Model, so the results are interpreted in terms of exclusion limits on various simplified supersymmetry-inspired models where gluinos are pair produced, as well as a mSUGRA/CMSSM model. The main background to the search is due to multi-jet production via the strong force. This motivates the second analysis presented in this thesis, which is a measurement of the cross section of four-jet events. The measurement is performed differentially in a series of variables which describe the kinematics and spatial configuration of the events. The results are compared to existing theoretical predictions.

Neste trabalho é proposto um modelo relativístico para o núcleon com graus de liberdade de quarks constituintes na Frente de Luz. Neste modelo fenomenológico, o acoplamento entre o campo dos quarks e do núcleon é expresso em termos de uma Lagrangeana efetiva. Diversos invariantes para este acoplamento são testados nos observáveis eletromagnéticos elásticos. A corrente eletromagnética do núcleon é formulada na aproximação de impulso, e expressa por um diagrama de Feynman triangular, que é projetado no plano-nulo x POT.+ = t + z = 0, através da integração analítica nas energias individuais associadas à translação temporal em x POT.+. Em seguida é introduzida a componente de momento da função de onda no computo da componente \"boa\" da corrente (J POT.+ = J POT.0 + JPOT.3). Comparamos o esquema de acoplamento de spins dos quarks formando o núcleon baseado na Lagrangeana efetiva, com a construção de Bakamjian-Thomas, largamente empregada em modelos de hádrons na Frente de Luz. Esta comparação foi realizada em um modelo do píon como um par qq-. A diferença é explicitada através dos argumentos de momento das rotações de Melosh dos spins individuais. Na construção de Bakamji an-Thomas, os argumentos de momento estão definidos no referencial de repouso do par, enquanto que na formulação Lagrangeana, dependendo de sua forma, pode ter os argumentos de momento no referencial de repouso do píon físico. Também no caso do núcleon discutimos em detalhes essas diferenças. As consequências fenomenológicas das diferentes formas de Lagrangeanas efetivas para o acoplamento dos quarks ao núcleon foram investigadas através das propriedades eletromagnéticas estáticas e fatores de forma. Foram testados os invariantes escalar, vetorial e misto para o acoplamento dos campos dos quarks no par isoescalar. As integrais nos momentos longitudinais e transversos no elemento de matriz da corrente do núcleon, após a redução tridimensional ao plano-nulo, foram realizadas numericamente. O fator de forma elétrico do nêutron depende fortemente da forma da Lagrangeana efetiva, para parâmetros do modelo que ajustem o momento magnético do nêutron. Os observáveis eletromagnéticos, para momentos transferidos quadrados até 1-2 (GeV /c) POT.2, são pouco sensíveis à componente de momento da função de onda. Generalizamos o estudo das correlações entre observáveis eletromagnéticos estáticos do núcleon para diferentes formas de Lagrangeana efetiva. As limitações do modelo em reproduzir os dados experimentais recentes para a razão entre os fatores de forma elétrico e magnético do próton são discutidas e são feitas propostas para melhorar o modelo do núcleon na Frente de Luz. / In this work, it is proposed a relativistic model for the nucleon with constituent quarks in the Light-Front. In this phenomenological model, the coupling between the nucleon and quark fields is expressed by an effective Lagrangean. Several diferent invariants for this coupling are tested in the elastic electromagnetic observables. The nucleon eletromagnetic current is described in the impulse approximation, which is expressed by a triangular Feynman diagram. The elastic amplitude is projected in the null-plane, x+ = t + z = O, through the analytical integration in the individual energies associated with the x+ -time translation. Then, the momentum component of the wave-function is introduced in the calculation of the \"good\"component of the current (J+ = J0 + J3). The quark spin coupling scheme based in the effective Lagrangean is compared to the Bakamjian-Thomas construction largely used in hadron models in the Light-Front. This comparision is performed in a qq- model of the pion. The difference is shown through the momentum arguments of the Melosh rotations of the individual spins. In the Bakamjian-Thomas construction, the momentum arguments are defined in the pair rest-frame. In the effective Lagrangean formulation, the momentum arguments may be defined in the physical pion rest-frame, depending the Lagrangean form. For the nucleon case, we also discussed these differences in detail. The phenomenological consequences of the different forms for the nucleon-quark effective Lagrangean were investigated through the nucleon static eletromagnetic properties and form-factors. The spin invariants for the isoescalar quark pair, given by a scalar, vector and mixed scalar-vector forms were tested in these observables. The longitudinal and transverse momentum integrations in the matrix element of the nucleon current, after the tridimensional reduction to the null-plane, were performed numerically. The neutron electric form-factor depends strongly on the form of the effective Lagrangean for model parameters fitted to the magnetic moment. The electromagnetic observables are quite insensitive to the momentum part of the wave-function for square momentum transfers below 1-2 (GeV/c)2. The study of the correlations between static eletromagnetic observables were generalized to different effective Lagrangeans. The limitation of the model to reproduce the recent experimental data of the ratio between the proton electric form-factor and the magnetic one is discussed and we propose forms to improve the present Light-Front nucleon model.

This thesis is a presentation of an analysis of the $ZZ\rightarrow 4\ell$ ($\ell = e,\mu$) process in proton-proton collisions with centre-of-mass energy $\sqrt{s}$ = 13 TeV at the LHC during 2015 and 2016 (a total integrated luminosity of 36.1 fb$^{-1}$), using the ATLAS detector. Candidate $ZZ \rightarrow 4\ell$ events are selected in the three decay channels, $4e$, $2e2\mu$ and $4\mu$. The cross section of the $ZZ\rightarrow 4\ell$ process is measured in four fiducial regions closely matching the detector acceptance: one for each decay channel ($4e$, $2e2\mu$ and $4\mu$) and one for the combination of all decay channels. The total cross section of $pp\rightarrow ZZ$ is measured in a phase-space in which both $Z$ bosons have a mass $m_{Z}$ in the range 66 GeV $ < m_{Z} < $ 116 GeV to be 16.5 $\pm 0.5$ (stat.) $\pm 0.4$ (syst.) $\pm 0.5$ (lumi.)$\,$ pb which is consistent with a next-to-next-to-leading-order prediction of $16.9^{+0.6}_{-0.5}$ pb. Observed event yields in four bins of transverse momentum are used to set 95{\%} CL$_{\text{s}}$ limits on four neutral triple gauge couplings ($f^{V}_{i}$, $V=Z,\gamma$, $i=4,5$) which parameterise an effective $ZZV$ vertex (assuming both $Z$ bosons are on shell). The obtained limits are of the order of $|f^{V}_{i}|<0.0017$.

Neste trabalho é proposto um modelo relativístico para o núcleon com graus de liberdade de quarks constituintes na Frente de Luz. Neste modelo fenomenológico, o acoplamento entre o campo dos quarks e do núcleon é expresso em termos de uma Lagrangeana efetiva. Diversos invariantes para este acoplamento são testados nos observáveis eletromagnéticos elásticos. A corrente eletromagnética do núcleon é formulada na aproximação de impulso, e expressa por um diagrama de Feynman triangular, que é projetado no plano-nulo x POT.+ = t + z = 0, através da integração analítica nas energias individuais associadas à translação temporal em x POT.+. Em seguida é introduzida a componente de momento da função de onda no computo da componente \"boa\" da corrente (J POT.+ = J POT.0 + JPOT.3). Comparamos o esquema de acoplamento de spins dos quarks formando o núcleon baseado na Lagrangeana efetiva, com a construção de Bakamjian-Thomas, largamente empregada em modelos de hádrons na Frente de Luz. Esta comparação foi realizada em um modelo do píon como um par qq-. A diferença é explicitada através dos argumentos de momento das rotações de Melosh dos spins individuais. Na construção de Bakamji an-Thomas, os argumentos de momento estão definidos no referencial de repouso do par, enquanto que na formulação Lagrangeana, dependendo de sua forma, pode ter os argumentos de momento no referencial de repouso do píon físico. Também no caso do núcleon discutimos em detalhes essas diferenças. As consequências fenomenológicas das diferentes formas de Lagrangeanas efetivas para o acoplamento dos quarks ao núcleon foram investigadas através das propriedades eletromagnéticas estáticas e fatores de forma. Foram testados os invariantes escalar, vetorial e misto para o acoplamento dos campos dos quarks no par isoescalar. As integrais nos momentos longitudinais e transversos no elemento de matriz da corrente do núcleon, após a redução tridimensional ao plano-nulo, foram realizadas numericamente. O fator de forma elétrico do nêutron depende fortemente da forma da Lagrangeana efetiva, para parâmetros do modelo que ajustem o momento magnético do nêutron. Os observáveis eletromagnéticos, para momentos transferidos quadrados até 1-2 (GeV /c) POT.2, são pouco sensíveis à componente de momento da função de onda. Generalizamos o estudo das correlações entre observáveis eletromagnéticos estáticos do núcleon para diferentes formas de Lagrangeana efetiva. As limitações do modelo em reproduzir os dados experimentais recentes para a razão entre os fatores de forma elétrico e magnético do próton são discutidas e são feitas propostas para melhorar o modelo do núcleon na Frente de Luz. / In this work, it is proposed a relativistic model for the nucleon with constituent quarks in the Light-Front. In this phenomenological model, the coupling between the nucleon and quark fields is expressed by an effective Lagrangean. Several diferent invariants for this coupling are tested in the elastic electromagnetic observables. The nucleon eletromagnetic current is described in the impulse approximation, which is expressed by a triangular Feynman diagram. The elastic amplitude is projected in the null-plane, x+ = t + z = O, through the analytical integration in the individual energies associated with the x+ -time translation. Then, the momentum component of the wave-function is introduced in the calculation of the \"good\"component of the current (J+ = J0 + J3). The quark spin coupling scheme based in the effective Lagrangean is compared to the Bakamjian-Thomas construction largely used in hadron models in the Light-Front. This comparision is performed in a qq- model of the pion. The difference is shown through the momentum arguments of the Melosh rotations of the individual spins. In the Bakamjian-Thomas construction, the momentum arguments are defined in the pair rest-frame. In the effective Lagrangean formulation, the momentum arguments may be defined in the physical pion rest-frame, depending the Lagrangean form. For the nucleon case, we also discussed these differences in detail. The phenomenological consequences of the different forms for the nucleon-quark effective Lagrangean were investigated through the nucleon static eletromagnetic properties and form-factors. The spin invariants for the isoescalar quark pair, given by a scalar, vector and mixed scalar-vector forms were tested in these observables. The longitudinal and transverse momentum integrations in the matrix element of the nucleon current, after the tridimensional reduction to the null-plane, were performed numerically. The neutron electric form-factor depends strongly on the form of the effective Lagrangean for model parameters fitted to the magnetic moment. The electromagnetic observables are quite insensitive to the momentum part of the wave-function for square momentum transfers below 1-2 (GeV/c)2. The study of the correlations between static eletromagnetic observables were generalized to different effective Lagrangeans. The limitation of the model to reproduce the recent experimental data of the ratio between the proton electric form-factor and the magnetic one is discussed and we propose forms to improve the present Light-Front nucleon model.

Attempting to find evidence for supersymmetry (SUSY) is one of the key aims of the ATLAS experiment at the Large Hadron Collider. This thesis is concerned with searching for supersymmetry in final states with 2-4 hadronic jets, missing transverse energy and no electrons or muons. In the first part, a search strategy is developed using 1.04 fb−1 of data from the first half of 2011. No excess over the Standard Model expectation is observed, so the data are used to set limits on two SUSY simplified models, in which pair-produced squarks or gluinos decay directly to neutralinos and jets. Good limits are achieved for scenarios where the neutralino is nearly as massive as the squark/gluino, compared to an earlier ATLAS analysis using the same dataset. For example, for pair-production of squarks decaying directly to neutralinos, all neutralino masses below 200 GeV are excluded at 95% confidence level when the squark mass is 300 GeV. Similarly, for pair-produced gluinos, neutralino masses below 300 GeV are excluded when the gluino mass is 400 GeV. The equivalent neutralino mass limits in the earlier analysis are 130 GeV and 240 GeV respectively. In the second part, the performance of the ATLAS missing transverse energy trigger is studied, and its suitability for use in the SUSY search is evaluated. The behaviour is found to be consistent with expectations, and the trigger strategy for 2010 data-taking is described.

The Muon Ionization Cooling Experiment (MICE) is the first technical demonstration of muon ionization cooling, using a prototype section of a Neutrino Factory cooling channel. MICE is currently under construction at the Rutherford-Appleton Laboratory in the UK and will make the first cooling measurements in 2015, in Step IV of the experimental programme. Cooling predictions in Monte Carlo simulations of Step IV were found to disagree with the predictions of the 'cooling formula', a widely-used approximation, by up to 30% in liquid hydrogen (LH2). This disagreement was shown to originate, largely, from the multiple scattering expression used in the cooling formula. It was necessary to go back to the fundamental physics of scattering to derive a more accurate expression that includes scattering from atomic electrons. A modified form of the cooling formula was derived using this expression and gave better agreement with the Monte Carlo in LH2. Predictions are given for the equilibrium emittance, using the new expression, for seven low Z materials at muon momenta of 140, 200 and 240 MeV/c. Theories which predict the distribution of multiple scattering angles are briefly reviewed, focusing on Moliere theory and its variants, which are the most widely-used theories. The distributions predicted by these theories are used in most Monte Carlo codes but their implementation is not transparent, especially regarding the treatment of scatters with atomic electrons, which are important in low Z materials. A simple Monte Carlo model to predict multiple scattering distributions was developed that correctly treats scatters off electrons. The model gives very good agreement with measurements by the MuScat Experiment. Investigations were made into the possibility of measuring multiple scattering in MICE Step IV, both with and without the magnetic field. Preliminary results suggest that measurements are easier with no magnetic field, where tracks are straight. Corrections to account for the resolution of the scintillating-fibre trackers are required in both cases, but these are substantially smaller when straight tracks are used.