This thesis presents two analyses of data recorded by the ATLAS detector during proton-proton collisions at the LHC. The first is the implementation of a vertex counting algorithm to measure the luminosity recorded by ATLAS during collisions at a centre-of-mass energy of √s = 8 TeV in 2012. This comprises a Monte Carlo closure test for validation of the method and its corrections, the calibration of the method using the van der Meer scans performed in 2012 and the application of the method to physics runs. It also includes tests of the internal and external consistency of the algorithm and the potential to use this algorithm to measure the luminosity of data collected during proton-proton collisions at √s = 13 TeV.The second analysis is the measurement of the inclusive and purely electroweak production of dijets in association with a Z boson, performed using the 3.2 fb−1 of data collected during collisions at a centre-of-mass energy of √s = 13 TeV in 2015. Cross-section measurements are presented for five fiducial regions, each of which has a different sensitivity to the electroweak component of the Zjj production. Data and Monte Carlo predictions are compared and found to be in reasonable agreement for most cases. The electroweak Zjj production cross-section is then extracted in a fiducial region where this contribution is enhanced. This measurement is also in good agreement with the Monte Carlo prediction. These first 13 TeV measurements will set the scene for studies of weak boson fusion, both within the Standard Model and in new phenomena searches, which will become even more important in Run 2 and the future of the LHC due to the electroweak sector not being as constrained yet, compared to the strong sector, and due to the larger enhancements as a result of a higher √s, where electroweak physics can be most easily extracted.

This thesis investigates the prospects of measuring anomalous triple gauge boson couplings in the ATLAS detector at the Large Hadron Collider (LHC). The most general(V ZZ, V = Z, g) vertex is parametrised by four couplings, fV = Z,gi=4,5 , all of which are zero in the Standard Model. Non-zero couplings would manifest themselves as an excess of events in ZZ diboson channels, and, if observed, would be a direct probe of new physics beyond the Standard Model. A set of criteria are outlined to select events recorded by ATLAS in two such channels, ZZ to llll(l = e, mu) and ZZ to llnunu. With 1 fb-1 of integrated luminosity at a centre of mass energy of sqrt(s) = 7 TeV, ATLAS can expect to observe 10+/-1 events in the ZZ to llllchannel, with 0.5+0.9-0.2 background events. In the ZZ to llnunu channel, 6.2+/-0.7 signal events are expected, with a background of 1.9+2.0-0.2 events. The expected sensitivity of ATLAS to non-zero anomalous couplings is calculated by performing extended, unbinned maximum-likelihood fits to the Z boson transverse momentum spectrum. For 1 fb-1 of integrated luminosity at sqrt(s) = 7 TeV, ATLAS has the potential to place constraints on the coupling parameters of |fZi | < 0.06 and |fig | < 0.07 at the 95% confidence level. These limits assume a form factor with a cutoff of Lambda_FF = 1.2 TeV. As a prelude to ZZ observation, criteria are defined to select Z to ll(l = e, mu) events in the first 315 nb-1 of ATLAS pp collision data at sqrt(s) = 7 TeV. In total 57 events are observed in the electron channel, with 109 in the muon channel, leading to cross-section measurements of sigma (Z to ee) = 0.70+/-0.09 (stat)+/-0.10 (syst)+/-0.08 (lumi) pband sigma(Z to mumu) = 0.90+/-0.09 (stat) +/-0.07 (syst) +/-0.10 (lumi) pb, both of which are consistent with the Standard Model predictions. In addition, this thesis presents a summary of developments made to the Data Acquisition (DAQ) system of the ATLAS Semiconductor Tracker (SCT). These include the construction of a test system, involving a scaled-down version of the entire SCT readout chain. The test system was subsequently used to develop a number of new DAQ features, including a hardware-based event simulator and monitoring framework.

No description available.

Thesis (Ph.D.)--Boston University / In this dissertation I use lattice gauge theory to study models of electroweak symmetry breaking that involve new strong dynamics. Electroweak symmetry breaking (EWSB) is the process by which elementary particles acquire mass. First proposed in the 1960s, this process has been clearly established by experiments, and can now be considered a law of nature. However, the physics underlying EWSB is still unknown, and understanding it remains a central challenge in particle physics today. A natural possibility is that EWSB is driven by the dynamics of some new, strongly-interacting force. Strong interactions invalidate the standard analytical approach of perturbation theory, making these models difficult to study. Lattice gauge theory is the premier method for obtaining quantitatively-reliable, nonperturbative predictions from strongly-interacting theories. In this approach, we replace spacetime by a regular, finite grid of discrete sites connected by links. The fields and interactions described by the theory are likewise discretized, and defined on the lattice so that we recover the original theory in continuous spacetime on an infinitely large lattice with sites infinitesimally close together. The finite number of degrees of freedom in the discretized system lets us simulate the lattice theory using high-performance computing. Lattice gauge theory has long been applied to quantum chromodynamics, the theory of strong nuclear interactions. Using lattice gauge theory to study dynamical EWSB, as I do in this dissertation, is a new and exciting application of these methods. Of particular interest is non-perturbative lattice calculation of the electroweak S parameter. Experimentally S ~ -0.15(10), which tightly constrains dynamical EWSB. On the lattice, I extract S from the momentum-dependence of vector and axial-vector current correlators. I created and applied computer programs to calculate these correlators and analyze them to determine S. I also calculated the masses and other properties of the new particles predicted by these theories. I find S > 0.1 in the specific theories I study. Although this result still disagrees with experiment, it is much closer to the experimental value than is the conventional wisdom S > 0.3. These results encourage further lattice studies to search for experimentally viable strongly-interacting theories of EWSB.

Malgré son impressionnant succès, le modèle standard de la physique des particules ne rend pas compte de toutes les observations expérimentales. De plus, des interrogations théoriques intrinsèques à sa formulation demeurent et requièrent des explications plus fondamentales. Par conséquent, au lieu d'être une théorie ultra-violette, le modèle standard doit plutôt être vue comme une description effective, valide à basse énergie, d'une théorie plus fondamentale et de la nouvelle physique devrait être introduite. On considère deux approches pour introduire la nouvelle physique. Dans la première, on étend le modèle standard avec seulement quelques nouveaux états en assumant que les états plus lourds sont découplés de l'échelle électro-faible. Cette approche est principalement phénoménologique car par hypothèse, les extensions que l'on considère sont une manifestation à basse énergie d'une théorie plus complète. On se concentre en particulier sur de nouveaux fermions car ils sont une prédiction commune à un ensemble de théories au delà du modèle standard fortement motivés. L'objet principal de cette approche est d'étudier l'effet de ces nouveaux fermions à la lumière des mesures récentes des couplages du Higgs. La seconde approche est plus théorique, elle est basée sur une théorie ultra-violette. On se concentre sur le modèles de Higgs composite qui ont pour vocation de résoudre le problème de hiérarchie de l'échelle électro-faible. Les modèles de Higgs composite étant réalisés dans le cadre d'un nouveau secteur fortement couplé constitué de nouveaux fermions fondamentaux, on étudie la dynamique non-perturbative avec le modèle de Nambu et Jona-Lasinio. On dérive par exemple dans ce contexte le spectre des résonances les plus légères que l'on pourra peut-être observer dans un proche futur au LHC ou à d'autre collisionneurs. / Despite its impressive success, the standard model of particle physics does not account for every experimental observations.In addition, it has some theoretical puzzles in its formulation that seem to require more fundamental explanations.Then, instead of an ultra-violet complete theory, the standard model should rather be viewed as an effective description of a more fundamental theory and new physics should be introduced.We follow two different approaches of new physics.In the first one, we extend the standard model by only few new states assuming that the heavier states are decoupled from the electro-weak scale.This approach is mostly phenomenological as by hypothesis the extensions that we consider are a low energy manifestation of a more complete theory.We particularly focus on new fermions as they are a common prediction of a lot of well-motivated beyond the standard model theories.The main purpose is to study the effect of these new fermions in light of the recent Higgs couplings measurements.%The second approach has a more theoretical origin and is based on an ultra-violet complete theory.We focus on composite Higgs models that aim to solve one theoretical puzzle of the standard model, that is, the hierarchy problem of the electro-weak scale.The composite Higgs paradigm being realised by a new strongly interacting sector made of new fundamental fermions, we study the non-perturbative dynamics using the Nambu and Jona-Lasinio model.We derive for instance the spectrum of the lightest resonances which could be observed in a near future at the LHC or at some other colliders.

One of the main motivating factors for the construction of the Large Hadron Collider (LHC) was the search for the Higgs boson, postulated to explain the origin of fundamental gauge boson masses. This thesis presents the results of the first search for the Higgs boson at the LHC, using 35 pb^−1 of proton-proton collision data with a centre of mass energy of 7 TeV collected by the ATLAS experiment throughout 2010. The search is performed in the H -> WW channel, since the branching ratio for Higgs boson decays to W bosons is large for a wide range of Higgs boson masses. Two different search methods are presented: a straightforward cut-based method and a method using a matrix-element-based discriminant to provide additional separation between signal and background. The matrix element method is shown to give better expected sensitivity at all Higgs boson masses. Using these methods, a SM-like Higgs boson with a mass of 160 GeV with a production rate of 1.2 times the SM rate is excluded at 95% Confidence Level and limits are placed on the production rate of the SM Higgs boson in the range of masses from 120 < mH < 200 GeV. In addition, a measurement of the SM WW cross section is performed. It is essential to understand this channel since it is the major background to the H -> WW search. SM WW production is also sensitive to new physics processes, which would enhance its cross section. The SM WW cross section is measured to be σ(WW) = 40+20−16(stat.)±7(syst.) pb, which is consistent with the NLO SM expectation of 46 ± 3 pb.

In 2012 a resonance with a mass of 125 GeV resembling the elusive Higgs boson was discovered simultaneously by the ATLAS and CMS experiments using data collected from the Large Hadron Collider (LHC) at CERN. Its observation finally confirms the mechanism for Spontaneous Electroweak Symmetry Breaking (EWSB) necessary for describing the mass structure of the electroweak (EW) gauge bosons. In 2013, Peter Higgs and Francois Englert were awarded the Nobel Prize in physics for their work in developing this theory of EWSB now referred to as the Higgs mechanism. The explanation for EWSB is often referred to as the last piece of the puzzle required to build a consistent theory of particle physics known as the Standard Model. But does that mean that there are no new surprises to be found? Many EW processes have yet to be measured and are just starting to become accessible with the data collected at the LHC. Indeed, this unexplored region of EW physics may provide clues to as yet unknown new physics processes at higher energy scales. Using the 2012 LHC data recorded by the ATLAS experiment, we seek to make the first observation of one such EW process, the massive tri-boson final state: WWW. It represents one of the first searches to probe the Standard Model WWWW coupling directly at a collider. This search looks specifically at the channel where each W boson decays to a charged lepton and a neutrino, offering the best sensitivity for making such a measurement. In addition to testing the Standard Model directly, we also use an effective field theory approach to test for the existence of anomalous quartic gauge couplings which could offer evidence for new physics at higher energies than those produced by the LHC.

No description available.

A complete renormalization scheme for the Glashow-Salam-Weinberg model is presented. As input parameters the scheme uses the fine structure constant, α, the muon decay constant, G_μ and the Z° mass, M_Z which are known or will be measured to high accuracy in the near future. These are used along with the Higgs and fermion masses to obtain the W mass, M_W. The full one-loop weak radiative corrections to the longitudinal polarization asymmetry, A_pol, and the forward-backward asymmetry, A_fb, in e⁺e^- → μ⁺μ^- are then calculated both on and off resonance. On resonance the results depend extremely sensitively on the Z mass, M_Z, and to a lesser extent on the top quark mass, m_t, and the Higgs mass, M_H, showing that this is a good place to test the standard model at the one loop level. The results are displayed over the full range of the parameters allowed by experimental and theoretical constraints. It is suggested that sufficiently accurate experiments may be able to set an upper bound on one of m_t or M_H if the other is known. It is noted that on resonance A_pol is only weakly dependent on the beam pipe and on the nature of the outgoing fermions. Comparison is made with other calculations of A_fb and the agreement is found to be good. The predictions are also compared with measurements of A_fb off resonance and are found in all cases to lie within the experimental errors. Previous analyses that indicated a possible discrepancy are shown to be incorrect.

First, we review the basics of baryogenesis and models for baryogenesis in the early literature. Then, we discuss the main theme in this thesis, electroweak baryogenesis, which utilizes the electroweak phase transition. We present how the phase transition can be used to achieve baryogenesis. Next, we develop our model, which is the Standard Model with a singlet field coupled to the Higgs doublet negatively and to the right-handed neutrinos. It is a simple extension of the singlet Majoron model. We focus on numerical analysis of the model. Parameter space satisfying all the conditions are found and relevant quantities, such as the critical temperature, the Higgs mass, and the mixing angle are found as well. It turns out that LEP bound on the Higgs mass and the mixing angle is a strong constraint, and only a small fraction of parameter space is acceptable for attaining a strong enough phase transition for baryogenesis.