Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / The Standard Model of particle physics provides an accurate description of all experimental data to date. The only unobserved piece of the Standard Model is the Higgs boson, a consequence of the spontaneous breaking of electroweak symmetry by the Higgs mechanism. An alternative to the Higgs mechanism is proposed by Technicolor theories which break electroweak symmetry dynamically through a new force. Technicolor predicts many new particles, called Technihadrons, that could be observed by experiments at hadron colliders. This thesis presents a search for two of the lightest Technihadrons, the ρT and ωT . The Low-Scale Technicolor model predicts the phenomenology of these new states. The ρT and ωT are produced through qq annihilation and couple to Standard Model fermions through the Drell-Yan process, which can result in the dimuon final state. The ρT and ω T preferentially decay to the πT and a Standard Model gauge boson if kinematically allowed. Changing the mass of the πT relative to that of the ρT and ωT affects the cross section times branching fraction to dimuons. The ρT and ωT are expected to have masses below about 1 TeV. The Large Hadron Collider (LHC) at CERN outside of Geneva, Switzerland, produces proton-proton collisions with a center of mass energy of 7 TeV. A general purpose high energy physics detector ATLAS has been used in this analysis to search for Technihadrons decaying to two muons. We use the ATLAS detector to reconstruct the tracks of muons with high transverse momentum coming from these proton-proton collisions. The dimuon invariant mass spectrum is analyzed above 130 GeV to test the consistency of the observed data with the Standard Model prediction. We observe excellent agreement between our data and the background only hypothesis, and proceed to set limits on the cross section times branching ratio of the ρT and ωT as a function of their mass using the Low-Scale Technicolor model. We combine the dielectron and dimuon channels to exclude masses of the ρT and ωT between 130 GeV - 480 GeV at 95% Confidence Level for masses of the πT between 50 GeV - 480 GeV. In addition for the parameter choice of m(π T ) = m(ρT /ω T )- 100 GeV, 95% Confidence Level limits are set excluding masses of the ρT and ωT below 470 GeV. This analysis represents the current world's best limit on this model. / 2031-01-01

The highly celebrated discovery of a new particle with a mass of 125 GeV in proton-proton collisions by the ATLAS and CMS experiments at the CERN Large Hadron Collider in 2012 has been shown to be compatible with the Standard Model description of the Higgs boson. However, in order to fully verify the Standard Model nature of the Higgs boson, most of its properties still remain to be measured. Such measurements include differential cross section measurements, which are shown here for the H→WW decay channel and the coupling of the Higgs boson to bottom quarks, for which a study of future prospects is presented. Differential fiducial cross section measurements of the Higgs boson were performed in the H → WW*→lvlv channel at the ATLAS detector with 20 fb−1 of √s = 8 TeV collision data. For Higgs bosons produced by gluon-gluon fusion, the cross section is measured as a function of kinematic variables, including transverse momentum and rapidity of the Higgs boson, as well as the number of jets associated with the Higgs event. The obtained distributions are unfolded to a fiducial volume using a two-dimensional iterative Bayesian algorithm. The measured fiducial differential cross sections are compared to predictions from Monte Carlo generators. The total cross section measured in the fiducial volume defined by the charged lepton and neutrino kinematic properties is 36.0 ± 9.7 fb. Additionally the jet-veto efficiency in the fiducial volume is extracted from the differential cross sections. An analysis is presented of Higgs boson production and decay into bottom quarks in association with a vector boson at the ATLAS detector for the future high-luminosity LHC with proton-proton collisions at √s = 14 TeV. The vector bosons are reconstructed from Z→l+l− or W→lv final states, where l is an electron or muon. The analysis uses generator-level Monte Carlo samples to which efficiency and resolution smearing functions are applied. These reproduce the expected resolution of the upgraded ATLAS detector for the foreseen amount of pile-up due to multiple overlapping proton-proton collisions. The analysis of the ZH(→ l+l−b¯b) channel is presented and results are combined with the WH(→lvb¯b) channel from a corresponding study. For an integrated luminosity of 300 fb−1 using an average pile-up of 60, the expected significance is 3.9 σ with an expected error on the signal strength of 25%. Likewise, for 3000 fb−1 using an average pileup of 140 the expected significance is 8.8 σ , and the error on the signal strength is expected to be about 15%.

La production des états charmonia (par exemple le J/ψ et le ψ(2S)) est l’une des sondes étudiées pour comprendre les propriétés du Plasma de Quarks et de Gluons (PQG) formé dans les collisions d’ions lourds à très haute énergie. En effet, la présence d’un milieu déconfiné est susceptible de modifier le taux de production des charmonia par effet d’écrantage de couleur de la paire de quarks charme-anti-charme. Une telle suppression fut déjà observée dans les collisions d’ions lourds aux énergies du SPS et du RHIC. Dans les collisions Pb-Pb à √sNN = 2.76 TeV au LHC, une suppression relative du taux de production du J/ψ par rapport à celui mesuré dans les collisions pp normalisé aux nombres de collisions binaires proton-proton (facteur de modification nucléaire RAA) fut aussi mesurée. Cette suppression est cependant moins importante que ce qui fut observé à plus petites énergies dans le centre de masse. Ceci peut s’expliquer par la présence d’un nouveau mécanisme de production, la recombinaison de quarks charme et anticharme déconfinés au moment de l’expansion hydrodynamique du PQG ou durant la phase d’hadronisation. Dans cette thèse, nous présentons les résultats de la production de J/ψ dans les collisions pp et Pb-Pb, mesurée par le détecteur ALICE à √sNN = 5.02 TeV à rapidité avant dans le canal de décroissance dimuonique. La section efficace de production mesurée dans les collisions pp est comparée à différents modèles théoriques et est utilisée pour calculer le RAA en collisions Pb-Pb, lui-même comparé à différentes prédictions théoriques ainsi qu’aux précédentes mesures. / The production of charmonium states (for instance J/ψ and ψ(2S)) is one of the probes studied to investigate the properties of the Quark-Gluon Plasma (QGP) formed in high-energy heavy-ion collisions. Indeed, the presence of a deconfined medium should modify the charmonium production yield, due to the color screening of the charm quark anti-quark potential. Such a suppression was already observed in heavy-ion collisions at SPS and RHIC energies. In Pb-Pb collisions at √sNN = 2.76 TeV at the LHC, a clear suppression of the J/ψ yield with respect to the one measured in binary-scaled pp collisions (nuclear modification factor RAA) was observed, but much smaller than that at lower collision energies. This observation can be explained by the presence of a new production mechanism, the (re)combination of deconfined charm and anti-charm quarks during the hydrodynamical expansion of the QGP or at the hadronization stage. In this thesis, we report on the results of the J/ψ production in pp and Pb-Pb collisions measured with the ALICE detector √sNN = 5.02 TeV at forward rapidity in the dimuon channel. The cross-section in pp collisions is compared to various model calculations, and is used to compute the RAA in Pb-Pb collisions, which is also compared to theoretical predictions as well as to previous measurements.

A search for heavy exotic diboson resonances decaying to llqq final states is presented using pp collision data collected with the ATLAS detector at the Large Hadron Collider. The analysis uses a data sample corresponding to an integrated luminosity of 20.3 fb^-1 at sqrt(s) = 8 TeV collected between April and December 2012. No significant excess of data events over the predicted Standard Model background is observed and 95% confidence level upper limits are set on the product of the production cross-section and the branching ratio for spin-2 Kaluza-Klein gravitons predicted by the bulk Randall-Sundrum model and for Extended Gauge Model W’ bosons. These results are subsequently combined with limits obtained from searches using the lvll, lvqq, and qqqq final states, and new mass limits are set on both signal models.

Es werden Messungen der totalen Wirkungsquerschnitte der Prozesse pp → W/Z → ll′ in Endzuständen mit Elektronen und Myonen präsentiert. Für diese Analyse werden Daten, die im Jahr 2013 mit dem ATLAS-Detekor am LHC bei einer Schwerpunktsenergie von √s= 2.76 TeV aufgezeichnet wurden und einer integrierten Luminosität von pb−1 entsprechen, verwendet. In dieser Analyse werden mehrere Quellen systematischer Unsicherheiten berücksichtigt. Die Untergründe werden sowohl mittels Simulationen (für elektroschwache und tt ̄ Un- tergründe) als auch mithilfe datengetriebener Methoden (für den Multi-Jet Untergrund) abgeschätzt. Die Resultate der Elektron- und Myon-Kanäle werden kombiniert und ergeben die folgenden Wirkungsquerschnitte: sigma_fid(W → lν) = 2206.3±20.2(stat.)±10.6(sys.)±68.4(lumi.)[pb], sigma_fid(Z → ll) = 200.4±6.1(stat.)±0.3(sys.)±6.2(lumi.)[pb]. Die kombinierten Wirkungsquerschnitte werden zur Berechnung der Wirkungsquerschnitts- Verhältnisse W/Z und W+/W− benutzt. Die Ergebnisse werden mit QCD-Berechnungen unterschiedlicher Ordnungen verglichen. Der Einfluss dieser Messung auf Partonverteilungsfunktionen (parton distribution func- tions, PDFs) wird abgeschätzt. Die gesamte Unsicherheit dieser Messungen ist vergleich- bar mit den Unsicherheiten aktueller PDFs, weshalb diese Analyse zur Einbeziehung in zukünftige PDF-Bestimmungen geeignet ist. Außerdem wurden Studien zur Frozen Showers Methode zur schnelleren Simulation von Ereignissen im ATLAS-Detektor durchgeführt. Es wurde eine neue Methode zur Gener- ierung einer Frozen Showers Bibliothek entwickelt, die eine Reduzierung der für diesen Abschnitt benötigten Zeit erlaubt. / Measurements of the total pp → W/Z → ll′ cross sections in the electron and muon s = 2.76 TeV with the ATLAS detector at the LHC (in 2013), corresponding to a total integrated luminosity of 4 channels are presented. Data collected at a center-of-mass energy pb−1, are used for this analysis. In this measurement, several sources of systematic uncertainties are estimated. The back- ground contribution is estimated using simulation (for electroweak and tt ̄ backgrounds) as well as data-driven methods (for the multijet background). The combined results for electron and muon channels provide the following cross sections: sigma_fid(W → lν) = 2206.3±20.2(stat.)±10.6(sys.)±68.4(lumi.)[pb], sigma_fid(Z → ll) = 200.4±6.1(stat.)±0.3(sys.)±6.2(lumi.)[pb]. The combined cross sections are used for the calculation of the W/Z and W+/W− cross section ratios. These results are compared with different orders of QCD calculations. The effect of the addition of the measured cross sections to the parton density functions (PDF) is estimated. The total uncertainty of the measurements is comparable to the current global PDF uncer- tainties, which makes them applicable for future PDF determination. Additionally, studies of the Frozen Showers method for fast simulation have been per- formed. A new method of Frozen Showers library generation, allowing to reduce time spend on this stage, is developed.

Die vorliegende Arbeit beschreibt die Suche nach der Produktion des Standardmodell Higgs-Bosons in Assoziation mit einem Top-Antitop-Quarkpaar ttH). Der verwendete Datensatz basiert auf einer integrierten Luminositat von 36.1 1/fb, aufgenommen mit dem ATLAS Detektor am Large Hadron Collider in den Jahren 2015 und 2016. Die selektierten Ereignisse enthalten entweder ein oder zwei Leptonen vom Zerfall des Top-Antitop-Quarkpaares. Die Sensitivität der Analyse wurde erhöht, indem die Ereignisse in unterschiedliche Regionen unterteilt wurden, basierend auf der Anzahl der Jets sowie der Wahrscheinlichkeit b-Jets zu enthalten. Methoden basierend auf multivariaten Analysetechniken wurden entwickelt, um ttH Signalereignisse vom Untergrund zu separieren, der von der Produktion von Top-Antitop-Quarkpaaren mit zusätzlichen b-Jets dominiert wird. Alle in der Analyse verwendeten Regionen wurden in einem Profile-Likelihood-Fit kombiniert, um die Vorhersagen des Untergrunds einzuschr{\"a}nken und die systematischen Unsicherheiten zu reduzieren. Ein Überschuss an Ereignissen über dem erwarteten Standardmodell-Untergrund wurde mit einer beobachteten (erwarteten) Signifikanz von 1.4 (1.6) Standardabweichungen gemessen. Die Daten schliessen ttH Signalstärken von mehr als 2.0 mit einem Konfidenzniveau von 95% aus. / This thesis presents the search for the Standard Model Higgs boson produced in association with a pair of top-quarks (ttH). The analysis uses a 36.1 1/fb dataset of proton-proton collisions collected with the ATLAS detector, at the Large Hadron Collider during 2015 and 2016. The selected events contain either one or two leptons from the decay of the top-quark pair. In order to improve the sensitivity of the search, events are split in regions according to the number of jets and how likely these events are to contain b-jets. Methods based on multivariate techniques were developed and applied in the signal-enriched regions to discriminate ttH events against background events being dominated by top pair production with additional b-jets. All analysis regions are combined in a statistical model using a profile likelihood fit to constrain the background predictions and reduce the systematic uncertainties. An excess of events over the expected Standard Model background is found with an observed (expected) significance of 1.4 (1.6) standard deviations. A ttH signal strength larger than 2.0 is excluded at the 95% confidence level.

Grâce à l’exploitation du Large Hadron Collider, débutée en 2010, le monde de la physique des particules espère enfin avoir une compréhension plus précise du mécanisme de brisure de la symétrie électrofaible et résoudre certaines questions expérimentales et théoriques que soulèvent encore le modèle standard. S’inscrivant dans cette effervescence scientifique, nous allons présenter dans ce manuscrit une paramétrisation largement indépendante des modèles afin de caractériser les effets d’une éventuelle nouvelle physique sur les mécanismes de production et de désintégration du bosons de Higgs. Ce nouvel outil pourra aisément être utilisé dans les analyses des grandes expériences généralistes comme CMS et ATLAS afin de valider ou d’exclure de manière significative certaines théories au delà du modèle standard. Ensuite, dans une approche différente, fondée sur la construction de modèles, nous avons considéré un scenario où les champs du modèle standard peuvent se propager dans un espace plat possédant six dimensions. Les nouvelles directions spatiales supplémentaires sont compactifiées sur un Plan Projectif Réel. Cet orbifold original est l’unique géométrie à six dimensions qui présente des fermions chiraux et un candidat de matière noire dit naturel. Le photon scalaire, particule la plus légère du premier mode de Kaluza-Klein, est en effet stabilisé par une symétrie résiduelle de l’invariance de Lorentz à six dimensions. En utilisant les contraintes actuelles fournies par les observations cosmologiques, nous avons déterminé l’ordre de grandeur de la masse de cette particule aux alentours d’une centaine de GeV. De ce fait les nouveaux états présents dans cette théorie sont suffisamment légers pour produire des signatures claires et observables au Large Hadron Collider. Avec une étude plus poussée du spectre de masses et des couplages du modèle, incluant les corrections radiatives à une boucle, nous avons pu ainsi donner les premières prédictions et contraintes sur la phénoménologie attendue au Large Hadron Collider. / With the start of the Large Hadron Collider runs, in 2010, particle physicists will be soon able to have a better understanding of the electroweak symmetry breaking. They might also answer to many experimental and theoretical open questions raised by the Standard Model. Surfing on this really favorable situation, we will first present in this thesis a highly modelindependent parametrization in order to characterize the new physics effects on mechanisms of production and decay of the Higgs boson. This original tool will be easily and directly usable in data analysis of CMS and ATLAS, the huge generalist experiments of LHC. It will help indeed to exclude or validate significantly some new theories beyond the Standard Model. In another approach, based on model-building, we considered a scenario of new physics, where the Standard Model fields can propagate in a flat six-dimensional space. The new spatial extra-dimensions will be compactified on a Real Projective Plane. This orbifold is the unique six-dimensional geometry which possesses chiral fermions and a natural Dark Matter candidate. The scalar photon, which is the lightest particle of the first Kaluza-Klein tier, is stabilized by a symmetry relic of the six dimension Lorentz invariance. Using the current constraints from cosmological observations and our first analytical calculation, we derived a characteristic massrange around few hundred GeV for the Kaluza-Klein scalar photon. Therefore the new states of our Universal Extra-Dimension model are light enough to be produced through clear signatures at the Large Hadron Collider. So we used a more sophisticated analysis of particle mass spectrum and couplings, including radiative corrections at one-loop, in order to establish our first predictions and constraints on the expected LHC phenomenology

The ATLAS experiment is one of the two general-purpose detectors at the Large Hadron Collider at CERN near Geneva. The goals of this particle physics experiment are the search for new physics phenomena and precision measurement of the particle interactions. Tilecal is the central hadronic calorimeter of the ATLAS. The calorimeter time calibration is important for the time-of-flight measurement of particles (used in some analyses) as well as for the actual energy reconstruction. 1

Doctor of Philosophy / Physics / Andrew G. Ivanov / In this thesis the measurement of production cross section of top-quark pairs in association with a photon in proton-proton collisions at a center of mass energy of 8 TeV is presented. The data was recorded at the CMS experiment at the LHC in 2012. This measurement aims to extend our knowledge of top quark properties and help to test consistency of the Standard Model (SM) of particle physics. Data-driven methods are used to estimate the photon identification efficiency and purity. The measured cross-section agrees with the standard model expectation.

Angular distributions of jet pairs (dijets) produced in proton-proton collisions at a centre-of- mass energy √s = 7 TeV have been studied with the ATLAS detector at the Large Hadron Collider using the full 2011 data set with an integrated luminosity of 4.8 fb−1, and reaching dijet masses up to 4.5 TeV. All angular distributions are consistent with QCD predictions. Analysis of the dijet angular distribution, using a novel technique simultaneously employing the dijet mass, is employed. This analysis is sensitive to both resonant new physics and phenomena with a slow-onset in mass. Using this technique, new exclusion limits have been set at 95% credibility level for several hypotheses of physics beyond the standard model including: quantum gravity scales, with 6 extra dimensions, below 4.11 TeV, quark contact interactions below a compositeness scale of 7.6 TeV, and excited quarks with a mass below 2.75 TeV. In a large and complex scientific experiment, such as ATLAS, the collection, management and usability of coherent data and metadata is a challenging operation. The availability of these data to physicists within the experiment is essential to all analysis efforts. A new web-based interface called “RunBrowser”, which makes ATLAS and LHC operations data available to the ATLAS Collaboration, is introduced.