A search for heavy fermionic top quark partners with charge 5/3 decaying to a pair of same-sign leptons with the CMS experiment

In the millennia of recorded human knowledge, no model for describing the workings of Nature is as elegant or complete as the Standard Model of Particle Physics (SM). However, the SM has several open questions and there exist multiple phenomena that it cannot explain. A pressing question is related to the mass of the Higgs boson, whose value the SM has no natural way of explaining, relying instead on the fine tuning of parameters to one part in 10^{28}. Many extensions of the SM propose new interactions and particles which solve this problem. A particularly common theme is that of new partners of the top quark, which in some models are fermionic and have vectorial couplings to the SM charged weak current. Such particles are referred to as vector-like quarks and represent a promising avenue of research. A search is presented for a vector-like quark with an exotic 5/3 charge (in units of the charge of the positron), referred to as an X5/3 particle. These particles are predicted in Composite Higgs theories, which rely on the masses of the X5/3 to be not more than ~2 TeV in order to solve the unnaturalness inherent in the mass of the Higgs boson. The search uses data collected by the CMS experiment in both 2015 and 2016 consisting of proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC. No significant excess of events is seen above the predicted background and limits are placed on the mass of the new top quark partner at 95% confidence level, excluding masses less than 1200 (1160) GeV for X5/3 particles that decay with right-handed (left-handed) couplings to W bosons. These are the most stringent limits to date on the mass of the X5/3 particle in this final state

Radiation-Hardness of PIN/VCSEL Arrays for the ATLAS Pixel Detector

I report on irradiation tests of PIN/VCSEL candidates for the ATLAS Pixel Detector's optical data-link, conducted at CERN by OSU's ATLAS group in 2006-2008.No embarg

Search for Supersymmetry in Final States with 2 or 3 Leptons with Data collected by the ATLAS Experiment

In this thesis I have presented a search for supersymmetry using data gathered by the ATLAS Detector from the Large Hadron Collider during the data-taking period from 2015 to 2016. In particular, this search looks for chargino and neutralino particles and targets several areas of phase space that haven’t been excluded yet. These particles are searched for in two different final states (the 2-lepton, 2-jet + E(miss)(T) final state and the 3-lepton + E(miss)(T) final state). This analysis also uses a novel method of variable construction called the “Recursive Jigsaw Reconstruction” method. This method has the potential to increase the sensitivity of analyses to signatures of SUSY particles without the need to dramatically increase the amount of data analysed.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202

Electron Identification, Electronics Upgrades, And Electroweak Supersymmetry At Atlas

This thesis presents two searches for direct production of “electroweakinos”, the supersymmetric partner particles (“superpartners”) of the Standard Model photon, W, Z, and Higgs bosons. These searches were performed using 36 fb⁻¹ of √s = 13 TeV data collected by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). The first search focuses on a final state with three prompt leptons (electrons or muons) from on-shell W and Z bosons produced in decays of the electroweakinos. The second search focuses on compressed scenarios where the electroweakinos decay via off-shell W and Z bosons to pairs of leptons with small transverse momenta (pT). This second search allowed for the first exclusions of directly produced compressed Higgsinos at the LHC. In describing these searches, particular emphasis is placed on the estimation of backgrounds which mimic prompt leptons. This thesis also details the ATLAS electron identification algorithm, which distinguishes prompt electrons from backgrounds using a likelihood-based method. Significant improvements have been made to this algorithm during Run 2 including reoptimization of the selection criteria for new detector conditions, extension of the algorithm to a wider range of electron pT, and adaptation of the algorithm for the real-time selection of events. The corresponding measurements of the efficiencies for electron identification and other electron selection criteria are also described. Finally, readout electronics designed for upgrades to the ATLAS tracking system for use during the high-luminosity phase of the LHC are discussed. During this high-luminosity phase, an expected 3000 fb⁻¹ of data will be delivered to ATLAS. It is therefore essential to ensure that ATLAS will be able to efficiently operate and collect data despite the significant radiation damage it will receive. The expected performance of one readout chip (“AMAC”) is demonstrated via functionality and irradiation tests of several prototypes, and good performance is observed

Supersymmetry or Universal Extra Dimensions? Utilizing the ATLAS Experiment at CERN

Supersymmetry and Universal Extra Dimensions are just two of an array of popular and enticing extensions to the Standard Model. The work presented in this thesis evaluates the feasibility of differentiating between these two models by measuring the spins of the new particles, utilising the ATLAS experiment at the Large Hadron Collider at CERN. The method presented depends upon the use of an angular variable, cos θ∗ ll, which is sensitive to the polar angle in the direct slepton pair production process: qq → Z0/γ∗ → ˜l± ˜l∓ → χ˜0 1l ±χ˜0 1l ∓. This angular variable is advantageous since it is longitudinally boost invariant along the beam axis. This allows it to be used at the Large Hadron Collider where the initial centre-of-mass frame of reference is unknown and unrecoverable owing to the presence of invisible particles in the event. The work invoked Monte Carlo events, fully simulated in accordance with the mSUGRA Supersymmetry model corresponding to a selected production point in mSUGRA space of the ATLAS detector. It was demonstrated that, using this method and given the Monte Carlo simulation at this test point, the Large Hadron Collider can distinguish between the supersymmetric production angular distribution and that of UED using 30 fb−1 of integrated luminosity. A further part of the work involved an investigation into part of the innermost detector system of ATLAS. The work investigated the operational performance of a certain fraction of semiconductor tracker barrel modules utilizing data acquired during cosmic commissioning tests carried out at CERN in 2006. This included measuring the noise occupancy levels and subsequently identifying any problematic modules in the process. Eight problematic modules were identified, forming three distinct classes, each class pointing toward a different type of problem. Furthermore, an anomaly, the socalled zeroth time bin anomaly, was found and prompted further investigation to its root cause

Seeing The Light (higgs): Searches And Measurements Of Higgs Boson Decays To Photons

This dissertation presents two results involving Standard Model Higgs bosons decaying to at least one photon. These analyses are performed using 79.8/fb of proton-proton collisions collected at √s=13 TeV with the ATLAS detector at the Large Hadron Collider. Differential and fiducial cross section measurements of the Higgs boson decaying to two photons are presented.The inclusive diphoton cross section was measured to be 55 ± 9 (stat.) ± 4 (syst.) ± 0.1 (theo.) fb compared with the Standard model prediction of 64 ± 2 fb. In addition to inclusive cross section measurement, fiducial cross section results are shown for regions targeting the vector boson fusion, VH, and ttH production modes of the Higgs boson as well as a region targeting beyond the Standard Model contributions. Differential results are given for multiple measured spectra involving kinematics of the diphoton system or variables with jets produced in association with the Higgs boson. No significant deviations from the Standard Model are observed and in their absence, limits are set on alternative hypotheses involving higher order couplings between Standard Model particles and the Higgs boson. Additionally, a search for a Standard Model Higgs bosons decaying into at least one photon and missing transverse momentum was performed.The photon can arise from a Higgs boson decaying into one or two neutralinos, which in turn decay into a gravitino and photon.Higgs bosons produced in association with a Z boson are considered in order to reduce the number of background events. No excess with respect to the Standard Model prediction is observed. Assuming a Standard Model ZH production cross section, the branching fraction of a Higgs boson to neutralinos or neutralino/gravitino is constrained to be less than 5-11% at the 95% confidence level for nearly massless gravitinos