The Large Hadron Collider is the most powerful particle accelerator built to date. It is a proton-proton and heavy ion collider which in 2015 and 2016 operated at an unprecedented center of mass energy of √s = 13 TeV. The Tile Calorimeter is the ATLAS hadronic calorimeter covering the central region of the detector. It is designed to measure hadrons, jets, tau particles and missing energy. In order to accurately be able to properly reconstruct these physical objects a careful description of the electronic noise is required. This thesis presents the work done in updating, monitoring and studying the noise calibration constants used in the processing and identication of hadronic jet in the 2011 data. Moreover the results of the searches for compressed supersymmetric squark-neutralino and large extra dimensions models are also presented in this thesis. The present work uses an experimental signature with a high energy hadronic jet and large missing transverse energy later often referred to as monojet signature. The search for supersymmetry is carried out using an integrated luminosity of 3.2 fb-1 recorded by the ATLAS experiment in 2015. The search for large extra dimensions presented in this work uses the full 2015 + 2016 dataset of 36.1 fb-1. No signicant excess compared to the Standard Model prediction has been observed on the production of squark pairs with the subsequent decay of the squark in a quark and a neutrino. Exclusion limits are set on squark production as a function of the neutralino mass. Squark masses up to 608 GeV are excluded for a mass difference between the squark and the neutralino of 5 GeV. In the second search for the presence of large extra spatial dimensions in the Arkani-Hamed, Dimopoulos and Dvali model scenario a good agreement between data and Standard Model prediction is observed and exclusion limits are set on the effective Planck scale MD of 7.7 and 4.8 TeV for two and six hypothesized large extra dimensions respectively signicantly improving earlier results.

Heavy Neutral Leptons (HNLs) are the heavier counterparts of the light neutrinos of the Standard Model of particle physics. HNLs can simultaneously solve several of the problems the Standard Model cannot yet resolve, one example being that they provide a candidate for Dark Matter. This thesis work aims to shed light on the nature of HNLs and study the displaced signature the particle gives rise to at colliders. The collider of interest is the Future Circular Collider that will be colliding electrons and positrons and the signal studied is the production of an HNL and a light neutrino from an intermediate Z boson, produced from the collision of an electron and a positron. The event generation was set up through MadGraph and PYTHIA and for the detector simulations DELPHES was used. Validation of three HNL samples were carried out in a standalone framework and in the FCC framework. The samples were validated by comparing theoretically calculated lifetimes with the lifetimes attained by simulation. Kinematic studies of the transverse momentum of the HNL and its decay particles showed correlation to the mass of the HNL. Reconstruction of the number of tracks created by the HNL decay was possible and the results of two track dominance were found to correlate with theory. For the vertexing study, the reconstruction of the production vertex of the decay particles was possible where displaced vertices were observed, hence proving the possibility of implementing displaced signatures in the FCC framework for the very first time. The next step in this trajectory of the study would be to investigate vertex fitting of the reconstructed vertices in order to carry out tracking studies of the HNL. This work hence sets the foundation for further exploration of HNLs and provides stepping stones for the possibility of discovery of HNLs in the FCC-ee.

Dark matter, the unknown matter that constitutes 85% of all matter in the universe, is one of the greatest mysteries in fundamental physics. One theory that might explain dark matter predicts that there are long-lived particles known as dark pions. If these were created in a particle accelerator, they could decay inside the detector, resulting in particles that seemingly "emerge" from nothing. This phenomenon is known as emerging jets. In this study, emerging jets are simulated with various values of the dark pion average lifetime, dark pion mass, and mediator particle mass. These simulations are compared with a search for displaced vertices conducted by the ATLAS collaboration, allowing one to reinterpret the ATLAS results to constrain the parameter values that the emerging-jets model can have. This study simulates and constrains the allowed values for the dark pion mass, dark pion average life time and mediator mass with 95% confidence level. This is the first study to use results from the ATLAS experiment to constrain the emerging-jets model, as well as the first study to exclude this region of the parameter space.

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The coupling of the Standard Model top quark to the Higgs boson is O(1), which leads to large quantum corrections in the perturbative expansion of the Higgs boson mass. Possible solutions to this so-called naturalness problem include supersymmetric models with gluinos and stop squarks whose masses are at the electroweak scale, O(1 TeV). If supersymmetry is realised in nature at this scale, these particles are expected to be accessible with the Large Hadron Collider at CERN. A search for gluino pair production with decays mediated by stop- and sbottom-squark loops in the initial 14.8 ifb of the ATLAS run 2 dataset is presented in terms of a pair of simplified models, which targets extreme regions of phase space using jet substructure techniques. No excess is observed and limits are set which greatly extend the previous exclusion region of this search, up to 1.9 TeV (1.95 TeV) for gluinos decaying through light stop (sbottom) squarks to the lightest neutralinos. A performance study of top tagging algorithms in the 20.3 ifb 2012 dataset is also presented, which includes the first measurements of substructure-based top tagging efficiencies and fake rates published by ATLAS, as well as a detailed comparison of tagger performance in simulation. A benchmarking study which compares commercially available cloud computing platforms for applications in High Energy Physics, and a summary of ATLAS liquid argon calorimeter data quality work focused on monitoring and characterising the sporadic phenomena of Mini Noise-Bursts in the electromagnetic barrel calorimeter are also included. / Graduate / 0798 / matt.leblanc@cern.ch

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Studies of minimum bias events at colliders probe the behavior of QCD in the non-perturbative regime. The phenomenology of events in this regime is described by empirical models that take many parameters, which all need to be tuned to the observed data. Measurements based on angular correlations between the highest transverse momentum charged particle track and the other charged particle tracks in collision events can, because of their robustness against experimental and detector effects, be a component of the tuning inputs for the models. We measure such observables in a variety of pseudorapidity ranges and at many center-of-mass energies at DØ and ATLAS. We observe that such observables are poorly described by current models and tunes that are used to produce simulated event samples, making them valuable information for the tuning process. The Matrix Element method is a powerful analysis tool to extract precise measurements from data samples of limited statistics. We have investigated the potential of the Matrix Element method to measure the mass of the charged Higgs in the exclusive decay H±→τ±ν→e±+3ν when produced in top quark decays at the Tevatron, with emphasis on the construction of transfer functions in the τ decay chain. We concluded that the τ decay chain can be successfully parametrized via a transfer function and that the method has the potential to provide an accurate charged Higgs mass measurement in this channel. Triggering on τ leptons is a key component for many beyond the Standard Model searches at ATLAS, such as the search for the charged Higgs boson. Events containing Z bosons can be used to measure the efficiency of the ATLAS τ hadronic-decay trigger. We have used a tag-and-probe method on simulated Z boson decays to 2 τ leptons where one decays to a μ while the other decays hadronically. The μ is used as the tag and the τ side is probed. We demonstrated that the efficiency of the τ hadronic-decay trigger can be accurately measured with this method using the first 100 pb-1 of ATLAS data. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 732