| 1. |
- Andrean, Stefio Yosse, 1991-
(author)
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Search for New Scalar Particles with ATLAS
- 2023
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Doctoral thesis (other academic/artistic)abstract
- The Large Hadron Collider (LHC) is currently the most powerful particle accelerator ever built. It accelerates protons and collides them at the center of mass energy of √s = 13 TeV. At one of the collision points of LHC, a general-purpose particle detector — ATLAS — is installed to measure the outgoing particles produced in the collisions allowing the study of interactions between the elementary particles. The work presented in this thesis uses the collisions data produced by LHC and the data collected by ATLAS during the period of 2015-2018 which amounts to the integrated luminosity of 139 fb-1.This thesis focuses on searches of spin-0 particles in two areas of Beyond Standard Model physics: supersymmetry and extended Higgs sector. Supersymmetry offers a solution to the hierarchy problem by introducing partners to every Standard Model particle. Stop — the superpartner of the top quark — is particularly interesting due to its ability to cancel the dominant top contribution in the Higgs boson mass loop correction, and therefore becomes the target of a search in this thesis. Many Beyond Standard Model scenarios extend the Higgs sector for they are motivated by neutrino oscillations, dark matter, and baryogenesis. One of the consequences of these scenarios is the prediction of extra Higgs-like scalar particles which may decay into the 125 GeV Higgs boson. This is the signature targeted by the second ATLAS data analysis documented in this thesis.The thesis also includes a performance study of the Tile Calorimeter. The Tile Calorimeter is part of the ATLAS calorimeter system whose main task is to measure the energy of hadrons. The study is conducted on the Tile Calorimeter using muons from W boson decay originating from proton-proton collisions. Each calorimeter cell response is measured in data and compared with detector simulation to verify that the energy scale in simulation matches that in the real detector.
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| 2. |
- Sunneborn Gudnadottir, Olga
(author)
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Of dark mesons and novel methods : A dark sector search in ATLAS data and development of new techniques for challenging final states
- 2024
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Doctoral thesis (other academic/artistic)abstract
- Studies of the interactions of elementary particles at high energies have been carried out at the Large Hadron Collider (LHC) at CERN for over a decade. Different quantities from the Standard Model (SM) of particle physics have been measured with increasing accuracy without substantial deviations from predictions. Searches for physics beyond the SM are similarly carried out, motivated by the existence of phenomena not yet described by it, such as dark matter. This thesis presents one such search in proton-proton collision data recorded by the ATLAS detector. The search is guided by a new, proposed addition to the SM, where the dark matter candidate arises as a composite particle of a new sector. If this were realised in nature, the same sector would give rise to other composite particles, dark mesons, that would be produced in proton-proton collisions and decay promptly to SM particles. This new model is largely free of previous constraints from searches and measurements. The full analysis targeting pair produced dark pions decaying to top and bottom quarks, tttb or ttbb, in the 1-lepton channel is described. It is carried out in the full Run 2 dataset of 140 fb−1 of proton-proton collisions at √s = 13 TeV center-of-mass energy. The analysis is sensitive to large parts of the parameter space of the model, and no significant excess was seen over SM predictions. Based on this, limits on the production cross-section of dark pions were set. By comparing with the theoretical cross sections of the model, these rule out dark pion masses up to 943 GeV in the most sensitive configuration.Further, several novel techniques that could aid with searches in similar phase-spaces are presented. First, the Extrapolation Engine fast simulation of the inner tracker for the high luminosity upgrade of ATLAS was used in the study of a proposed hardware track trigger (HTT). This could be crucial to retaining efficiency in similar phase-spaces in the extreme conditions at the high luminosity LHC (HL-LHC). Second, the fully scalable multi-dimensional density estimate in SparkDensityTrees was applied on background and signal similar to those in the dark meson analysis and was shown to efficiently find signal-enriched regions. Third, the unsupervised clustering algorithm UCluster which can be trained with any clustering objective, such as signal extraction, anomaly detection or jet tagging was developed to run on multiple cores for arbitrary scalability. Lastly, a Boosted Decision Tree (BDT) was applied for signal and background discrimination in the dark meson analysis, yielding promising results for future iterations of it.
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| 3. |
- Sidebo, P. Edvin
(author)
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Measurements of the Standard Model Higgs boson cross sections in the WW* decay mode with the ATLAS experiment
- 2018
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Doctoral thesis (other academic/artistic)abstract
- This thesis summarises measurements of the Standard Model Higgs boson production cross sections based on proton–proton collision data at √s = 13 TeV produced by the Large Hadron Collider at CERN. By analysing data collected during 2015 and 2016 by the ATLAS experiment, corresponding to an integrated luminosity of 36 fb−1, the Higgs boson gluon–gluon fusion and vector boson fusion production cross sections are measured in the WW* decay mode. To obtain a high signal to background ratio, the data is filtered for final states with one electron (positron) and one anti-muon (muon) and missing transverse momentum. A major part of the thesis concerns the estimation of backgrounds with misidentified leptons. These backgrounds originate from the production of a W boson and an associated object mistakenly identified as an (anti-)electron or (anti-)muon, and are estimated with data driven techniques. A maximum likelihood fit is performed and the cross sections times branching ratios are simultaneously measured to be σ ·B = 12.6+2.3/-2.1 pb and σ ·B = 0.50+0.30/-0.29 pb for the gluon–gluon fusion and vector boson fusion modes, respectively. Both systematic and statistical uncertainties are taken into account in the confidence intervals. The corresponding Standard Model predictions are 10.4 ± 0.6 pb and 0.81 ± 0.02 pb. The observed (expected) significance of the gluon–gluon fusion mode is 6.3 (5.2) standard deviations above the Standard Model background. For the vector boson fusion mode, the observed and expected significances are 1.9 and 2.7 standard deviations, respectively.A smaller part of the thesis investigates the prospects for measuring the luminosity in the high-luminosity phase of the Large Hadron Collider, to begin in 2026. ATLAS will build and insert a timing detector with silicon pixel technology into the forward region, to cope with the harsh pileup environment present at high luminosity. The capabilities of this detector to provide luminosity measurements are investigated. The number of detector hits is observed to scale linearly with collision multiplicity across the full range of expected multiplicities.
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