| 1. |
- Andrean, Stefio Yosse, 1991-
(författare)
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Search for New Scalar Particles with ATLAS
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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. |
- Klimek, Pawel, 1985-
(författare)
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Search for Charginos and Sleptons in ATLAS and Identification of Pile-up with the Tile Calorimeter
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The standard model of particle physics (SM) describes the elementary particles and their interactions. Supersymmetry (SUSY), a symmetry beyond those included in the standard model could resolve some of the SM shortcomings. It can provide a candidate for Dark Matter and a solution to the hierarchy problem. The Large Hadron Collider (LHC) has the potential to produce the particles predicted by SUSY. This thesis presents two searches for SUSY particles in proton-proton collision data recorded by the ATLAS experiment.The first search described in this thesis looks for direct production of chargino and slepton pairs in a final state characterized by the presence of two leptons and missing transverse energy. The second search looks for production of chargino pairs via vector boson fusion (VBF) in a final state containing of two leptons, two jets and missing transverse energy. This is the first attempt in ATLAS to search for supersymmetric particles produced via VBF. A possible observation of such process would prove that the exchanged neutralino is a Majorana particle. These analyses are done using L=20.3 fb-1 proton-proton collisions at √s=8 TeV collected in 2012. No significant excess over background is observed. New exclusion limits at 95% confidence level on chargino, neutralino and slepton masses and cross section for chargino pair production via VBF are set.The energy measurements of the particles created in LHC collisions are performed by the ATLAS calorimeters. Energy deposits from different collisions in the same read-out window and in the same calorimeter channel (pile-up) can spoil the energy measurements by the calorimeter. It is shown that the quality factor computed offline for each collision and for each channel in the Tile Calorimeter (TileCal) can be used to identify channels that need a special treatment to account for large energy depositions from pile-up. Efficient criteria to detect pile-up in TileCal are proposed.
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| 3. |
- Pasuwan, Patrawan, 1990-
(författare)
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Searches for dark matter in association with top quarks with the ATLAS experiment
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a search for dark matter with the ATLAS experiment using 139 fb−1 of pp collision data at √s = 13 TeV delivered by the LHC from 2015 to 2018. The search targets a simplified model where a spin-0 scalar or pseudoscalar mediator particle, decaying to a pair of dark matter particles, is produced in association with a pair of top quarks. The final state of interest includes one charged lepton, multiple jets and missing transverse momentum. No excess over the Standard Model background is observed and new exclusion limits at 95% confidence level are placed on the mediator mass. Mediator masses up to 200 GeV are excluded for a dark matter mass of 1 GeV and a coupling strength of one. The one-lepton analysis is statistically combined with those targeting final states with zero or two leptons. The combined results extend the exclusion limits for mediator masses up to 370 GeV. These results are also used to set an upper limit on the Higgs boson branching ratio into invisible particles of 0.40 at 95% confidence level. Luminosity is a key measure of the LHC performance which relates the number of events of a physics process to its cross-section. This thesis presents systematic studies of the track-counting algorithm which is used to measure the luminosity in ATLAS.
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| 4. |
- Andrean, Stefio Yosse, 1991-
(författare)
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Search for Stop Using the ATLAS Detector and Performance Analysis of the Tile Calorimeter with Muons from W Decays
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a search for the supersymmetric partner of the top quark in the final state with one lepton. The search focuses especially in the region of the parameter space where the 2-body decay dominates. The analysis is performed using LHC full Run 2 data at √s = 13 TeV as recorded by the ATLAS detector. No significant excess above the backgrounds is observed, and 95% confidence level exclusion limits are calculated in the stop-neutralino mass plane. Stops are excluded up to 1200 GeV in the low neutralino mass scenario of below 400 GeV.The Tile Calorimeter is part of the ATLAS calorimeter system whose main task is to measure the energy of hadrons. A performance study is conducted on the Tile Calorimeter using muons from W boson decay originating from proton-proton collisions. Each calorimeter cell response is measure in data and compared with detector simulation. The azimuthal cell response uniformity is also investigated using a likelihood method. Overall, a good data to detector simulation agreement and azimuthal uniformity is observed which shows well-calibrated cells and uniform responses among the calorimeter modules.
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| 5. |
- Bertoli, Gabriele, 1983-
(författare)
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Search for Supersymmetry and Large Extra Dimensions with the ATLAS Experiment
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 6. |
- Bertoli, Gabriele, 1983-
(författare)
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Search for Supersymmetry in Monojet Final States with the ATLAS Experiment
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Large Hadron Collider is the most powerful particle accelerator built to date. The LHC is a proton–proton and heavy ion collider, in 2015 it operated at an un- precedented center of mass energy of sqrt(s) = 13 TeV. This thesis presents the results of the search for physics beyond the Standard Model of particle physics in a compressed supersymmetric squark–neutralino model. The present work uses an experimental sig- nature with a single high energetic jet and large missing transverse energy, so called monojet signature. The search is carried out using an integrated luminosity of 3.2 inverse femtobarn recorded by the ATLAS experiment in 2015. No significant excess compared to the Standard Model prediction has been observed thus a 95% CL limit has been set on the production of squark pairs with the subsequent decay ~q -> q + chi01 (q = u, d, c, s). Squark masses up to 608 GeV are excluded for m(~q) - m(chi01) = 5 GeV significantly improving earlier results.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 of data and the identification of hadronic jets. These studies showed an unexpected variation over time of the cell noise and further investigation led to discover that the tile noise filter was not behaving as expected in some situations in approximately 5% of the detector cells.
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| 7. |
- Klimek, Pawel, 1985-
(författare)
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Search for Charginos and Sleptons in ATLAS and Identification of Pile-up with the Tile Calorimeter
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Large Hadron Collider (LHC) located at the European Organization for Nuclear Research (CERN) is the most powerful particle accelerator in the world in terms of collision energy and luminosity. This thesis presents a search for supersymmetric particles in proton-proton collision data recorded by the ATLAS experiment.A search for direct production of chargino and slepton pairs in a final state characterised by the presence of two leptons and missing transverse momentum is presented. This analysis is done using L = 20 fb-1 proton-proton collisions at √s = 8 TeV collected in 2012. No significant excess over background is observed. Exclusion limits at 95% confidence level on chargino, neutralino and slepton production are set.In 2011-12 the LHC was providing collisions every 50 ns. This puts very strong requirements on the energy measurement in presence of energy deposits from different collisions in the same read-out window and in the same calorimeter channel (pile-up). A quality factor computed offline for each collision and for each channel in the ATLAS Tile Calorimeter (TileCal) is studied. It is shown that the quality factor can be used to select channels that need a special treatment to account for large energy deposition from pile-up. Efficient criteria to detect pile-up in TileCal channels are proposed.
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| 8. |
- Lundberg, Olof, 1984-
(författare)
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Searches for Dark Matter and Large Extra Dimensions in Monojet Final States with the ATLAS Experiment
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents searches for evidence for Weakly Interacting Massive Particles (WIMPs) and Extra Dimensions in proton-proton collisions recorded by the ATLAS experiment at the CERN Large Hadron Collider (LHC). The WIMP is one of the main candidates to constitute the particle content of Dark Matter. Extra Dimensions are introduced in several theories in order to explain the apparent weakness of gravity when compared to the other interactions in Nature. Theories with WIMPs as well as Extra Dimensions can manifest themselves at the LHC, with experimental signatures characterized by an energetic hadronic jet associated with large missing momentum. These signatures are known as monojet signatures, and are investigated in this thesis. The first analysis is performed using L = 20.3 fb-1 of proton-proton collisions at √s = 8 TeV recorded in the ATLAS Run 1. The second analysis is performed using L = 3.2 fb-1 of proton-proton collisions at √s = 13 TeV recorded in the ATLAS Run 2. No significant excess over the expected background is found in either of the analyses. New exclusion limits are set at 95% confidence level on Dark Matter particle production. New limits are also set on graviton production in the so-called ADD scenario with Extra Dimensions.
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| 9. |
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| 10. |
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| 11. |
- Tylmad, Maja, 1986-
(författare)
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Search for Weakly Produced Supersymmetric Particles in the ATLAS Experiment
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Large Hadron Collider located at CERN is currently the most powerful particle accelerator and ATLAS is an experiment designed to exploit the high energy proton-proton collisions provided by the LHC. It opens a unique window to search for new physics at very high energy, such as supersymmetry, a postulated symmetry between fermions and bosons.Supersymmetry can provide a solution to the hierarchy problem and a candidate for Dark Matter. It also predicts the existence of new particles with masses around 1 TeV, thus reachable with the LHC. This thesis presents a new search for supersymmetry in a previously unexplored search channel, namely the production of charginos and neutralinos directly decaying to electroweak on-shell gauge bosons, with two leptons, jets, and missing transverse momentum in the final state. The search is performed with proton-proton collision data at a center of mass energy of √s = 8 TeV recorded with the ATLAS experiment in 2012. The design of a signal region sensitive to the new signal is presented and a data driven technique to estimate the Z+jets background is developed.Precise measurements of hadronic jet energies are crucial to search for new physics with ATLAS. A precise energy measurement of hadronic jets requires detailed knowledge of the pulse-shapes from the hadron calorimeter signals. Performance of the ATLAS Tile Calorimeter in this respect is presented using both pion test-beams and proton–proton collision data.
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