The theoretical development of the Standard Model in the 1960s and 1970s has led to the discovery of its missing piece, the Higgs boson, at the ATLAS experiment (together with its sister experiment, CMS) at the Large Hadron Collider at CERN in 2012. In order to solve the problem of large quantum corrections to the leading expansion of the Higgs boson mass, additional symmetries need to be added to the Standard Model. A possible solution to this problem of Higgs mass quantum corrections is the theory of Supersymmetry, which includes partners to each Standard Model particle, such as gluinos, squarks, and the lightest supersymmetric particle. This dissertation describes a search for pair-produced gluinos, where each gluino decays via a top squark or a bottom squark to the lightest supersymmetric particle (a neutralino). Events which contain a large jet multiplicity in the final state (i.e. at least four jets of which at least three must be b-jets), and large amounts of missing transverse energy, are obtained for further studies. The dataset used for this search includes 139 fb^−1 of integrated luminosity recorded by the ATLAS experiment between 2015 and 2018 at a center-of-mass energy √s = 13 TeV. No significant excess is observed in data above the Standard Model backgrounds, and gluino masses up to 2.3 TeV for both the Gtt and the Gbb models are excluded at a 95% confidence level. This dissertation also contains an overview of, and the author’s work on, the data quality monitoring of the ATLAS liquid argon calorimeter during 2017 data taking. / Graduate

Title: Study of fake-tau background with the ATLAS experiment Author: Gabriela Martinovicová Institute: Institute of Particle and Nuclear Physics Supervisor: Mgr. Vojtěch Pleskot, Ph.D., Institute of Particle and Nuclear Physics Abstract: The τ-leptons are the important final-state components, not only in the Standard Model processes but also in the processes beyond the Standard Model studied at the ATLAS experiment at CERN. They are characterized by mostly decaying into hadrons with one or three charged particles and, in most cases, with at least one neutral pion in the final-state. Due to their short decay length, only their decay products are observed in the detector. Jets naturally fake hadronically decaying τ leptons, so it is necessary to estimate such a fake-τ background. The Fake Factor method uses a correction factor, called fake factor (FF), measured from the data and applied to the data to estimate the fake-τ background in a given signal region. One of the complications is that FF differs for τ candidates faked by jets derived from quarks or gluons and thus must be measured in the control region with the same fraction of quark jets as in the signal region. The solution to this problem is the universal FF method, which proposed that from the FFs measured in samples with a large difference in the...

Les collisions d'ions lourds ultra-relativistes permettent d'étudier les propriétés de la matière nucléaire soumise à des conditions extrêmes de température et/ou de pression. Dans ces conditions, celle-ci pourrait subir une transition de phase vers un nouvel état appelé : Plasma de Quarks et de Gluons (QGP). Un premier indice de la formation du QGP a pu être observé au SPS du CERN par l'expérience NA50 à travers la suppression anormale du taux de production du méson J/$\psi$, via sa désintégration en deux muons, dans les collisions Pb - Pb.<br />L'étude des paires de leptons joue un rôle crucial dans les expériences d'ions lourds. En effet, l'absence significative de ré-interactions avec l'environnement hadronique fait des dileptons une sonde idéale des premiers instants de la collision. De plus, les mésons vecteurs produits dans les collisions ($\rho$, $\omega$, $\fi$, J/$\psi$) peuvent être détectés à travers leur désintégration en paires de leptons. L'expérience NA60, qui succède à NA50, étudie la production des dimuons dans les collisions In - In à 158 GeV/c/nucléon. Les motivations de l'expérience ainsi que ses différentes étapes seront décrites.<br />Le dispositif expérimental de NA60 comporte dans la région cible du spectromètre à muons de NA50 un ensemble de détecteurs de pixels (le télescope de vertex) disposés dans un champ magnétique. Ils permettent d'accéder aux caractéristiques cinématiques des traces des muons avant leur passage dans l'absorbeur hadronique et de déterminer avec précision le vertex d'interaction. Ce télescope de vertex permet également de mesurer la distance dans le plan transverse passant par le vertex de chacun des muons de la paire par rapport au vertex. Cette capacité est essentielle pour séparer des phénomènes prompts de ceux provenant de la décroissance de résonances.<br />Nous présenterons une méthode de reconstruction des traces dans le télescope de vertex dans les collisions d'ions lourds basée sur l'Analyse en Composantes Principales (ACP). Les principaux sujets d'étude de l'expérience NA60 décrits dans ce mémoire concernent les modifications du méson $\rho$ dans le milieu dense et chaud créé par la collision, l'origine de l'excès de dimuons observé pour des masses intermédiaires entre celles du f et du J/$\psi$ et l'étude de la suppression anormale du J/$\psi$. En particulier, le taux de production du J/$\psi$ est étudié en fonction de la multiplicité des traces dans le télescope de vertex. Pour chacun des sujets abordés, les résultats fournis pas NA60 ont permis une avancée significative dans la compréhension de la physique des ions lourds aux énergies du SPS.

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This thesis presents the results for two searches for new physics performed with the ATLAS experiment. The first, a search for the rare B-meson decay Bs → μμ and measurement of its branching ratio, uses 25 fb⁻¹ of √s = 7 and 8 TeV data recorded during 2011 and 2012. After observing a small number of these decays, a branching ratio of B(Bs → μμ) = (0.9⁺¹·¹₋₀.₈) x 10⁻⁹ is measured, assuming non-negative event yields. This is compatible with the Standard Model at the 2σ level. The second, a search for direct pair production of the supersymmetric top quark partner, is performed using 36.07 fb⁻¹ of √s = 13 TeV data recorded during 2015 and 2016. Final states with a high jet multiplicity, no leptons and large missing transverse momentum are selected to target these decays, with several signal regions designed to cover a wide range of particle masses. No excess is observed, with all signal regions being compatible with the Standard Model within 2σ. Limits are set on the stop mass, excluding up to mt̃1 = 940 GeV for values of mx̃⁰₁ below 160 GeV, assuming a 100% branching fraction to t̃1 → tX̃⁰₁ decays. In addition two reinterpretations of this data are presented, for a gluino-mediated stop production scenario and a direct dark matter production scenario. No excess is observed for either model, and limits are set on the mass of the relevant particles. Finally a viability study into using machine learning techniques to improve on existing SUSY search methods has been performed, with the initial results proving promising.

Significant work has been performed on the development of STEFF (SpecTrometer for Exotic Fission Fragments), a 2E2V (2-Energy 2-Velocity) spectrometer built by the University of Manchester Fission Group. The majority of this work was in the development of the time-of-flight systems, in particular the stop detector; with the main goals of improving the timing resolution and the detection effciency of the fission fragments. Further development of the STEFF spectrometer was done to enable 2E2V measurements of the 235U(n,f) reaction with coincident measurements using a white neutron spectra of energies ranging from 10 meV to200 MeV provided by the n TOF (neutron Time Of Flight) facility, CERN. The STEFF spectrometer was successfully operated twice on the Experimental Area-2 high flux pulsed neutron beam line resulting in 2E2V measurements for fission events with neutron energies ranging from 20 meV to 10 MeV. The first experiment received 1.36x10^18 POT (Protons On Target) with stable conditions and the second received 1.53x10^18 POT with stable conditions. The development of the stop detector resulted in a replacement MWPC (Multi-Wire Proportional Counter) detector for the second of the two experiments. This allowed direct comparison for the timing coincidence resolution, sigma_c, between the start and stop detectors and gave sigma_c = 0.81+/-0.08 ns for the prior PPAC (Parallel Plate Avalanche Counter) detector and sigma_c = 0.40 +/- 0.04 ns for the MWPC. The MWPC gave improved the detection efficiency per fission fragment of Eff = 0.67 compared to Eff = 0.43 for the PPAC. The methods and research described in this work also provided alternate stop detector designs with greater performance. This work produced two large data sets from the two successful deployments of the STEFF spectrometer on the n TOF beam line that will be the future work of many nuclear structure scientists to come.

In the ATLAS experiment software plays a central role in data taking, reconstruction, and analysis. This thesis presents a set of analysis algorithms written for the ATLAS software framework Athena, towards H+ searches in the τ+jets channel. Data recorded by the ATLAS experiment for 900 GeV proton-proton collisions from 2009 and 7 TeV proton-proton collisions from 2010 are analyzed with these algorithms and compared to Monte Carlo. A search for top quarks is performed on 36 pb-1 of data, and the results are found to agree well with Monte Carlo.

University, Diss., 2007--Frankfurt (Main). / Zsfassung in engl. und dt. Sprache.

University, Diss., 2000--Mainz. / Zeichendarstellung im Hauptsacht. teilw. nicht vorlagegemäß wiedergegeben.

University, Diss., 2002--Heidelberg.